ORNL-3804.txt

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ORNL-3804
UC-4 — Chemistry
TID-4500 (3%9th ed.)

 

RARE-EARTH HALIDES

R. E, Thoma

 

 

 

OAK RIDGE NATIONAL LABORATORY
operated by

UNION CARBIDE CORPORATION
for the
U.S. ATOMIC ENERGY COMMISSION

 

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ORNIL-3804

Contract No. W-7405-eng-26
Reactor Chemistry Division
RARE-EARTH HALIDES

R. E. Thoma

MAY 1965

OAK RIDGE NATIONAL LABORATORY
Oak Ridge, Tennessee
operated by
UNION CARBIDE CORPORATION
for the
U.S5. ATOMIC ENERGY COMMISSION

NAL LABORATORY LIBRARIES

OAK RIDGE NATIO | | .I || ||

3 445b 054d42ke 5

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
iii

CONTENTS

Abstract « ¢ ¢ ¢ 4 ¢ o s 4o e e s v e

Introduction « « o ¢ ¢ ¢ ¢« ¢« o « 4 &

Preparation and Purification . . « . &
The Trihalides « « ¢« ¢ ¢« o« o o+ &
The Dihalides =« « & o ¢ ¢ « o o &
The Tetrahalides « « « « + & « + &

Properties of the Simple Halides . . .

I~ W

o

Oxyhalides

Rare-Earth Metal-Trihalide Systems
Complex Halides and Phase Diagrams

Spectroscopy of the Rare-Earth Halides

References
Tables .

Figures .

*

-

O 0

10
12
15
19
25
40

 
RARE-EARTH HAIIDES

 

R. E. Thoma

ABSTRACT

A review is presented of the chemistry of the anhydrous
rare-earth halides. Topical emphasis is placed on the current
state of chemical development in this field, preparation and
purification methods, properties of the simple halides, the
oxyfluorides, rare-earth metal-trihalide systems, complex ha-
lides, phase diagrams, and absorption spectra. Tebular data
are included which summarize the currently available data on
physicel properties, oxidation potentials, crystallographic
properties, and phase equilibria.

 

INTRODUCTION

In the early stages of the United States nuclear reactor develop-
ment it became gquite clear that it would be necessary to separate the
rare-earth elements from spent fuels. Under the aegls of the USAEC,
separations processes were then devised from which the rare-earth ele-
ments were ultimately recovered via reduction of the halides. Develop-
ment of production-scale operations for this effort was achieved at the
Ames Laboratory, which became the chief producer of rare-earth elements
in this country. As production methods were refined and adapted for
commerce, individual rare earths of high purity became widely available.
Their availability has given impetus to numerous scientific and techno-
logical advences. Accomplishments of these vigorous development efforts
are now appearing in commercial applications such as magnesium alloy
production, new phosphors, masers, refractory ceramics, electronic com-
ponents, and in chemical reprocessing of nuclear reactor fuels. Indus-
trial consumers have, in general, preferred to obtain rare earths in

their elemental state, the principal modes for the production of which
have entailed reduction of the chlorides or fluorides. For this reason,
the earliest and most intensive investigation of the chemistry of the
rare-carth compounds was confined to the halides. This does not imply,
however, that the fundamental chemistry of the halides is more extensive-
ly known than for other rare-earth compounds, for the metal production
methods did not require highly intricate chemistry for success.

At the present time there is a fast-growing body of literature con-
cerning the chemistry and physics of the rare-earth halides, generated
by new fundsmental and technological interests in the rare earths. It
is being generated primarily independently by workers in the United States
and in Russia, if anything at a more rapid rate in Russia than in this
country. Some of the salient chemical interest in the rare-earth halides
now lies in their potential uses in laser device development, their role
in new methods for chemically reprocessing nuclear reactor fuels, and in
the systematics of the chemical relationships with the transuranium ele-
ments. One unfortunate characteristic of the present development is its
unsystematic approach to the chemistry of the rare-earth halides, caused
probably in part by the lack of liaison between U.S. and Russian workers.
Prospects that this situation will gradually improve appear brighter as
the growing number of international conferences such as the Rare Earth
Conferences and the IUPAC Congress, last held in Moscow, afford new ave-
nues of exchange. In addition, the approach is unsystematic because the
rare-earth halides have become employed in so wide a variety of research
programs and technical uses that new data on their properties are often
generated as incidental but necessary parts of other efforts. As a con-
sequence, comprehensive collections of property data have not heretofore
been readily accessible.

One of the purposes of this report is to provide a collection of
currently available chemical data for the rare-earth halides rather than
a discussion of the disciplines in which the materials are used. A re-
view of the current state of rare-earth halide chemistry calls primarily
for a comprehensive collection of the best property data at hand, and to
some extent for a perspective on the trends in research efforts. Although
the field has become quite active, many kinds of data are still required

before an appreciation of the significance of many current investigations

 
can be gained. In several broad areas, so little information is availa-
ble that a genuine perspective on the rare-earth halides must await and
anticipate the results of future researches. There is, for example, little
known yet concerning the bromides in unusual oxidation states, the crys-
tal chemistry of most of the halides, both simple and complex, and the
relation of electronic states to many of the chemical and physical proper-
ties of the halides. It may be anticipeted, therefore, that if the devel-
opments in rare~earth halide chemistry proceed at the current rate of
expansion, it will soon be possible to establish generalizations concern-
ing many phenomena about which we now have little insight. For these
reasons, there will clearly be occasion for additional reviews of the

rare-carth halides in the near future.

PREPARATTION AND PURIFICATION

As is exemplified in several recent chemical discoveries, e.g., the
noble gas compounds, a priori correlations between electronic structure
of some elements and the apparent oxidation state which they may exhibit
in the formation of compounds sometimes appears to be fortuitous. As
Moellerl has pointed out, this is found to be true with regard to the
oxidation states exhibited by the rare-earth ions in various anionic
(1igand) environments. The fact, therefore, that the rare-earth elements
are known to exhibit multiple oxidation states has in the past had little
value in predicting the chemical behavior of the elements in forming com-
pounds. Nevertheless, the character of a number of experiments conducted
in recent years indicates that it will soon be possible to specify the
identity and properties of all the stable halides of the rare-earth ele-
ments.

The fact that only fragmentary data exist which show the occurrence
and properties of rare-earth halides in their unusual oxidation states is
evidence of the small amount of attention which has been given to this
interesting aspect of rare-earth chemistry in the past. All of the tri-
valent halides have been prepared, although description of their physical

and chemical properties is, for the most part, in preliminary stages.

 
¥

and Asprey andwKEenanB appear

to have established the extent to which tetravalent rare-earth fluoride

Experiments conducted by Cunningham gg'gl.z

species may be produced. Tetravalent fluorides are now known for the lan-
thanons, Ce, Pr, Nd, and Tb. Until recently, the tetravalent halide state
was known only among the fluorides. That the chlorides may also be oxi-
dized to the (IV) state as is indicated in reports of the formation of
RbaPrCle and CsaPrClg by Pajak.o:f‘f,4 leads to the speculation that the list
of In(IV) halide compounds may soon be extended. Investigations of the
subhalides have scarcely begun. Apparently no attempts have been made to
investigate the thermodynamic stability of the monovalent halides. Numer-
ous reports, however, have described the preparation and partial charac-
terization of the halides in the divalent state. As late as 1960 only
Those involving Sm2+, Eu2+, Tm2+, and Yb2®* appeared to be supported by
convincing evidence. In the last few years a variety of subhalides have
been prepared both as pure single crystalline phases and as reduced spe-
clies dispersed within suitable host crystals. Some evidence exists5 that
all of the trivalent ions from cerium to ytterbium (and probably also
lanthanum) can be reduced in situ to the divalent state in CaFz by gamma

radiation.
The Trihalides

General techniques for the preparation and purification of the rare-
earth trihalides have become sufficiently extensive in their development
and application that several reviews of synthesis methods are now availa-
ble.é"8 Most recent reports appear to have emphasized methods for obtain-
ing the pure anhydrous trihalides.9"13 That the trihalides, except for
the fluorides, are water soluble suggests that conventional synthesis
methods, such as evaporation of saturated aqueous solutions, should afford
a sultable means for obtaining crystals of the halide. The saturating
phases in such solutions are, however, generally in the form of hydrates,
and the removal of the last traces of water is very difficult to accom~
plish without causing some hydrolysis to occur. Apparently the equilib-

rium constants for the hydrolysis reactions

 
InX3 + OH = InOX + HX

are very large for the forward reaction when X is any one of the halide
ions.

