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Phase equilibria in the system CsFZrF4
J. Inorg. N u c l Chem., 1965, Vol. 27. pp. 559 to 568. Pergamon Press Ltd. Printed in Northern Ireland
PHASE EQUILIBRIA IN THE SYSTEM CsF-ZrF~* G. D. ROBaINS,t R. E. THOMA and H. INSLEY** Reactor Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee (Received 24 July 1964)
Abstract--The phase equilibrium diagram of the condensed system CsF-ZrF, has been constructed from a combination of results obtained using thermal analysis, visual observation of crystallising fluoride mixtures, and quenching techniques. Phase identification was accomplished by petrographic and X-ray diffraction analysis. In the system CsF-ZrF4 three intermediate compounds were isolated and identified: Cs3ZrFT, Cs2ZrFs, and CsZrFs. The compounds CssZrF7 and CsZrF5 melt congruently at 784 and 518°C respectively; Cs~ZrFs melts incongruently to CssZrF7 and liquid at 530°C. Polymorphism is exhibited by Cs2ZrF6 and CsZrFs, the latter compound showing a markedly exothermic inversion on cooling below 330°C. Crystals of the high-temperature equilibrium forms of Cs2ZrF6 and CsZrF~ were not retained on quenching; crystallographic verification of liquid-solid transitions was therefore unobtainable in the low-melting region of the system. The phase CssZrF7 was observed to vary in composition. As a homogeneous single phase, the solid solution extends to the composition 80CsF-20ZrF4 (mole ~o). X-ray diffraction data obtained from single crystals of the stoichiometric phase revealed that the compound is face-centered cubic with a0 = 9.70 .~., and the space group is F~3m, F432, or Fm3m. This compound is apparently isotypic with KsZrFT,(NH,)sZrFT, and KaUF7 as regards the metal ions, but no deduction can be made from this work as to the exact arrangement of the fluoride ions. INTRODUCTION PRELIMINARYinvestigations of the system CsF-ZrF4, conducted at this L a b o r a t o r y by BARTON and co-workers/1~ employed cooling-curve techniques exclusively for obtaining the data used in the construction o f the phase diagram. The complexity in solid phase relationships which prevails a m o n g the other alkali fluoride-ZrF 4 phase diagrams and the paucity o f data available for the complex c o m p o u n d s in the C s F Z r F 4 system suggested the need for a more detailed study o f this system, employing a variety o f additional techniques. EXPERIMENTAL Apparatus and methods
The phase equilibrium data were obtained from heating and cooling curves data, by direct observation of melts crystallising at known temperatures, and bY identifying phases present in mixtures which were equilibrated and quenched. The thermal effect techniques used for measurement of the temperatures which define the CsF-ZrF4 phase diagram, as well as the X-ray and microscopic techniques used in identifying phases, have been discussed previously/2.3~ * Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation. t Temporary student employee; present address: Chemistry Department, Princetown University, Princetown, New Jersey. +*Consultant to Oak Ridge National Laboratory. ,l~ C. J. B A R T O N , L. M. BRATCHERand W. R. GRIMES, Phase Dia,q,rams of Nuclear Reactor Materials, p. 58. (Edited by R. E. THOMA)ORNL-2548 (Nov. 2, 1959). ,2~ C. J. BARTONet al., J. Amer. Ceram. Soe. 42, 63 (1958). ~a, H. A. FRIEDMAN,G. M. HERaERT and R. E. THOMA, Thermal Analysis and Gradient Quenching, Apparatus and Techniques for the lnvestig,ation of Fused Salt Phase Equilibria, ORNL-3375 (Dec. 18, 1962). 559
560
G. D. ROBBINS, R. E. THOMA and H. INSLEY
The visual-thermal apparatus employed in determining liquidus temperatures permitted observation of the formation of the first crystals on cooling the liquid melt. Samples were heated in a glove box which contained a helium atmosphere and were stirred manually with a shielded thermocouple. The thermal effects recorded on cooling the melt usually occurred within 2°C of the visually observed liquids, the thermal effects occurring at the lower temperature. The accuracy of the temperature measurements reported in these studies is limited by the characteristics of the ChromelAlumel thermocouples used. Manufacturers estimate this accuracy to be within 5° in the temperature range 400-800°C, and the accuracy and precision of the determinations reported here are considered to be within this limit of error. Some commercial caesium fluoride reagents used in this study contained approximately 1 wt. ~o water and were therefore predried before use in preparation of specimens for phase studies. Zirconium fluoride is easily converted to oxyfluoride or oxide at elevated temperatures, and caesium fluoride is extremely hygroscopic. Therefore, it was necessary to remove small amounts of water and oxygen as completely as possible from starting materials To facilitate removal of these substances, ammonium bifluoride was added to mixtures of CsF-ZrF4 before initial heating in the thermal analysis experiments. The same mixtures were used later for quenching experimehts. As in previous fluoride phase studies conducted at this Laboratory, all mixtures were protected from exposure to the atmosphere after purification at high temperatures. Materials