So little success is achieved by attempts to prepare and purifyy the
trihalides by methods which involve aqueous steps that high-temperature
conversion reactions using the methods described by Taylor,6 Carlson and

Schmid‘l:,'7 and Asprey and co-workerst©

are now generally considered to be
most effective. One general complaint which continues to be voiced regard-
ing rare~carth halides is that even though numerous properties of rare
earths are reported in the literature, synthesis methods are very often
neglected entirely, and those listed do not frequently produce an isolated
product for which property data can be obtained. Clearly, the greatest
impediment to successful synthesis of the pure chlorides, bromides, and
iodides is that imposed by their hygroscopic character. In the fluorides
as well, the sensitivity of the salt to hydrolysis makes it difficult to
free the product from contaminant oxide ion (as oxyfluoride). Analytical
methods for the detection of contaminant oxide ion in the rare-earth
halides have improved continuously in the last few years. Recent devel-
opments in chemical methods of analysis, e.g., vacuum fusion, polarographic,
and activation methods, now make it possible to obtain accurate oxide
analyses below the 100-ppm range. Other methods have evolved which now
give comparably accurate assays for cationic impurities. These methods
have yet to be used routinely in the researches with rare-earth halides.
The difficulties experienced in obtaining quite pure rare-earth halides
in the past indicate that it may be some time before the commonly used
values for the rare-earth halide properties can be considered to be well
established. It is an interesting aspect of rare-carth halide chemistry
that in earlier days the research chemist faced numerous experimental
frustrations connected with the difficulties of obtaining cationically
pure materials. Now that he has cationically pure rare-earth compounds
in abundance, his frustrations focus on the difficulty of obtaining anion-
ically pure phases.

The preparation and purification of the trihalides of the rare

earths is now standardized to a large extent, and the methods have been

 
adequately reviewed elsewhere. We shall turn then to the methods which
have been described for preparing the halides in their:usual oxidation

R
states. .

The Dihalides

Early investigators prepared the dihalides by reduction of the tri-
valent species from aqueous solutions with alkali metal amalgams,l4’l6
electrolytic reduction at a mercury cathode,l7’l8 and by action of magne-
sium metal on alcoholic salt solutions.19 Some success has been achieved
in reducing the anhydrous trihalides with hydrogen or ammonia2o at ele-
vated temperatures or obtaining the dihalide by thermal disproportionation
of a suitable trihalide.?l Recently, A. D. Kirshenbaum and J. A. Cahi1122
observed that they might reduce SmF3 by graphite at elevated temperatures.
This method has also been used by Brunton23 to produce the difluorides of
om and Eu. The difluorides produced were cubic, with lattice constants,
ap = 5.7% and 5.836 A, and refractive indices, 1.632 and 1.551, respec-
tively. Several industrial institutions appear to be producing divalent
rare earths as dopants in host fluorides such as CaFs for development of
optical lasers. Until recently, the only known means for producing diva-
lent ions of all the lanthanides in host fluoride cyrstals was by ionizing
radiation.”? All of the rare earths are reducible in this manner, but to
only a small fraction of the rare earth in the divalent state, 5 to 10%.

Kiss and Yocom24

have found recently that reduction of a large fraction
of the trivalent rare-earth ions can be achieved by heating the trivalent-
containing crystal in an atmosphere of the appropriate alkaline earth
metal. Crystallographic data for identifying the rare-earth subhalides
and knowledge of the extent to which they may occur as stable phases are
less well developed than for any of the other halide compounds. Many
investigations are currently under study in this field, and the previous

paucity of information is expected to disappear in the near future.

 
 

The Tetrahalides

Only the lanthanides, Ce, Pr, Nd, and Tb, are known to form tetraha-
lides. Although the pure tetrafluorides CeF,; and TbFz have been produced,2’25
tetrafluorides and tetrachlorides of praseodymium and neodymium were ob-
tained only by stabilization in solid solutions or complex halides-4’26’27
Bryan26 found that gaseous fluorination of solid solutions of PrF3 in CeFj
completely converted the ce3" to Ce4+, but fluorination of PrF3 was com-
pleted only when the mole fr%%tig%zof Pr was about 0.1l. With further in-
crease in the Pr content theiilafii'oxidation was reduced, becoming zero
for pure PrF3. Similar lack of success was encountered by other worker53’28
in attempts to prepare NdFg. Stabilization of the higher fluorides of Pr
wa.s ach:i.eve<129-’30 by preparing complex fluorides of the rare earths with
alkali fluorides. In this way, even dysprosium was partially oxidized to
the tetravalent state. The highly oxidizing conditions required for the
production of the tetrafluorides of the rare-earth fluorides and the re-
quirement for stabilizing ligands to preserve the ion in the tetravalent
state probably account for the few data on the tetrachlorides and the
absence of data for the tetrabromides and tetraiodides.

The special emphasis on purity and quality of crystal growth which
pervades much of current chemical research is brought about by technolog-
ical needs such as those which arise in the transistor, optical maser,
and nuclear reactor industries. Such needs will, to be sure, bring about
Turther imaginative efforts for obtaining pure halides and extend our
knovledge of the materials even more rapidly. Unquestionably, the crys-
tallographic properties of the rare-earth trifluorides are more accurately
characterized than for any other class of the halides. The elegant crys-

25,31-33 1as pointed up the way for

tallographic work in the fluorides
many profitable new investigations. These have in turn made more rapid

advances possible in the basic and applied sciences.

 
 

PROPERTIES OF THE SIMPLE HALIDES

Development of practical methods for obtaining pure rare-earth
metals and compounds for research and industrial use has focused on the
compounds which are the most stable and least hygroscopic and which do
not introduce either difficulties in their preparation and purification
or undue expense in their production. As a result, the halides which
are best known and most accurately characterized are the fluorides and
chlorides. A careful survey of the available data on the halides reveals
that for even these classes of materials there is not yet complete con-
currence on the exact properties of the halides. A summary of the physi-
cal property data which is currently available for the halides is shown
in Table 1. Various other fragmentary data have been obtained and are
listed in other reviews.>* 37 Measured and estimated values for the oxi-
dation potentials of several of the II-III and II-IV couples are listed
in Table 2.

It mey be noted that a considerable fraction of the data given in
Table 1 are estimated values based on approximations of thermodynamic
values. Many of the other data are obtained from reagents of questionable
purity. New and more accurate data will be needed if the chemistry of
the rare-earth halides is to have the increased significance that its
intrinsic elegance suggests is possible.

Crystallographic data are available for many of the simple di-, tri-,
and tetrahalides of the rare earths. Virtually all of the deductions re-
garding symmetry of these crystals were made by U.S. workers. All cur-
rently available data are listed in Teble 1. Relatively few optical
identification data are available for the halides, and these are almost

32 showed

exclusively for the fluorides (see Fig. 1). Zalkin and Templeton
that the rare-earth trifluorides are dimorphic, occurring in hexagonal

and orthorhombic modifications. Some evidence that there are significant
differences in the lattice energies of these two forms was evidenced in
the thermal data obtained by Spedding and Daane,38 who observed that solid
state transitions occurred for some of the trifluorides. 1In current in-
vestigations of camplex fluorides at the Oak Ridge National Laboratory39

the dimorphic relationships of the trifluorides were determined from
thermal gradient quenching experiments and by high-temperature diffractomet-
ric measurements. The relative stabilities of the two crystalline forms

of the trifluorides at temperatures above 400°C are shown in Fig. 2. The
pattern of the hexagonal-orthorhombic transition temperatures through the
trifluoride series suggests that while orthorhombic modifications of Iala,
CeFa, PrF3, and NdF3 may exist, the inversion reaction probably occurs so
slowly that it will not be detected in conventional equilibration experi-

ments.

OXYHALIDES

Relatively few data describing the chemical properties of the rare-
earth oxyhalides appear in the literature. There arec adequate reasons
that this condition should continue to prevail in the future. A plethora
of such solid phases does exist; they are encountered by many experimen-
tal investigators, annoying evidence that insufficient care was given to
purification of a halide or that moisture-laden atmospheres were in con-
tact with the halide at some point. The contaminating phases have been
observed most frequently with the fluorides and chlorides largely because
of the lesser emphasis placed on bromide and iodide researches. In being
intermediate in size between the fluoride and chloride ion, the oxide ion
may be substituted through a wide concentration range in these heavy metal
fluorides and chlorides. As a result, compositional variability of the
rare-earth oxyhalides, with respect to the anionic ligands, is very exten-
sive, and accurate property data are very limited. Research problems
connected with these materials firstly require information as to the ex-
tent of oxide-halide miscibility before other data may be obtained. Recog-
nition of the generally indiscrete composition of the solid phases Just
then vitiates continuing interest in attempts to obtain other accurate
data.

Apart from scattered fundamental investigations of oxyfluorides, the
impelling interest in the rare-earth oxyhalides seems to arise in connec-
tion with the solubilities of the oxides, as slag or ore phases, in molten
solvents, i.e., in technological applications similar to those of signifi-

cance in electrolytic or pyrophoric methods of metal production.

 
 

10

Interest in the oxyfluorides and -chlorides has been recorded in this

country by Vorres in his examinations of oxyfluoridesz*o-’ZPl and in Russia

by Batsanov and co--wcf»rorkersz"2 and Aksel'rud-éB Riviello and Vorresd"l re-
ported that they had extended the known list of rare-earth oxyfluorides

from the lighter members of the series'g’l*'48 to include the entire series,
and that all of the rare-earth oxyfluorides were hexagonal and isostruc-

42,43

tural with LaOF.43 Russian workers measured variables which they

considered to be the controlling parameters governing the compositions
and properties of the rare-earth oxyf‘luor:i.desd"2 and basic chlorides;43
the results do not appear, however, to be definitive. Unit-cell data for
the oxyfluorides were measured by Baenziger et §£.45 Lattice constants
for the rhombohedral and cubic structures are listed in Table 3. As
Finkelnberg and Stein49 noted, the unit-cell constants of the fluorite-
type structure varied with the amount of fluorine in the lattice.

Aksel'rud and Spivakovskii®O-2%

studied the composition and solubil-
ities in agueous media of the basic rare-earth chlorides. They concluded
that in such solutions the saturating phases were In(0H)2Cl (for In = La,
Sm, Er, and Yb) and In(OH)1.75C1l1.25 for In = Y and Dy. After aging for
some time the solids became Ln(OH)z,sclo,s and for Sm and Dy the pure
nydroxides.

The chief impediment toward genuine characterization of the oxyhalides
of the rare earths is the improbability that single crystals studies will
be made of these phases. Development of theoretical estimates of the ex-
tent and mechanism of anionic miscibility in these materials must await
the results of such studies, and these are improbable for the reasons

stated above.

RARE-EARTH METAL-TRIHALIDE SYSTEMS

Much interest has arisen concerning the interactions of metals with
their molten halides. Two reviews of the status of such research are

available.53’54 As Bredig points out, "though not well known to most
chemists, the subject is at least 150 years old." That the scope of chem-

ical research in this area has come to include the rare-earth metals and

 

 
11

their halides is cause for same exuberance among rare-earth chemists.

The principal reason lies in the certainty that the research methods de-
veloped by the principal investigators, Bredig, Corbett, and Cubicciotti,
afford direct means for producing all of the equilibrium subhalides of
-7 of the

unusual oxidation states of the rare earths, cite experimental attempts

the rare-earth elements. Asprey and Cunningham, in their review

to produce the rare-earth tetrafluorides. The high oxidation potentials
of the In(III)-(IV) couples suggest that the tetrachlorides, -bromides,
and -iodides would be impossible or difficult to prepare. A recent
report5 indicates, however, that tetravalent chlorides of praseodymium
can be obtained, in the complex compounds RbaPrClg and CsaPrCle, if not
as the simple halides.

Of the 60 lanthanide metal-trihalide systems some 13 have been inves-
tigated. The subhalides observed in those studies are shown in Toble 4.
The results of the measurements of magnetic susceptibility and resistivity
measurements by workers in this field lead to the interesting view that
the mere existence of a definite intermediate compound in the phase dia-
grams of the rare-earth metal-trihalide systems cannot be taken as a state-
ment of the actual oxidation state of the rare-earth ion in the compound.
The subhalides of the rare earths have been observed to vary extensively

in their salt-like character. Mee and Corbett conclude56

from magnetic
properties of the reduced solids as well as from cryoscopic behavior of
the dilute melts that the divalent state becomes increasingly stable in
~the lanthanide halides from lanthanum through europium but that with gad-
olinium a sharp decrease in the reducibility of the tripositive ion would
be anticipated. The same reasoning leads to the expectation that the
heavier lanthanides would form subhalides much as occurs in the lighter
group. The results of a current investigation of the Er-ErClsz and Er-ErIs

systems57

show, surprisingly, that subhalides are not formed in either
system, even though substantial solubilities are observed. Theoretical
reasons for the behavior in the metal halide systems are understandably
incomplete. The appearance of subhalides corresponding to apparently
unusual oxidation states is generally attributed to trapped electrons in

the compound. As Bredig states it,58

 
12

"One might distinguish two subgroups, depending on the nature
of the second, metal-rich phase (solid or liquid) that, at
saturation, is in equilibrium with the molten salt-rich phase:
(a) The reaction goes so far as to lead to the crystalliza-
tion of a solid 'subhalide.' (b) The metallic element itself
(with a small amount of salt dissolved) forms the second
phase. 1In this group, the lower valence state of the metal
is stable only in the molten solution.

"The division between [those systems in which the metal
imparts metallic character to its solution and those in which
strong interaction occurs between metal and salt] is not
clear cut, and both mechanisms of dissolution may describe
a single system. The distinction is that between relatively
mobile electrons and electrons which attach themselves to,
and become part of, ions to produce a lower valence state.

There are likely to be intermediate cases which are not

clearly defined, and the attachment of electrons {(or 'subha-

lide'! formationj may be a matter of degree only, rather than

a matter of an equilibrium between two distinctly different

states of the electron attached and unattached. Also, this

may vary with temperature and composition."

The conclusion to be drawn from the work on the subhalides and the
tetrahalides is that chemically stable compounds can be expected as true
di-, tri-, and tetrahalides of the rare earths. No data yet support the
view that the monovalent oxidation state is stable, nor “he pentavalent
or higher oxidation states. Interoxidation compounds, like those formed

59,60 or from the chromium fluorid_es6l appear to

in the rare-earth oxides
have been observed. The excellent quality of the experimental work being
reported in this field today portends that most interesting and definitive
new results may be expected on the rare-earth subhalides in the near fu-

ture.

COMPLEX HALIDES AND PHASE DIAGRAMS

Since highly pure rare earths have become readily available in the
last few years, it would have been expected that numerous investigations
of the complex halides might have been initiated. It is a remarkable
aspect of rare-earth halide chemistry that so few of such investigations
have actually begun. Except for a few early reports showing that complex
halides, generally hydrates such as CsLaClg°4H20, CsSmCle*6H20 and

 

 
13

CsPrle,-5H20,62 have been obtained from aqueous solutions, compound forms-
tion in anhydrous systems does not appear to have been investigated before
Dergunov examined the complex compound formation in the binary systems of
the alkali fluorides with some of the lanthanide trifluorides.®3s%% yntil
very recently, nearly all of the data concerning the complex halides of
the rare earths was obtained in connection with attempts to examine the
stability of the tetravalent lanthanides-4 As a result, the crystallo-
graphic and melting point data required for characterizing the compounds
are not generally available. A list of the known complex halides and
available crystal property data is given in Table 5. The rudimentary state
of knowledge concerning the complex halides is displayed by the fact that
data are available only for the complex compounds involving some of the
tri- and tetravalent fluorides, a few trichlorides, but no bromides or
iodides, or divalent species.

That so few investigations of the anhydrous halide systems of the
rare earths have been made is somewhat surprising, because there are un-
questionably many easily synthesized complex compounds in these systems
which have not yet been described. There is, for example, almost certainly
a large class of stable 3:1 compounds which will form in all of the KF-,
RbF-, and CsF-LnF3 binary systems, in the KCl-, RbCl-, and CsCl-LnCl 3,
and RbBr- and CsBr-InBrj binary systems, and possibly in the CsI-InIj3 sys-
tems. Perhaps one reason for the dearth of experimental results on the
complex halides has been the tacit assumption that differences in behavior
among the lanthanides would be imperceptible. 1In a study correlating the
effects of cation size on the occurrence of complex fluoride compounds in
the binary systems of the alkali fluorides and the rare-earth trifluorides,
the author65 estimated that these systems would fsll into three groups,
simple eutectic systems without intermediate compounds or solid solutions,
systems forming a single equimolar intermediate compound, and systems form-
ing a congruently melting cryolite-like phase. The LiF-~-InFj3 systems (Fig-
3) fit this category rather well, as do the KF-, RbF-, and CsF-based sys-
tems (Figs. 5-16). The NaF-InF3 systems, however, are found to display
remarkable complexity (Fig. 4), possibly related to the ease with which
Nat and In3* may become randomized in the high-temperature solid phases
in these systems. The phase regions in these systems (Figs. 3 and 4) cor-

respond to those in the somewhat analogous systems L:‘LF—YF366 and NaF-YF3-67

 
14

The phase diagram series, LiF-InF3 and NaF-InFj3, shown in Figs. 3 and
4, comprise the only detailed descriptions of the phase behavior involving
any one class of rare-earth compounds. The availability of information
for these two complete series now makes it possible to interrelate behav-
iorial differences in such systems with minor differences in the sizes of
the rare-earth ions. Data are listed in Table 5 for the lattice constants
and refractive indices of the tetragonal LiF-InF3, cubic NaF:InFj3, and
hexagonal NaF:InFj3 crystal phases. The low-temperature form of the com-
pound S5NaF-:9InF3 occurs as crystals which appear to be an ordered modifi-
cation of the disordered fluorite phase, stable at higher temperatures.
The structure of this phase has not yet been solved; it is known, however,
to be of low symmetry.68

The contrast in complexity of the LiF- and NaF-based systems with the
trifluorides raises questions as to whether the KF-, RbF-, and CsF-based

behave as simply as de-
scribed. The fact alone that cubic and hexagonal forms of KF+LaFj3 have
been described (see Table 5) suggests that equilibria in the KF-LaF3 sys-
tem may be more nearly related to those in the NaF-LnF3 systems than would
be inferred from the KF-LaF3 phase diagram.