The mixtures used in the phase equilibrium studies reported here were prepared from commercially available caesium fluoride and ..from hafnium-free zirconium fluoride. In the course of the investigation, three grades of caesium fluoride were used. In the initial thermal analysis experiments, a commercial product was employed which contained approximately 1 per cent contaminant as alkali metal cations, principally as rubidium, potassium, and sodium. It provided satisfactory data for confirming liquid-solid transitions described earlier, c~ A higher quality of product was used in subsequent thermal-gradient-quenching experiments in order to ascertain the effect of the alkali metal impurities on the CsF-ZrF4 transition temperatures. For these experiments a reagent was obtained in which the concentration of metallic contaminants did not exceed 0.2 per cent. This product was purified further by heating it above its melting point in a dry helium atmosphere and allowing it to cool slowly, thereby effecting segregation of impurities in the melt. The best crystals of the salt were then selected in a dry box and mixed with ZrF, to form specimens of the appropriate compositions. Zirconium oxide and oxyfluorides are virtually insoluble in CsF-ZrF4 melts. In these melts contaminant oxides, in concentrations even as small as 2000-3000 p.p.m., are converted to ZrO2 and zirconium oxyfluorides, thereby introducing considerable uncertainty into the determination of liquid-solid transitions where visual or quenching methods are employed. Removal of chemisorbed water and oxide contaminants from CsF by reaction with ammonium bifluoride is much more difficult than is corresponding purification of the less hygroscopic alkali fluorides. Consequently, highly accurate determinations of liquidus determinations, using the visual method, require reagents of very high purity. For these experiments, melts were prepared from optical-grade single crystals of CsF and freshly sublimed ZrF4. The melting point' of the two better grades of CsF was determined to be 703°C, in good agreement with a previous determination by BREDI~ and co-workers,~ who obtained data from a very pure specimen. RESULTS AND DISCUSSION T h e liquidus d a t a o b t a i n e d in the c u r r e n t investigation o f the C s F - Z r F 4 system essentially confirm the liquidus described earlier by BARTON a n d co-workers, tl~ M u c h new i n f o r m a t i o n has been o b t a i n e d concerning the p h a s e relations o f the three interm e d i a t e c o m p o u n d s in the system. T h e phase d i a g r a m o f the system, based o n the d a t a o b t a i n e d in the c u r r e n t investigation, is shown in Fig. 1. T h e i n v a r i a n t equilibria a n d solid-state t r a n s i t i o n p o i n t s are listed in T a b l e 1. E x p e r i m e n t a l d a t a are s h o w n in T a b l e 2. O p t i c a l a n d X - r a y identification d a t a are listed in Table 3. A s a consequence o f the occurrence o f the two c o n g r u e n t l y melting c o m p o u n d s , Cs3ZrF 7 a n d C s Z r F s , the system C s F - Z r F 4 is c o m p r i s e d o f essentially three subsystems. T h e p h a s e d i a g r a m is described below in terms o f the subsystems. ~J M. A. BREDIG,H. R. BRONSTEINand W. T. SMITH,Jr., J. Amer. C/tern. Soc. 77, 1454 (1955).
Phase equilibria in the system CsF-ZrF4
561
950
900
850
I 800
750
700 I o v Ld tr :) I'--
650
hi I3-
600
! r
-4-
I L~J I--
i
./
550 518 °
500 465 ° 450
400
350
i
I
I
300
CsF
~0
20
30
40
50
60
70
- 80
90
J ZrF 4
ZrF 4 (mole %) FIO. 1 . - - T h e system CsF-ZrF,.
The subsystem CsF-Cs3ZrF 7 The components of the subsystem, CsF and CsaZrF 7, melt congruently at 703 and 784°C respectively. The eutectic formed from these solid phases occurs at 10 mole% ZrF 4 and at 646°C. Single crystals of Cs3ZrF7 were prepared from melts at the compound composition, and an X-ray diffraction study was made using a Buerger precession camera. (5) Results of the investigation revealed that CsaZrF 7 is face-centred cubic with a = 9.70 4= 0.02 A; the space group is F2t3m, F432, or Fm3m. Although the fluoride-ion arrangement is disordered, the Z 6 + and Cs + are at (or very near) (5) G. D. ROBBINS and J. H: BURNS, 1962).