Apart from the fluoride systems described above, few phase diagrams
are available for the rare-earth halides, and these pertain exclusively
to the chlorides. The lack of concurrence regarding equilibria among the
reported alkali chloride-lanthanon trichlorides is so pronounced that com-
parative properties of the systems are listed (Table 6) rather than the
phase diagrams themselves. These systems were apparently investigated as
part of the efforts to establish the phase diagrams for the ternary sys-
tems NaCl-CaClz-LaCls,®? Nac1-CaClz-Ndcl3,®9 KC1-MgCla-CeCls, 0 and NaCl-
KCl-PrCl3l7 (Figs. 17-20). The ternary systems are of considerable
interest in Russia for the development of treatment methods for ores con-
taining the rare esrths and for the preparation of rare-earth metals by
molten bath electrolysis.

In other investigations of rare-earth trichloride binary systems
Novikov 92.2&'72 found from thermogravimetric and tensiometric studies
that FeClz-InCls (In = La, Ce, Pr, Nd.) pairs formed simple eutectic sys-
tems without intermediate compounds. Sun and Morozov'70 found much the

same behavior for the system MgClz~CeCl 3.

 

 
 

15

In the only mass spectrometric analysis reported for the alkali

73 observed the ionic

chloride=-InClj systems, Semenov and Gavryuchenkov
species, K, kc1', Ercl™, Ercl,t, KErcl,*, and KErCls®, in the vapor
state at temperatures up to 1000°C.

That the results described above comprise almost all of the recorded
efforts with mixed systems of the rare-earth halides is indicative of the
primitive state of research in this area. As rare-earth science and tech-
nology advances it is expected that multicomponent system investigations

will receive much more attention.

SPECTROSCOPY OF THE RARE-EARTH HALIDES

Much of the fundamental theoretical development from which an under-
standing of the chemical properties of the rare-earth elements arises is
produced from studies of absorption spectra. The rare-earth halides come
to have special interest in these studies for some of the following rea-
sons. FElectronic transitions in infrared, visible, and ultraviolet radia-
tion occur as (1) intraconfigurational transitions (infrared), i.e., f-f
transitions, which are forbidden by LaPorte's rule, and (2) Rydberg or
electron-transfer transitions (ultraviolet), e.g., those involving 3f-4d
transitions. In the lower transition series of the elements, spectra of
the gaseous ions are generally sufficiently amenable to study that the
lines in the absorption spectrum can be identified unequivocally. 1In the
rare-earth ions, however, this condition does not prevail, for in these
elements the number of electron transitions produced from the variety of
species in the ionized gases is so greét and the spectra so complex that
advances cannot be made in identifying the spectra. If rare-earth ions
are available in an established (single) oxidation state and in a known
configuration, then the problem of assigning absorption at low temperature
is very much simplified. Of unique benefit is that the f-electrons in
the rare earths are so well shielded that the contribution to the spectra
by electron replusion and spin orbit coupling is nearly the same as in
the gaseous ion, and the spectra approach those in the gaseous state. 1In
crystals, of course, crystal field effects produce splittings of the free-

ion states. Because the halides are probably the best characterized of

 
 

16

the rare-earth compounds, and because good crystals of the pure materials
can be prepared in a straightforward manner, much of the current rare-earth
absorption research is employing the halides and complex halides of these
elements.

In the last few years there has arisen a great deal of interest in
the rare-earth spectra in the near infrared and visible regions arising
from developments of the optical maser. For development of masers, exten-
sive information concerning electron transition probabilities is required,
a fact which seems to account for a considerable number of current reports
on absorption spectra. Rapid progress in maser technology has generated
a wave of intense research which promises to continue and perhaps even en-
large in the next few years. It will be fitting, therefore, that a thorough
review of this field be made in the relatively near future.

Two tantalizing ramifications of absorption spectra studies concern
the prospects that (1) the probability of monovalent stability among the
lanthanides may be appraised, and (2) that information gained from lantha-
nide spectra will be of correlative value for future investigations of the
actinides. The intrinsic stability of the monovalent state can apparently
be estimated only from knowledge of the high energy electron transfer states,
a field of research for the rare earths which has been neglected except for
the work of J;firgensen-74 It may be some time before contributions to this
appraisal are made fram spectra data. There is good reason to anticipate
that absorption spectra data obtained from the lanthanide halides will con-
tribute much to the future elucidation of actinide electronic structures.
As Sancier and Freed75 have noted, for example, studies of the crystal
absorption spectra of y* through cm®" show that there is marked resemblance
with the lanthanide spectra. Other investigators as well, e.g.y Carnall
and Fields,76 have expressed some enthusiasm in connection with the use of
anhydrous halides and for obtaining useful information as to the electronic
structures of the actinides.

In an earlier review, K':r'umholz,7r7 referring to aqueous solution chem-
istry of the rare-earth halides, cited Freed's78 inference that the absorp-
tion spectra of Eu?* ions in aqueous solutions are very similar to those
of the ions in crystals. Both consist of very sharp lines clustered into

separate groups at about the same wavelengths. Many factors are involved

 
17

in obtaining experimental data for undistorted spectra. To obtain as near
free-ion spectra as possible, many workers have turned to anhydrous crystal-

13 that signifi-

line media after finding, as Batsanova and co-workers did,
cant shifting and widening of the absorption bands takes place among the
rare-earth fluorides if the fluorides are hydrated. Such effects were at-
tributed to hydrogen bonding. Typical of the studies with anhydrous halides
are Krupke and G-ru'ber's79 analysis of the absorption spectra of Er3* in
1aF3. In this work lines were identified for 21 experimentally observed
intraconfigurational 4f electronic transitions, all of which were in agree-
ment with those expected from 411 free-ions.

The last comprehensive treatment of lanthanide absorption spectra was
that given by McClure in 1959.80

are currently available, even though the rate at which reports are being

No formal reviews of rare-earth halides

published on the subject now exceeds 50 a year. In lieu of a current re-
view, the recent surveys by McClure and K’iss,2 Dieke and Pandey,8l and
Smith82 will suffice. It is becoming evident from the numerous reports

in the literature concerning the near infrared, visible, and ultraviolet
absorption spectra of the rare-earth halides that these anhydrous materials
afford convenient media for use in locating and assigning the many lines
arising from intraconfigurational 4f transitions. That the actinide ha-
lides possess similar attributes, i.e., good radiation stability, trans-
parency, and relative ease of avallability as single crystals or polycrys-
talline materials, has made it possible for a good start to be made on

studies of the actinide absorption s;pectra.83’84

 
10.

1l.

i2.

13.

14.

15.
16.
17.

18.

19

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Table 1. Physical Propertics of the Rare Earth Halildes

 

Crystal Structure Data Thermochemical Data (kéal/mole)

 

 

 

 

Compound ?éfij ?6§5 Symnmetry Space Lattice Parameters, A ?§7§2§y AHformation AHfusion vaporizationa

Group aq by Cq (298°K)

LaF3 1490 hexagonal P63/mem  7.186 7. 7.352 5.936 (~421)P

IaCls 852 . 2700 hexagonal P63/m 7.483 4o 364 3.848 -255.9 13.0 ¢

1aBrs 783% (-233) 13.0

Ials 761 orthorhombic Cemm 437 14.01 10.04 2.246 -189

1als 830 c

CeFy dec . e monoclinic I2/c 12.6 10.6 8.3 (=442)

CeFs 1437,1430 2327 hexagonal P63/mem 7.112 7.279 6.157 (-416)

CeCla 802 1925 hexagonal 7450 44315 252.8

CeBr 5 722 1705 (~228)

Cels 761 1397 orthorhombic Cemm 4 o341 14.00 10.015 2.273 -185 12.4

Celz c

Pr¥y, monoclinic 12/c

Prfi 1395 2327 hexagonal P63/mem  7.075 7.238 6,14 (-413) 8 62

PrCls 786 1905  hexagonal 7,422 4.275 -252.1 12.1 52.3

PrCla -163

aVapor pressure data may be found in reports by J. A. Gibson et al., The Properties of the Rare Farth Metals and Compounds,
Battelle Memorial Institute, Columbus, Ohio, 1954, and by E. Shimazaki and K. Niwa, Z. anorg. und allgem. Chem. 314, 21 (1962).

bFigures in parentheses are estimates.

 

CLalz, Celo, and PrIz are isostructural but of a different structure than NdIpz, SmIz, and YbIp which corprise a second
isomorphous series [G. I. Novikov and 0. G. Polyachenok, Usp. Khim. 33, 732 (1964)].

G

 
 

Table 1. (continued)

 

Crystal Structure Data Thermochemical Data (kecal/mole)

 

 

 

 

Compound %6§5 %égi Symmetry gpace Lottice Paramelers, A %;?izgy formation AHfusion fllHvaporization

TOoup 3o by Co (2980K)
PrBr 3 693 1547 -225 11.3 45
Prls 733 1377 orthorhombic Cemm 44309 13.98  9.958 2.309 -152 12.7 4 41
PrIa 760 c
NdFg monoclinic I2/c
NdF 3 1374 hexagonal P63/mem  7.030 7.200 (-410) 62
NACl 3 760 -245.6 12.0 - 51.8
NdCla 835 -163.2
NdBr 3 684 (-223) 10.8 46.8
NAT s 775 orthorhombic Cerm 4,284 13.979 9.948 2.342 (-158.9) 9.7 ¥ 41
NdIa 565 c
SmF 3 1306  (2327) hexagonal P63/mem  6.956 -- 7.120 6.925 (-405) 8

orthorhombic Prima. 6.669 7.059 4.405 6.643
Sml'2 (1377) (2427)  cubic Fm3m 5.79 - -- (-2%90) 5
SmC1 3 678 hexagonal 7.378 . 4,171 =243
SmCla 859,740 (2027)  orthorhombic 8.973 7.532 4497 -195.6
SmBr 3 664 1645 (-216)
SmBra 700  (1877) a - - (-182)
SmI 3 820 hexagonal R3 7 .490 - 20.80 3.141 (=174)
Smlz (527) (1577) e (-155)
uF3 1276  (2277) hexagonal P63/mem  6.916 - 7.091 7.088 (-391)
orthorhombic Pnma 6.622 7.019 4.396 6.793

EuF (1377) (2527) cubic Fm3m 5.842 (-300)

dSrBrz structure.

eSmIz and Fulz are considered to be isostructural, although neither structure has been determined.

oc
 

 

 

 

 

 

Table 1. (continued )
Compound E%P ] 5P, Cry;;zieSt ruc giiggt;armemrs - Pensity Ther.'mochemical ]‘)ata (kcal/moli:) -
c) (°¢) Symmetry Group = = é (g/cc) formation fusicn vaporization
° ° o (298°k)

EuCls 774,623 dec. hexagonal 7.369 -- 4.133 (-233)
EuCla 738  (2027) orthorhombic Pnmb 8.914 7.499 4.493 (-210)
EuBr 3 (705)  dec. (-203)
EuBra (702) (1873) orthorhombic (-187)
Fuls (880) dec. (-159)
Fula (527) (1573) e
GdF3 1228,1231 (2277) hexagonal P63/mem (7.064) (6.900)

\ orthorhombic  Prma 6.571 6.985 4.393 7.056 (=404) 8 60
GACls 609,602; (1577) hexagonal 7.363 -- 4.105 (-240.1 9.6 45
GdBr 3 785  (1487) (-214) 8.7 b,
GdIs 931 (1377) hexagonal R3 7.539 _— 20.83 3.138 -170 10 40
Gdla 831
TbF. monoclinic 12/c 12.1 10.3 7.9
TbF 3 1172 (2277) hexagonal P63/mem (7.035) - (6.875) 8 60

orthorhombic Pnma 6.513 6.949 4.384 7.236 (-400)
ToCl3 588  (1547) (-241) 7 45
ToBr 3 (830) (1483) (-211) 9 4y
ToI 3 955 (1327) hexagonal R3 7.526 -- 20.838 3.155 (-169) 10 40
DyF3 1154  (2227) hexagonal P63/mem (7.010) .- (6.849) (-398) 8 60
orthorhombic Pnma 6.460 6.906 4.376 7465

DyCls 654  (1627) monoclinic 6.91 11.97  6.40 (-236) 45
DyBr 3 881 (1473) (-209) 9 by
DyI; 955 (1317) Thexagonal R3 7.488 - 20.833 3.210 -166 10 41

LC

 
 

 

 

 

 

 

Table 1. (continued)
Crystal Structure Data . Thermochemical Data (kecal/mole
Compound ?6?5 ?5P3 Symmetry rygpace lLattice Parameters, A ?Z?iigy Formation Fusion ( viporiiation
roup aq be Co (2989K)
HoF'3 1143 (2227) hexagonal P63/mem  6.833 - 6.984 7.829
orthorhombic Pnma. 6.404 6.875 4.379 7 . 644, (-395) 8 60
HoCl 3 720 (1507) monoclinic 6.85 11.85 6.39 -233 7.0 44,
HoBr 3 914 (1467) (-207) 10 43
Hol 3 1010 (1297) 3.240 ~164 10 41
ErFs 1140  (2227) hexagonal P63/mem (6.952) - (6.797)
orthorhombic Prioma, 6.354 6.848 4.380 7.814 (=392) 8 60
ErCls 776 (1497)  monoclinic 6.80 11.79  6.39 211.4 7.8 44,
ErClp =150
ErBrs 950  (1457) (-205) 10 43
ErT s 1020 (1277) hexagonal R3 7.451 - 20.78 3.279 ~162 10 40
TinF 3 1158  (2223) hexagonal P63/mem  6.763 -- 6.927 8.220 (-391)
orthorhombic Prma 6.283 6.811 4.408 7.971 60
TmCl 3 821 (1487) monoclinic 6.75 11.73 6.39 -229 9 4,
TmBr 3 (955) (1437) (-203) 10 43
TnT 3 (1015) (1257) Thexagonal R3 7.415 -- 20.78 3.321 -137.8 10 40
T hexagonal 4.520 - 6.967
YbF3 1157 (2227) hexagonal P63/mem (6.897) - (6.745) (~376)
orthorhombic Pnma 6.216 6.786 4.434 8.168 8 60
YbF 1477  (2377)  cubic 5.571 - - (-280) 5 75
YbCl3 854  dec. monoclinic 6.73 11.65 6.38 -228.7, =223 9 ~—
YbCla 723  (1927)  orthorhombic 6.53 6.68 6.91 184.5 6 50
YbBr3 940  dec. (~185) 10 --

8¢
Table 1. (continued)

 

Crystal Structure Data Thermochemical Data (keal/mole)

 

 

 

 

Compound I?c')gj }(BéP ) Symmetry gfqgie lattice Pa;-a.meters s A I(DZI}E;CM);}[ Mo mation 8Ha sion AHvap orization
T ao o C o (2980}{)

YbBra 677  (1827) (-157) 6 48
YbIa (1030) dec. hexagonal R3 7 43 -- 20.72 3.331 (-143) 10 -—
YbI 527  (1327) hexegonalfl c6 4503 - 6.972 (-135) 5 37
TuF3 1182  (1427) hexagonal P63/mem (6.87) - (6.72)

orthorhombic Pnma 6.181 6.731 4446 8.44 (-392) 8 60
TuCl 3 892  (1477) wmonoclinic 6.72 11.60 6.39 -227.9 9 43
TuBrs 960  (1407) («200) 10 42
TuT 3 1045  (1207) hexagonal R3 7.395 - 20.71 3.386 ~133.2 11 38

fIsostructural with Tmla.
Bibliography

t

L. B. Asprey and B. B. Cunningham, "Unusual Oxidation States of Some Actinide and Lanthanide Elements," p. 267-302 in Progress in
Inorganic Chemistry, ed. by F. A. Cotton, Vol. II, Interscience, New York, 1960.

 

L. B. Asprey, T. K. Keenan, and F. H. Kruse, Inorg. Chem. 3, 1137 (1964).

H. Bormer and E. Hohmann, Z. anorg. Chem. 248, 357 (1941).

D. T. Cromer, J. Phys. Chem. 61, 753 (1957).

R. B. Cunningham, D. C. Feay, and R. M. Rollier, J. Am. Chem. Soc. 76, 3361 (1954).
W. DOll and W. Klemm, Z. fir anorg. allgem. Chemie, 241, 239 (1939).

L. F. Druding and J. D. Corbett, J. Am. Chem. Soc. 81, 5512 (1959).

A. S. Dvorkin and M. A. Bredig, J. Phys. Chem. 67, 697 (1963).

 

6c
 

(Bibliography for Table 1, continued)

. S. Dworkin and M. A. Bredig, J. Phys. Chem. 67, 2499 (1963).

A. Gibson et al., The Properties of the Rare Farth Metals and Compounds, Battelle Memorial Institute, Columbus, Chic, 1954.
Jantsch and N. Skalla, Z. anorg. Chem. 193, 391 (1930).

Kubachewski and E. L. Evans, Metallurgical Thermochemistry, Wiley, New York, 1956.

 

R. Machlan, C. T. Stubblefield, and L. Eyring, J. Am. Chem. Soc. 77, 2975 (1955).

E. Mee and J. D. Corbett, Inorg. Chem. 4, 88 (1965).

. I. Novikov and O. G. Polyachenok, Usp. Khim. 33, 732 (1964).

Porter and E. A. Brown, J. Am. Ceram. Soc. 45, 4 (1962).

L. Quill, ed., Chemlstry and Metallurgy of Miscellaneous Materials, Thermodynamics, McGraw-Hill, New York, 1950.
Shimazaki and K. Niwa, 7. anorg. und allgem. Chem. 314, 21 (1962).

H. Spedding and J. P. Flynn, J. Am. Chem. Soc. 74, 1474 (1954).

H. Spedding and A. H. Daane, eds., The Rare Earths, Wiley, New York, 1963. Covee o . © s

. Zalkin and D. H. Templeton, J. Am. Chem. Soc. 75, 2453 (1953).

og
Teble 2. Oxidation Potentials for Some In(II)-(III) and In(III)-(IV) Couples

 

 

 

Reaction E Reference

pr3t = pr4t 4+ &7 2.9 v L. Eyring, H. Lohr, and B. B. Cunningham, J. Am. Chem. Soc.
74, 1186 (1952).

sm?" = sm3t + e~ -1.55 v A. Timnick and G. Glockler, J. Am. Chem. Soc. 70, 1347 (1958).