X-ray Diffraction Study of CsaZrFT, ORNL TM-310 (Aug.
7,
G. D. ROBBINS,R. E. THOMAand H. INSLEY
562
(0, 0, 0) ~ face centering and (½, ½, ½; ¼, ~, ¼; t, ], ]) q- face centering, respectively. The X-ray density of the compound is 4.53 g/cm -a. This compound is apparently isotypic with KaZrFT, (NH4)aZrF ~, and KaUF7 as regards the metal ions, but no deduction can be made from this work as to the exact arrangement of the fluoride ions. Powdered samples of CsaZrF7 which are not protected from the atmosphere a r e hydrolysed rapidly; the only crystalline phase which appears in the residue is that of the low-temperature form of Cs2ZrF s. The diffraction pattern of C%ZrF s is not observed for samples prepared in a dry box, sealed in a protective mount, and examined TABLE I.--INVARIANT EQUILIBRIA IN THE SYSTEM CsF-ZrF4
Composition
Invariant
(mole ~. ZrF4) temp. (°C) Type of equilibrium l0 25
646 784
37
530
--
465 425 518 330 465
42 50 -55
Eutectic Congruent melting point Peritectic
Phase reaction invariant temperature
L* ~ CsF + CssZrF7 ss
L ~ CsaZrF7 L + CssZrF7 ss ct-CssZrF6 Inversion of Cs2ZrFo et-CsaZrF6 ~ fl-Cs2ZrF6 Eutectic L ~ Cs~ZrF6 + ~t-CsZrFs Congruent melting L ~ ct-CsZrF5 Inversionof CsZrF5 ~t-CsZrF6 ~ fl-CsZrF6 Eutectic L ~- ct-CsZrF5 + ZrF4
* L refers to liquid. immediately after heat treatments. However, after several hours the sealed samples show mixtures of Cs3ZrF 7 and Cs2ZrF 6. The X-ray photographs of single crystals of Cs3ZrF7 always showed some trace of C%ZrFs, even though considerable effort was made to exclude moisture. The reciprocal lattice of Cs~ZrF e was always axially related to that of CssZrFT, and twinning was evident. These relation.s suggest that CsaZrF 7 decomposes into CszZrF 6 and CsF at an equilibrium temperature well below the temperature ranges encountered in this study and that this reaction is greatly accelerated in the presence of water vapour. CsF is not identifiable in the reaction product because of its extreme hygroscopicity. The compound CsaZrF ~ was observed to vary in composition as shown in the phase diagram, accommodating slightly more than 20 per cent CsF into its lattice at the solidus. Refractive indices of the solid solution were observed to vary in this composition range from 1.470 at the stoichiometric composition to 1.480 near the CsF-rich terminus of the solid solution. Solid solution of an alkali fluoride in 3:1 compounds has not been observed previously in any alkali fluoride-MF4 system. That cation sites in CsaZrF7 are completely occupied in an ordered arrangement by Cs ~ and Zr 4+ ions excludes the possibility that this solid solution is substitutional, but the solution mechanism has not been identified. One of the purposes of CsaZrF 7 single-crystal studies was to learn the nature of the anionic configuration in the solid phase. This goal was precluded by the occurrence of the disordered anionic lattice. In an attempt to obtain information about ionic association in the molten state, we measured the effect of ZrF 4 on CsF freezing-point depression from 0 to 10 mole % ZrF 4. On the assumption that the activity of CsF
Phase equilibria in the system C s F - Z r F 4
563
TABLE 2 . - - C s F - Z r F . PHASE TRANSITION TEMPERATURE DATA Composition (mole ~ ZrF~) 0 2.0 4"0 5"35 6-0 6.20 7.0 8-05 9.1 10.1 !1.2 13 14 15 17 20 21 22.2 23 25 26 30 32 33.3 34"5 36 37 38 39 40 41 42 43 44 45 46 47 50 51 52 53 54 55.6 60 61 62.3 63 63-7 65 66.7 68 69-2 75
Liquidus temp. (°C) TA* TGQ'[" VO~ 702 693 683 675 672 670 666 659 652 643 640 683 705 720~4§
646
754
761
530 486 464 473
784 t: 2
625
644£4 634 656 628
734:]::4 778 784 777 746 714 645 566 563 557
Other transition temperatures (°C) TA TGQ
646 638 645
720
726 754::[:4
Solidus temp. (°C) TA TGQ VO
700:[:4 708 i 4
780
700
517 444 :L 2
523 502 526 525 504 421 401 411 425 417 425
530~2
530 526~3
427 413 :~ 2
464 530, 452, 320 450, 329 463, 331
462 464 :t: 464i2
425 416 i 2
317 33O
416 :L 2 471 487 501 498 510 506
499:[:2 512:]:2 515::[:2
426 414 425 399
317 329
518 518 513 505 488 492
547
485 449 461 467 465 465 477
443 425,328 325
559 ! 2 550
475 572 473 623
658 664
476 465
326
741
* Thermal analysis of cooling curve data. t Thermal gradient quenching data. ~: Phase transition temperature determined by direct visual observation of melt during freezing, § Temperature limits indicate temperature interval between segments of annealing tube.
564
G. D. ROBBINS, R. E. THOMA and H. INSLEY TABLE 3.--OPTICAL AND X-RAY DIFFRACTION DATA FOR C s F - Z r F , COMPOUNDS CssZrF~ Equilibrium stability range: <784°C Optical character: isotropic Refractive index: 1.471 X-Ray diffraction data Symmetry: Space group: Density: d(A), obs. d(A),
fcc, a = 9.70 A* F~[3m, F432, or Fm3m 4.53 g/cm (calculated) talc. Lobs. hkl
3"43
3'43
vs
2-92
2"92
vw
220 311
2.803
2.779
vw
222
2"424
2"424
s
400
1"980
1.989
s
422
1"715
1'714
m
440
1.535
1"533
ms
620
1.297
1.296
ms
642
fl-Cs~ZrFe Equilibrium stability range: <465°C Optical character: uniaxial ( - ) Refractive indices: No, = 1.482, Ne = 1-460 X-ray diffraction data d(A), obs.
Symmetry: hexagonal, P~ml,t a = 6-41 A, c = 5.01 A d(A), calc.:~ /, obs. hkl d(A), obs. d(A), calc.~ /, obs. 5"551
100
2.098
210
5"01
110
1.974
112
4'11 3'73
hkl
m 3'720
3.54
vs
101
1-932
1"935
m--
211
1"857
1"860
m--
202
1"850
300
1.670
w+
3-20
3-205
m+
110
1.667
w÷
003
2-771
2"776
w+
200
1-601
w~
103
2.687
2-700
w+
111
}-480
w÷
2.501
2"505
w~-
002
1.336
w
2.430
2"428
m-I-
201
1"308
w--
2.286
2"284
m --
102
1.236
w--
Phase equilibria in the system C s F - Z r F 4 Table 3
(contd.) ~-CsZrF 5 Equilibrium stability range: 518-330°C Optical character: biaxial ( + ) N~ -- 1.464 N~ -- 1.476 2V = 75 ° X-Ray diffraction data d(A), obs,
1
5.80
m
4-59
m
3.88
vs
3.78
s
3-68
m
3.50
vs
3.28
s
2.417
m
2.097
m
fl-CsZrF5 Equilibrium stability range: < 3 3 0 ° C Optical character: uniaxial ( - ) , 2 V variable, N,o = 1"476, N~ = 1"464 X-ray diffraction data d(A), obs.
I
d(A), obs.
I
6.65 6.15 4.33 4"27 3.98 3-88 3-81 3-70 3.59 3.50 3"42 3-23 3"15 3"097 2.996
s w w+ w w+ m -m-vs s w÷ m~ vs w÷ m vw
2.680 2.572 2.513 2'320 2.258 2.247 2'210 2,179 2.159 2'074 2.000 1.850 1'738 1'466
vw vw w+ w w w w w w w w w vw vw
* G. D. RoaatNs and J. H. BURNS, X-Ray Diffraction Study ofCsaZrFT, O R N L T M - 3 1 0 (July 7, 1962). t V. H. BODE and G. TEUFER, Z. anorg. Chem. 283, 18 (1956). :~ Calculated on basis of parameters given in above Refe'rence.