Bt = Bu?t + e” -0.43 v H. N. McCoy, J. Am. Chem. Soc. 58, 1577 (1936).
W. Noddack and A. Bruckl, Angew. Chem. 50, 362 (1937).

™3 = m4" + e~ -2.9 v L. B. Asprey and B. B. Cunningham, "Unusual Oxidation
States of Some Actinide and Lanthanide Elements,' in
Progress in Inorganic Chemistry, Vol. IT, ed. by F. A.
Cotton, Interscience, New York, 1960.

Y2t = yu3" e~ -1.15 v Noddack, op. cit.

 

e

 
32

Table 3. Unit-Cell Dimensions of Rare-Earth Oxyfluorides,
Rhombohedral LaOF Structure Type®

 

 

ain & a in degrees
LaOF 7.132 £ 0.001 33.01 £ 0.01
LaOF 7.132 +* 0.001 32.99 % 0.01
ProF 7.016 £ 0.004 33.03 £ 0.03
NdOF 6.953 £ 0.001 33.04 £ 0.01
SmOF 6.865 + 0.002 33.07 £ 0.02
EuOF 6.827 * 0.002 33.05 * 0.02
GAOF 6.800 * 0.001 33.05 £ 0.01
TbOF 6.758 * 0.011 33.02 £ 0.09
CeOF” 6.985 * 0.001 33.56
CeOF 5.703 % 0.001 cubic, face centered
CeOF 5.66 % 0.01 to 5.73 * 0.01

 

®N. C. Baenziger, J. R. Holden, L. E. Knudson, and
A. I. Popov, J. Am. Chem. Soc. 76, 4734 (1954).

thombohedral cell dimensions corresponding to the
face-centered cubic cell actually observed.
33

Table 4. Rare-BEarth Metal-Trihalide Systems

 

 

 

System Subhalides Formed Crystal Data Reference
1a~IaCls none 1
La-laBr3 none 2,3
La-Lals lal2,42, Lal2.00 4
Ce=CeCl3; none 5
Ce-CeBra none 2,3
Ce=Cels Cela.4y Celz.o 6
Pr-PrCls PrCla, 31 fee, ag = 7.00 A 7,8,9,10
Pr-PrBra PrBrs, 338 2
Pr-Prls PrIs.s0, PrIz.oo 4
Nd-NdCl 3 NdClz.ay, NACla.27 fece, ay = 7.00 A 8,10
Nd-NdT 3 NdI;:.os 8
Sm-SmCl 3 SmCla, SmClz, 10 11
Gd~-GdCls GACl;.s3 orthorhombic, a, = 8.98, 12
by = 7.22, ¢cg = 6.72 A
Gd-GdI 3 GdIz2.04 hexagonal, a, = 8.67, 12
co = 5.75 A
References
1. F. J. Keneshea and D. Cubicciotti, J. Chem. and Engr. Data, 6, 507 (1961).

2. R. A. Sallach and J. D. Corbett, Inorg. Chem. 2, 457 (1963) .
3. G. Cleary and D. Cubicciotti, J. Am. Chem. Soc. 74, 557 (1952).

4- Jo D-

Corbett, L. F. Druding, and C. B. Lindahl, Discussion Faraday Soc.

32, 79 (1961).

5. G+ W.
6. G. W.
7. G. I.
8. L. F.
9. L. F.
10. G. I.

Mellors and S. Senderoff, J. Phys. Chem. 63, 1111 (1959).

Mellors and S. Senderoff, J. Phys. Chem. 64, 294 (1960).

Novikov and O. G. Polyachenok, Zh. Neorg. Khim. 7, 1209 (1962).
Druding and J. D. Corbett, J. Am. Chem. Soc. 83, 2462 (1961).
Druding, J. D. Corbett, and B. N. Ramsey, Inorg. Chem. 2, 869 (1963).
Novikov and O. G. Polyachenok, Zh. Neorg. Khim. 8, 1053 (1963).
Polyachenok and G. I. Novikov, Zh. Obshch. Khim. 33, 2797 (1963).
Mee and J. D. Corbett, Inorg. Chem. 4, 88 (1965).
Table 5. Crystal Property Data for Camplex Halides of the Rare-Earth Elements

 

 

 

 

ggmmpncfizd Symme try Sp(a;.]c::.e Tg;:up Lattico:e Constant sc c() A) Ref;:act ive Ifiiices Reference
IiF+EuFa tetragonal I41/a 5.21 11.02 1.477 1.510 1
LiF*GdF3 tetragonsal I41/a 5.21 11.00 1.474 1.502 1,2
LiF-TbF3 tetragonal 141/a 5.19 10.89 1.476 1.502 1,2
IiF-DyF3 tetragonal I41/a 5.19 10.81 1.476 1.504 1,2
LiF-HoF3 tetragonal T41/a 5.16 10.75 1.472 1.498 1,2
LiF-ErFj tetragonal 141/a 5.16 10.70 1.474 1.502 1,2
LiF+TmF, tetragonal I41/a 5.15 10.64 1.470 1.498 1,2
LiF+YbF3 tetragonal I41/a 5.14 10.59 1.470 1.496 1,2
LiF+IuF3 tetragonal I41/a 5.13 10.55 1.468 1.494 1,2
NaF:LaF3 hexagonal P& 6.157 3.822 3,4
NaF:CeF3 hexagonal P& 6.131 3.776 1,3,4,5
NaF:PrF32 cubic FinJm 5.695-5.702 6
NaF+PrF3 hexagonal P6 6.123 3.822 3
NaF:NdF3 cubic Fm3m 5.670-5.678 6
NaF+NAF, hexagonal P6 6.100 3.711 3
NaF:SmF 3 cubic Fm3m 5.605-5.628 6
NaF*SmF3 hexagonal P& 6.051 3.640 3
NaF:EuFs cubic Fm3m 5.575-5.605 6
NaF:EuF3 hexagonal P& 6.044 3.613 3

e
 

 

 

 

Table 5. (continued)

Compound Space Group Iattice Constants (A) Refractive Indices

Formila Symmetry or Type ag e Né T, Reference
NaF:GdF3 cubic m3m 5.552-5.583 6
NaF:GdF3 hexsgonal P6 6.020 3.601 3
NaF:TbF3 cubic Fm3m 5.525-5.563 6
NaF+TbF3 hexagonal P6 6.008 3.580 3
NaF:DyF3 cubic Tm3m 5.508-5.545 6
NaF:DyF3 hexagonal P6 5.985 3.554 3
NaF:HoF3 cubic Fm3m 5.480-5.526 6
NaF+«HoF3 hexagonal P6 5.991 3.528 3
NaF:ErFi cubic Fin3m 5.455-5.510 6
NaF+ErFi hexagonal P6 5.959 3.514 3
NaF:TmF 3 cubic Fm3m 5.435-5.495 6
NaF-TmF 3 hexagonal P6 5.953 3.494 3
NaF:YbF3 cubic Fm3m 5.418-5.480 6
NaF+YbF3 hexagonal P6 5.929 3.471 3
NaF:LuFs cubic Fim 5.400-5.465 6
NaF+LuF3 hexagonal P6 5.912 3.458 3
KF-LaFj cubic Fin3m 5.9321 4

hexagonal Co2m 6.526 3.791

RbF+1alF3 7
3CsF+laFj3 7
KF-PrFa 8
3RbF-PrF3 8

GE

 
 

Teble 5. (continued)

 

 

 

 

ggfizfid Symmetry szge ngzup Lat‘cize Constant iO(A) Revazactive Ing_:ces Reference
3CsF+PrFj 8
3KF+SmF 3 8
3RbF+Smk3 8
3CsF+SmFa 8
3KF-ErF3; 8
3RbF-ErF3 8
3CsF*ErF3 8
2NaF-:CeFy 9
2KF'-CeFy 9
3RbF-CeFy, 9,10
3CsF-CeFy cubic (NH,) 3A1Fs ~10 2,9
2CsF+CeFy 9
2NaF“PrFy hexagonal C32 9
NaF-PrFy, 9
2KF*PrF, hexagonal C32 9
2RbF-PrFy, hexagonal C32 9
3RbF-PrF, 10

ot
Table 5. (continued)

 

Compound

Space Group

Tattice Constants {(A)

 

Refractive Indices

 

 

Formula Syrmetry or Type o ey T, N, Reference
3CsFPrF, cubic (NHg) 3A1Fg ~10 9
3CsF-NdF, tetragonal (NHg ) 32rFy ~10 9
3CsF-DyFs;  tetragonal (NH,) 3ZrFq ~10 9
3CsF+TbF, cubic (NHg ) 3A1Fg ~10 9

 

aNaF:Lan denotes the compositionally variable fluorite phases shown in Fig. . Lattice constants
are given for the fluorite phase at the solubility limits.

References

1. G. D. Brunton et al., Crystallographic Data for Some Metal Fluorides, Chlorides, and Oxdides,
ORNL-3761 (in press).
2. K. S. Vorres and R. Riviello, Proceedings of the Fourth Rare Farth Research Conference, Phoenix,
Ariz., April 22-25, 1964.

3. J.
4o W.
5. C.
6. R.
7. E.
8. E.
9. R.
10. H.

H.
H.
Je
E.
P.
P.

Burns, Inorg. Chem. (in press).