565
G. D. ROBBINS, R. E. THOMAand H. INSLEY
566
can be deduced from the expression
acs F = [(Nc,+) / (Ntot, l catton)] [( NF-) / (Ntotal ,nion) ] where a -- activity and N = number of moles, ideal liquidus curves can be generated for each ZrF~-' complex assumed to be the predominant species in the melt. For 7~o
i
700 690 u
68o
W
e:: 670 ::D w
660
\
tad
~- 650
]
640
[
\
-"
\
\
i
630
!EXR
\ \
I
CURVE
ZrF84 -
\ \ ZrF95\ \ ZrF~o ~-
620 CsF
2
4 5 6 7 8 ZrF4 CONCENTRATION (mole %)
9
~0
~4
FIG. 2.--Comparison of calculated and observed liquidus curves in the system CsF-ZrF~ example, assuming that Zr 4-" is the complex species of zirconium present in the melt, the equation In acsF = In (1 -- 4Xzrr,)(l -- ZXzrF, ) = [ - - A H # ( r -1 - - To-1) +
@dt
dt]/R
can be used to calculate a corresponding ideal liquidus curve. Although ACp for CsF is not known experimentally, ACp for the alkali fluorides decreases approximately linearly within the alkali fluoride series. Extrapolation of these values indicated that ACpcsF is approximately zero. Using this approximation, ideal liquidus curves were constructed for several ZrF~ -x complexes assumed to be the stable complex in the melt. Figure 2 shows these results and the experimentally determined liquidus. The data presented here indicate a coordination number near eight for the tetravalent ion in molten C s F - Z r F 4 mixtures. In similar determinations CANTOR(6) inferred that zirconium is also 8-coordinated in molten N a F - Z r F 4 mixtures. Further evidence of 8coordination by zirconium is found in the investigation of the K~ZrF 6 structure by BODE and TEUFER(Tj and in the complete determination of the structure of the compound LiaBeF4ZrFs by SEARS and BURNS.(8) (6) S. CANTOR, J. Phys. Chem. 65, 2208 (1961). ('~ V. H. BoDe and G. TEUFER, Aeta Cryst. 9, 929 (1956). is} D. R. SEARS and J. H. BURNS, J. Chem. Phys. 41, 3478 0964)
Phase eqmlibria in the systemCsF-ZrF4
567
The subsystem CssZrF7-CsZrF5 Two invariant points are located in the subsystem Cs3ZrF7-CsZrFs: the peritectic at 37 mole % ZrF 4 and at 530°C and the eutectic at 42 mole % ZrF 4 and at 425°C. In quenched specimens, a progressive diminution in the ambunt of quench growth below the liquidus at 26 mole ~/o ZrF4 from high to low temperatures was observed along with a corresponding increase in the index of refraction of the primary phase. These data show that a small region of CssZrF7 solid solution exists, although the persistence of quench growths, observed in small amounts at temperatures just above the solidus, indicates that the composition of the solid solution area does not extend to 26 mole % ZrF 4. Crystals of the incongruently melting compound CszZrF6 obtained from slowly cooled or from rapidly quenched specimens were routinely found to be hexagonal with lattice constants, a = 6.41 and e = 5.41 A, identical with those of the phase reported by BoDE and TEUrER.~9~ The thermal data as well as the optical data obtained in our experiments show that Cs2ZrF 6 undergoes a solid state transition at 465°C, the hexagonal form occurring as the equilibrium phase at the lower temperature. Crystals of the high-temperature modification were not retained even in 15- to 20-mg specimens which were rapidly quenched. As noted by BODEand TEtrFER,RbzZrF6 is isomorphous with the hexagonal form of CszZrF6. The hexagonal form of RbzZrF6 is also the lower-temperature modification of the phase. Even though crystals of the high-temperature form of CszZrF6 could not be retained during the quenching act, determinations of liquidus transitions in the primary phase field of the compound could be made with the polarizing light microscope. As the primary phase, the crystals appeared in a polycrystalline and highly twinned form, as though they were relic structures of crystals which had inverted via a solid-state transition. That the relic structures were those of hexagonal Cs2ZrF 6 was confirmed by X-ray diffraction analyses. Inversion of Cs2ZrF 6 on cooling appears to be accompanied by such a small energy change that detection of the thermal effect accompanying the inversion was noted in 50-g melts only in the presence of several mole per cent of the liquid phase, and the inversion was not detected either at the compound composition or in the two-solidphase region Cs3ZrFT-CS2ZrF6. Quenched specimens at 34.5 mole % ZrF 4 were observed to contain crystalline Cs2ZrF 6 and quench growths, indicative of the presence of rapidly cooled liquid. If only the thermal data were considered, it could be inferred that the high-temperature form of CszZrF6 can accommodate a small amount of dissolved ZrF 4 and that the inversion temperature of the pure phase is depressed by solid solution resulting in a eutectoid located at app.roximately 36 mole ~/o ZrF4 and at 465°C. Optical and X-ray diffraction data obtained from quenching experiments were identical regardless of the composition from which the crystals were obtained. The presence of quench growths in quenched specimens at 34-5 mole % ZrF 4 (see Table 3) and the uniformity of identification data indicate the improbability of eutectoid formation. The compound CsZrF 5 exhibits a solid-state transition as does Cs2ZrF 6. The optical data obtained from the quenching studies indicated that the high-temperature modification was nearly as difficult to retain in cooled melts as was the high-temperature form of Cs~ZrF6, and crystals sufficiently well formed for optical measurements ~g~ V. H. BODE a n d G. TEUFER, Z. anorg. Chem. 283, 18 (1956). 6
568
G. D. ROBBINS,R. E. THOMAand H. INSLEY
were not obtained. Specimens quenched from temperatures above 330°C produced X-ray diffraction patterns containing a group of poorly resolved lines, differentiable, however, from those of the low-temperature form (see Table 3). Optical determination of the liquidus in the primary phase field of the compound with the use of the quenching method was not successful because of the rapid inversion of CsZrFs, for relic structures were not produced on inversion of the solid phase. The heat of fusion for the compound was sufficiently large that the liquidus transition could be detected by thermal means, and liquid-solid transitions involving CsZrF 5 as the primary phase were determined by this means. Visual determination of the melting point of CsZrF 5 using optical-grade CsF gave the value of 518°C, somewhat lower than that obtained from either the direct determination using less-pure CsF or from extrapolation of liquidus points obtained with the less-pure material. If formed from very pure components, the compound CsZrF 5 tends to supercool somewhat more than if prepared from impure components.
The subsystem CsZrFs-ZrF 4 Equilibrium crystallization reactions take place in CsF-ZrF 4 mixtures containing more than 50 mole % ZrF 4 simply by precipitation of ~-CsZrF s or monoclinic ZrF 4 from the liquid phase. Final freezing occurs at the eutectic invariant point at 55 mole % ZrF 4 and at 465°C. Experimental determinations of equilibrium transitions in this subsystem are difficult because of several factors: 1. Minute amounts of contaminant oxides or oxyfluorides sharply depress the apparent liquidus in the ZrF 4 primary phase field. 2. In the presence of even very small concentrations of oxide contamination a heretofore unrecognized metastable face-centred cubic form of ZrF a may be stabilized. The refractive index of the cubic form of ZrF 4 is 1.560, in contrast with the refractive indices of the monoclinic form, 1.560, 1:598 and 1.606. The lattice constanL a 0, offcc ZrF 4 is 7-88 A. The unit cell contains eight molecules of ZrF 4 and has a calculated density of 4.539 cm-3; ~1°~ 3. At atmospheric pressure pure ZrF 4 sublimes at ,~905°C. Thus the relatively high vapour pressure of CsZrFs-ZrF 4 mixtures further complicates high-temperature measurements with these materials. Although at atmospheric pressure ZrF 4 sublimes rather than melts, it vaporizes congruently. The condensed subsystem CsZrFs-ZrF 4 is therefore quasibinary.
Acknowledgements--The authors are pleased to acknowledge the assistance of J. H. BURNS,who contributed much to the determination of crystal structures of Cs3ZrF~ and cubic ZrF4. We are also indebted to T. N. MCVAYfor assistingin the measurementsof the optical properties of CsF-ZrF4 phases, and to E. H. GUINN and G. M. HEBERTfor assisting in some of the experimental investigations. ~x0~j. H. BURNS,ORNL, unpublished work.
PHASE EQUILIBRIA IN THE SYSTEM CsF-ZrF~* G. D. ROBaINS,t R. E. THOMA and H. INSLEY** Reactor Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee (Received 24 July 1964)
Abstract--The phase equilibrium diagram of the condensed system CsF-ZrF, has been constructed from a combination of results obtained using thermal analysis, visual observation of crystallising fluoride mixtures, and quenching techniques. Phase identification was accomplished by petrographic and X-ray diffraction analysis. In the system CsF-ZrF4 three intermediate compounds were isolated and identified: Cs3ZrFT, Cs2ZrFs, and CsZrFs. The compounds CssZrF7 and CsZrF5 melt congruently at 784 and 518°C respectively; Cs~ZrFs melts incongruently to CssZrF7 and liquid at 530°C. Polymorphism is exhibited by Cs2ZrF6 and CsZrFs, the latter compound showing a markedly exothermic inversion on cooling below 330°C. Crystals of the high-temperature equilibrium forms of Cs2ZrF6 and CsZrF~ were not retained on quenching; crystallographic verification of liquid-solid transitions was therefore unobtainable in the low-melting region of the system. The phase CssZrF7 was observed to vary in composition. As a homogeneous single phase, the solid solution extends to the composition 80CsF-20ZrF4 (mole ~o). X-ray diffraction data obtained from single crystals of the stoichiometric phase revealed that the compound is face-centered cubic with a0 = 9.70 .~., and the space group is F~3m, F432, or Fm3m. This compound is apparently isotypic with KsZrFT,(NH,)sZrFT, and KaUF7 as regards the metal ions, but no deduction can be made from this work as to the exact arrangement of the fluoride ions. INTRODUCTION PRELIMINARYinvestigations of the system CsF-ZrF4, conducted at this L a b o r a t o r y by BARTON and co-workers/1~ employed cooling-curve techniques exclusively for obtaining the data used in the construction o f the phase diagram. The complexity in solid phase relationships which prevails a m o n g the other alkali fluoride-ZrF 4 phase diagrams and the paucity o f data available for the complex c o m p o u n d s in the C s F Z r F 4 system suggested the need for a more detailed study o f this system, employing a variety o f additional techniques. EXPERIMENTAL Apparatus and methods