Zachariasen, J. Am. Chem. Soc. 70, 2147 (1948).
Barton, J. D. Redman, and R. A. Strehlow, J. Inorg. Nucl. Chem. 20, 45 (1961).
Thoma et al., unpublished work.

Dergunov, Akad. Nauk SSSR, 60, 1185 (1948).
Dergunov, Akad. Neuk SSSR, 85, 1025 (1952).

Hoppe (Univ. of Munster, Germany), unpublished work.
Bode and E. Voss, Z. anorg. Chem. 290, 1 (1957).

Le

 
 

 

 

 

 

 

Table 6 . Properties of Alkali Chloride - Rare-Earth Chloride Binary Systems
In-Chzhu %;353?2 Morozov Novikov and Baev {1961)P Shevtsova et al. (1962)C Novikov et al. (1964)d
System M.P. or Max. M.P. or Max. M.P. or Max. M.P. or Max.
Compound Temp. of Compound Temp. of Compound Temp. ofo Compound Temp. of
Stability (°¢) Stability (°C) stability (°c) stability (°c)
NaCl-CeCls Eut., 510°C at 60NaCl-40CeClja
mele %
NaCl-PrCli Eut., 480°C at 59NaCl-41PrCli But., 468°C at 62NaC1-42PrCls
mole % mole %
NaCl-NdCl 3
KC1-I1aCls 3KC1+1aCls (625) -
_— 2KC1-IaCls (645)
- KCl-3LaCls (620)
KC1-CeCl3 B-3KC1:CeCls (628) 3KC1+CeCl3s (640)
Q=KC1-CeCl3 (512)
2KC1+CeCl3s (623) -
- 3KC1°2CeCls (675)
- KC1-3CeCla (548)
KC1-PrCl; 3-3KC1+PrCls (682) 3KC1*PrCl s (675) 3KC1+PrClj {658)
Ct=3KC1-PrCl 3 (512) --
- 2KC1+PrCls (620) 2KC1*PrCl, (612)
- 3KC1 +2PrCl 4 (615) -
KC1-NdCl 3 B=3KC1-NACl 3 682 3KC1°NACl 3 (690)
0-3KC1-NACl 5 345 2KC1+NaCls (640)
3KC1+2NdCl3 (590)
KC1-5mCl 3 3KC1+SmCl 3 (750)
2KC1+SmCl 3 (574)

KC1+29mCl 3 (530)

8¢
 

 

 

 

 

 

 

Table 6. {continued)
Tn-Chehn 28 e orome Novikov and Baev (1961)P Shevtsova et al. (1962)C Novikov et al. (1964)%
System M.P. or Max. M.P. or Max. M.P. or Max. M+sP. or Max.
Compound Tm@-oflj Campound Twm-ofi) Compound Tmm-ofij Compound Temp. of
Stability (°C) stability (°C) stability (°c) Stability (°C)

KC1-SmCl 2 KC1+25mCly (620)

KC1-YbCl 3 0-3KC1 *YbCl 3 (800)

B-3KC1+YbCl 3 (385)

KC1-YbClz KC1+YbCly (640)
(sCl-LaCls B-3CsCl+TaCls (777)
0-30sC1Talls (389)
CsCl-CeCls B-3CsCl-CeCls (800)
030501 +CeCl 3 (400)
CsC1-NdCla B-3CsCl*NACl 3 {813)
0t=3CsC1+NACL 3 (345)

 

#In-Chzhu Sun and I. S. Morozov, Zhur. Neorg. Khim. 3, 1916 (1958).

b

®7. N. Shevtsova, E. N. Korzina, and B. G. Korshunov, Zh. Neorg. Khim. 7, 2596 (1962).

dG- I. Novikov, 0. G. Polyachenok, and S. A. Frid, Zh. Neorg. Khim. 9, 472 (1964).

G. I. Novikov and A. K. Baev, Vestnik Leningrad. Univ. No. 22, Ser. Fiz. i Khim., No. 4, 116 (1961).

6t

 
REFRACTIVE INDEX

.63

40

ORNL—-DWG 64-2797R

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 1.

Refractive Indices of the Rare-~Farth Trifluorides.

¢
.62 N |—2
5
.61
0
o AQ
N
.60 f>—<>—$—(z +
o Q 0
(L fi)
.59
C
.58
9
Q
.57 9O
e STARITZSKY AND ASPREY, AR
ANAL. CHEM., 29,857 (1957) al |
.56
C
.55 T
L P P E Tb H T L
“ce T Nd "sm Y 6d oy CEr "y U

 

 
TEMPERATURE (°C)

41

ORNL-DWG 65-1781

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

T T T T T T T T
o F.H. SPEDDING AND A.H. DAANE, THE RARE
1600 EARTHS, WILEY, NEW YORK, 1961, p.78. __
o R.E. THOMA, efa/., UNPUBLISHED DATA
(\
1400 T~
LIQUID
1200 A
—
HEXAGONAL LnF 0
1000 /J,
800 //
/ ORTHORHOMBIC LnFs
600 /
J
400 /,
200 /

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 2.

0
Lla Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

LANTHANON

Dimorphism Among the Rare-Earth Trifluorides.

 
42

ORNL-DWG 65-187R

BN ‘TT[llTII

1300

 

  

H0C

900 [ -

 

 

 

 

700 — : — —

50C

 

 

300

 

\

 

 

 

TEMPERATURE (°C)

 

500:|_iF NdFy
LYt

BT T T T T T 7]

 

EI .

 

300

 

T

1100 — —

900 +—

700 —\/ —

500 -

[
W\

 

 

<

 

 

 

 

L L 1]

 

 

 

[ E—

oo L L Ll

 

 

 

BT T T T1]
(100

900

 

 

 

{
Ll
T T T
\

 

700 ]

 

 

{

TEMPERATURE (°C)

P00 T LiF DyFs | | LiF HoFy |
oLl Ll B bty

O T T T T T T T ]

|
|
|

 

 

 

 

 

 

OO —
900 —
700 N

500 ™ LiF TmF,

 

 

 

 

 

 

 

INRENARE

LiF YbFy

S
20 40 60 80 100
LnF3 {mole %)

 

 

 

 

 

 

 

 

—

 

300

 

O
o
o
3
D
O
@
o
8
o

Fig. 3. Phase Diagrams of the Lithium Fluoride-Rare-Earth Trifluoride
Systems.

 
ORNL- DWG €5-1BBR

 

 

1300

1100

900

 

 

 

 

 

700

 

500

 

 

 

 

 

300
1300

 

 

1100 T

900

 

 

700 {—

 

TEMPERATURE (°C)

500 -~

 

 

 

 

 

300

 

1300

 

1100 [

900

 

 

 

 

700 -

500 | -

 

 

 

 

300

 

1300

 

 

3
S

900

8

 

 

 

 

 

TEMPERATURE (°C)

500

 

 

 

 

 

 

 

 

 

300

 

 

1300

{100 -

900

700

 

 

500

 

 

 

 

 

 

 

 

 

 

 

 

300
0 20 40 60 80 100 O 20 40 60 80 00 O 20 40 60 80 {00

LnFy (mole %)

Fig. 4. Phase Diagrams of the Sodium Fluoride-Rare-Earth Trifluoride
Systems.

 
ORNL-DWG 65-705

/

1400

| /1
{300
/
1200 /

LIQUID /

100 /

 

 

 

 

 

1000

 

 

LaF5 +LIQUID

 

900
/
800 \\ // 770°

KF + \ ,@ LaF
00 | lQuiD \\/ +LIQUID

 

 

TEMPERATURE (°C)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

620° W
600 h
'—f KF-LQF3 +LaFy
500 KF + KF - LaF
400

 

 

 

KF {0 20 30 40 50 60 70 80 S0 L0F3
LaF5 (mole %)

Fig. 5. The System KF-laF3 After E. P. Dergunov, Doklady Akad. Nauk
5SSR, 60, 1185 {1948).

 
 

45

ORNL-DWG 65-706

1400 /
1300 /.
1200 /
1100 LIQUID /

1000 /

900 _ LaF 5+ LIQUID

800 /

700 N // 68i4°
RbF + \
teun N\ —~——-RbF - LaF5 + LIQUID

 

 

 

 

 

 

 

 

 

TEMPERATURE (°C)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

600 N 582°
M
5 RbF - LaF; + LaF.
1 ’ 3
500 RbF + RbF -LaF, - 3
0
o
400
RbF 10 20 30 40 50 e0 70 80 90 LaFs
LaF3 (mole %)

Fig. 6. The System RbF-LaF3 After E. P. Dergunov, Doklady Akad. Nauk
SSSR, 60, 1185 (1948).
 

46

ORNL-DWG 65-707

 

 

 

1400 /
1300 //
1200 /

100 LIQUID //
1000

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

e
~ /
L
5 / LaF, + LIQUID
5 900 /
8 3CsFrLaF3 +LIQUIDY  /
5 800 /] D/
— N
s \/ 726°
700\
600 // \/6000 LLO
CsF+LIQUID| 19 3CsF-LaFs+LaFy
500/ CsF + 3CsF+LaF 31 %
0
400

CsF 10 20

W
O

40 50 60 70 80 90 LaFy
LaF, (mole %)

Fig. 7. The System CsF-LaF3 After E. P. Dergunov, Doklady Akad. Nauk
SSSR, 60, 1185 (1948).