The phase equilibrium data were obtained from heating and cooling curves data, by direct observation of melts crystallising at known temperatures, and bY identifying phases present in mixtures which were equilibrated and quenched. The thermal effect techniques used for measurement of the temperatures which define the CsF-ZrF4 phase diagram, as well as the X-ray and microscopic techniques used in identifying phases, have been discussed previously/2.3~ * Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation. t Temporary student employee; present address: Chemistry Department, Princetown University, Princetown, New Jersey. +*Consultant to Oak Ridge National Laboratory. ,l~ C. J. B A R T O N , L. M. BRATCHERand W. R. GRIMES, Phase Dia,q,rams of Nuclear Reactor Materials, p. 58. (Edited by R. E. THOMA)ORNL-2548 (Nov. 2, 1959). ,2~ C. J. BARTONet al., J. Amer. Ceram. Soe. 42, 63 (1958). ~a, H. A. FRIEDMAN,G. M. HERaERT and R. E. THOMA, Thermal Analysis and Gradient Quenching, Apparatus and Techniques for the lnvestig,ation of Fused Salt Phase Equilibria, ORNL-3375 (Dec. 18, 1962). 559
560
G. D. ROBBINS, R. E. THOMA and H. INSLEY
The visual-thermal apparatus employed in determining liquidus temperatures permitted observation of the formation of the first crystals on cooling the liquid melt. Samples were heated in a glove box which contained a helium atmosphere and were stirred manually with a shielded thermocouple. The thermal effects recorded on cooling the melt usually occurred within 2°C of the visually observed liquids, the thermal effects occurring at the lower temperature. The accuracy of the temperature measurements reported in these studies is limited by the characteristics of the ChromelAlumel thermocouples used. Manufacturers estimate this accuracy to be within 5° in the temperature range 400-800°C, and the accuracy and precision of the determinations reported here are considered to be within this limit of error. Some commercial caesium fluoride reagents used in this study contained approximately 1 wt. ~o water and were therefore predried before use in preparation of specimens for phase studies. Zirconium fluoride is easily converted to oxyfluoride or oxide at elevated temperatures, and caesium fluoride is extremely hygroscopic. Therefore, it was necessary to remove small amounts of water and oxygen as completely as possible from starting materials To facilitate removal of these substances, ammonium bifluoride was added to mixtures of CsF-ZrF4 before initial heating in the thermal analysis experiments. The same mixtures were used later for quenching experimehts. As in previous fluoride phase studies conducted at this Laboratory, all mixtures were protected from exposure to the atmosphere after purification at high temperatures. Materials
The mixtures used in the phase equilibrium studies reported here were prepared from commercially available caesium fluoride and ..from hafnium-free zirconium fluoride. In the course of the investigation, three grades of caesium fluoride were used. In the initial thermal analysis experiments, a commercial product was employed which contained approximately 1 per cent contaminant as alkali metal cations, principally as rubidium, potassium, and sodium. It provided satisfactory data for confirming liquid-solid transitions described earlier, c~ A higher quality of product was used in subsequent thermal-gradient-quenching experiments in order to ascertain the effect of the alkali metal impurities on the CsF-ZrF4 transition temperatures. For these experiments a reagent was obtained in which the concentration of metallic contaminants did not exceed 0.2 per cent. This product was purified further by heating it above its melting point in a dry helium atmosphere and allowing it to cool slowly, thereby effecting segregation of impurities in the melt. The best crystals of the salt were then selected in a dry box and mixed with ZrF, to form specimens of the appropriate compositions. Zirconium oxide and oxyfluorides are virtually insoluble in CsF-ZrF4 melts. In these melts contaminant oxides, in concentrations even as small as 2000-3000 p.p.m., are converted to ZrO2 and zirconium oxyfluorides, thereby introducing considerable uncertainty into the determination of liquid-solid