 
 

47

ORNL-DWG 65-710

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1400 /
e
1300 -
1200 //
—~ 100 ,/
$ /
W 41000
x
- /
2 /
= 900
§ N LIQUID PrF, +LIQUID
W 800
\\\\ 740°
v |
700 /’ L KF-Prfy + LIQUID
{ 610°
600\
KF +LIQUID .
W KF-PrF; + Prig
500 o
KF + KF - PrFy i
sool— |
KF 10 =20 30 40 50 60 70 80 90 Prfy
Pre, (mole % )

Fig. 8. The System KF-PrF3 After E. P. Dergunov, Doklady Akad. Nauk
SSSR, 85, 1025 (1952).

 
 

ORNL-DWG 65-714

P

1400

 

1300 s

1200 /
Ve
LIQUID /
1400

4000 //
900 / PrFz + LIQUID
800 797 /

NN/
700 _\ \ \764}0\:& \/

 

 

 

 

 

 

 

 

 

TEMPERATURE (°C)

 

 

 

660°

 

 

 

~3RbF-PrFz + LIQUID

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

600 _RbF‘+L|QL‘J|D
RbF + 3RbF - Pri5 3RbF -PrFy + Prfz
500
~3RbF-PrF3
400

RbF 10 20 30 40 50 60 70 80 90 Prfs
PrFz (mole %)

Fig. 9. The System RbF-PrFi3 After E. P. Dergunov, Doklady Akad. Nauk
SSSR, 85, 1025 (1952).

 
49

ORNL-DWG 65-742

e

1400

 

 

1300
/
1200 yd

LIQUID /
1100

 

 

 

 

 

e 7

. /

£ 1000 /

l_

= 920° / PrFy + LIQUID
W 900 AT

B / \\ o

~ 800 783

 

 

 

 

“3CSF'PrF3 + LIQUID

 

 

 

700 654°

\CSF +LIQUID

 

 

 

 

600

SCSF : Pl"F3

3CsF-PrFz + Prfy
CsF + 3CsF - PrFg

 

500

 

 

 

 

 

 

 

 

 

 

 

 

 

400
CsF 10 20 30 40 50 ©0 7O 80 90 Prfs

Pr F3 (mole 70)

Fig. 10. The System CsF-PrF3 After E. P. Dergunov, Doklady Akad. Nauk
SSSR, 85, 1025 (1952‘)/.
50

ORNL—DWG 65—713
1400

 

 

1300 p

1200 LIQUID ~

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1100 /
o 882° /
émoo i W— 3KF - SmF, + /o
o LIQUID/ SmF, + LIQUID
+
< 900 _(KF+LIQUID ' mF,
L
o 7|\ .
= 800 |-\ . 800
— 758 1
700 ‘
Ll_m
600 — KF +3KF- —UE) ———— SKF - SmF3 +SmF3
SmF3 .
L
500 x
400

KF 10 20 30 40 50 60 70 80 90 SmF3
SmFy (mole %)

Fig. 11. The System KF-SmF'3 After E. P. Dergunov, Doklady Akad.
Nauk SSSR, 85, 1025 {1952).

 
51

ORNL-DWG 65-714

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1400
1300 —
LIQUID -

1200 /,
_ 1100 //
s 3RbF-SmF,+LIQUID L
W 1000 V4
2 RDF+LIQUID | oge / SmF, +LIQUID
< 900 T/
o- v /\L /
a
S / \/ 800°
i 800
- /

700 N/ 700

3RbF-SmF; +SmFy
600 |
«——RbF + 3RbF - SmF 4
500
3 RbF-SmF
400 | |

 

 

 

 

 

 

 

 

 

 

 

 

RbF 10 20 30

40 50 60
SmFy (mole %)

70

80 90 SmF

Fig. 12. The System RbF-SmF3 After E. P. Dergunov, Doklady Akad.

Nauk SSSR, 85, 1025 (1952).

 
52

ORNL-DWG 65-715

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1400
1300 ———
LIQUID /

1200 //
__ 1100 /
& 3CsF-SmFy +LIQUID
w1000 9700 /
> //\\h //// SmF,+ LIQUID
é 9200 // \ / ‘
L \/ 855°
s
w800 /

/ _———CsF +LIQUID
645°
J 3CsF-SmF3+ SmF3
600 |
——CsF + 3CsF - SmF3
500
~—3CsF-SmF4
400 ! i

 

 

 

 

 

 

CsF 10 20 30 40 50 60 70 80 90 Sm F3
SmF, (mole %)

Fig. 13. The System CsF-SmF3 After E. P. Dergunov, Doklady Akad.
Nauk SSSR, 85, 1025 (1952).

 
 

53

ORNL-DWG 65-716

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1400

300

LIQUID
1200
a-Erf3 +LIQUD~ -
1100 =
3KF - Erf +LIQUID —

= 1012° //
£ 1000 N -
" /Y
5 KE+ A
E 900 LIQUID \ ~ B-ErF3+L|QU|D
S 7
o
L
S 800\/ / \/ 790°
= N/ 756°

700

600 - . ;

KF+3KF-Erfy [£ 3KF -ErFz+B-ErFs
500 v
400
KF 10 20 30 40 50 60 70 80 90 ErFs

ErFz (mole %)

Fig. 14. The System KF-ErF3 After E. P. Dergunov, Doklady Akad.

Nauk SSSR, 85, 1025 (1952).

 
 

54

ORNL-DWG 65-717

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1400
1300
1200 LIQUID
a-ErFy+ LIQUID —
1400 —— 3RbF - ErFz+LIQUID 3 /\(
G 1034° /
2 1000 AN A
Ll / -~
: [AN
S5 900 pr 4 A B-ErFz+LIQUID |
& LIQUID e
> ) 800°
= 800
|_
\\/ 732°
700
LL.m
600 W 3RbF -ErF5+3-Erfy
RbF +3RbF -Erfz ?é
500 rO
400

 

 

 

 

 

 

 

 

 

 

 

 

 

RbF 10O 20 30 40 50 60 70 80 30 ErfFs
ErFz (mole %)

Fig. 15. The System RbF-ErFj After E. P. Dergunov, Doklady Akad.
Nauk SSSR, 85, 1025 (1952).

 
55

ORNL-DWG 65—718

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1400 l l
1300 LIQUID
\ a-ErF, + LIQUID
1200 3CsF - ErF, + LIQUID
1100 ‘ ‘ N
, roag® -~ 3CsF - Erfy+ LIQUID
O x\\\» //,//’
1000 <
m
o O /
Sl s /N LINL
900 & _ —
< = / N~ B-Erf, + LIQUID
a + " °
> 800 |6 800
W O
|.—
700 <;/
\/ 643° |
600 e - 3CsF-ErF3 + B—ErF3
CsF +3CsF -  |Y
500 Erfy L
O
I
400 |
CsF 10 20 30 40 50 60 70 80 90 Efi%
ErfFy (mole %)
Fig. 16. The System CsF-ErF3 After E. P. Dergunov, Doklady Akad.

Nauk SSSR, 83,

1025 (1952).

 
56

ORNL-DWG 65—703

LaCl

  
 
  

TEMPERATURES IN °C
COMPOSITIONS IN mole %

700
750

N/
Cc:CI2

 

NaCl
£ -494°

Fig. 17. The System NaCl-CaClz-LaClz After I. S. Morozov, Z. N.
Shevtsova, and L. V. Klyukina, Zhur. Neorg. Khim. 2, 1640 (1957).

 
57

ORNL-DWG 65-704

 

 

NdCl g
TEMPERATURES IN °C
COMPOSITIONS IN mole %
700
650
600
550 7 .
£E-430
£-585°
550
700 700
750 50
COC|2 NaCl
£ —-494°

Fig. 18. 'The System NaCl-CaClp-NdCli After I. S. Morozov, Z. N.
Shevtsova, and L. V. Klyukina, Zhur. Neorg. Khim. 2, 1639 (1957).

 
58

ORNL—DWG 65—701

CeCi3
TEMPERATURES IN °C
COMPOSITIONS IN mole %
/
750
700
650 /
X 420° 550
/ ~3KCI-CeCl4
~445°
4552
428
430%,
N/

 

 

KCl
MgCl,,

/ /
KCI-MgCl,  2KCI-MgCl,

Fig. 19. The System KC1-MgCla~-CeCls After In'-chzhu Sun and
I. S. Morozov, Zhur. Neorg. Khim. 3, 1923 (1958).

 
PI"C|3

59

ORNL-DWG 65-1782

TEMPERATURES IN °C
COMPOSITIONS IN mole %

5—468\

—~SS MINIMUM-640

600

 

 

700 o8
VoLV \/ N/ KC|
£-483/ p-e12/ / \£-610
2KCI- PrClg 3KCI - PrCly

Fig. 20. The System NaCl~-KCl-PrCls After Z. N. Shevtsova, E. N.

Koszina, and B. G. Korhunov, Zhur. Neorg. Khim. 7, 25% (1962).
6l

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