transitions where visual or quenching methods are employed. Removal of chemisorbed water and oxide contaminants from CsF by reaction with ammonium bifluoride is much more difficult than is corresponding purification of the less hygroscopic alkali fluorides. Consequently, highly accurate determinations of liquidus determinations, using the visual method, require reagents of very high purity. For these experiments, melts were prepared from optical-grade single crystals of CsF and freshly sublimed ZrF4. The melting point' of the two better grades of CsF was determined to be 703°C, in good agreement with a previous determination by BREDI~ and co-workers,~ who obtained data from a very pure specimen. RESULTS AND DISCUSSION T h e liquidus d a t a o b t a i n e d in the c u r r e n t investigation o f the C s F - Z r F 4 system essentially confirm the liquidus described earlier by BARTON a n d co-workers, tl~ M u c h new i n f o r m a t i o n has been o b t a i n e d concerning the p h a s e relations o f the three interm e d i a t e c o m p o u n d s in the system. T h e phase d i a g r a m o f the system, based o n the d a t a o b t a i n e d in the c u r r e n t investigation, is shown in Fig. 1. T h e i n v a r i a n t equilibria a n d solid-state t r a n s i t i o n p o i n t s are listed in T a b l e 1. E x p e r i m e n t a l d a t a are s h o w n in T a b l e 2. O p t i c a l a n d X - r a y identification d a t a are listed in Table 3. A s a consequence o f the occurrence o f the two c o n g r u e n t l y melting c o m p o u n d s , Cs3ZrF 7 a n d C s Z r F s , the system C s F - Z r F 4 is c o m p r i s e d o f essentially three subsystems. T h e p h a s e d i a g r a m is described below in terms o f the subsystems. ~J M. A. BREDIG,H. R. BRONSTEINand W. T. SMITH,Jr., J. Amer. C/tern. Soc. 77, 1454 (1955).
Phase equilibria in the system CsF-ZrF4
561
950
900
850
I 800
750
700 I o v Ld tr :) I'--
650
hi I3-
600
! r
-4-
I L~J I--
i
./
550 518 °
500 465 ° 450
400
350
i
I
I
300
CsF
~0
20
30
40
50
60
70
- 80
90
J ZrF 4
ZrF 4 (mole %) FIO. 1 . - - T h e system CsF-ZrF,.
The subsystem CsF-Cs3ZrF 7 The components of the subsystem, CsF and CsaZrF 7, melt congruently at 703 and 784°C respectively. The eutectic formed from these solid phases occurs at 10 mole% ZrF 4 and at 646°C. Single crystals of Cs3ZrF7 were prepared from melts at the compound composition, and an X-ray diffraction study was made using a Buerger precession camera. (5) Results of the investigation revealed that CsaZrF 7 is face-centred cubic with a = 9.70 4= 0.02 A; the space group is F2t3m, F432, or Fm3m. Although the fluoride-ion arrangement is disordered, the Z 6 + and Cs + are at (or very near) (5) G. D. ROBBINS and J. H: BURNS, 1962).
X-ray Diffraction Study of CsaZrFT, ORNL TM-310 (Aug.
7,
G. D. ROBBINS,R. E. THOMAand H. INSLEY
562
(0, 0, 0) ~ face centering and (½, ½, ½; ¼, ~, ¼; t, ], ]) q- face centering, respectively. The X-ray density of the compound is 4.53 g/cm -a. This compound is apparently isotypic with KaZrFT, (NH4)aZrF ~, and KaUF7 as regards the metal ions, but no deduction can be made from this work as to the exact arrangement of the fluoride ions. Powdered samples of CsaZrF7 which are not protected from the atmosphere a r e hydrolysed rapidly; the only crystalline phase which appears in the residue is that of the low-temperature form of Cs2ZrF s. The diffraction pattern of C%ZrF s is not observed for samples prepared in a dry box, sealed in a protective mount, and examined TABLE I.--INVARIANT EQUILIBRIA IN THE SYSTEM CsF-ZrF4
Composition
Invariant
(mole ~. ZrF4) temp. (°C) Type of equilibrium l0 25
646 784
37
530
--
465 425 518 330 465
42 50 -55
Eutectic Congruent melting point Peritectic
Phase reaction invariant temperature
L* ~ CsF + CssZrF7 ss
L ~ CsaZrF7 L + CssZrF7 ss ct-CssZrF6 Inversion of Cs2ZrFo et-CsaZrF6 ~ fl-Cs2ZrF6 Eutectic L ~ Cs~ZrF6 + ~t-CsZrFs Congruent melting L ~ ct-CsZrF5 Inversionof CsZrF5 ~t-CsZrF6 ~ fl-CsZrF6 Eutectic L ~- ct-CsZrF5 + ZrF4
* L refers to liquid. immediately after heat treatments. However, after several hours the sealed samples show mixtures of Cs3ZrF 7 and Cs2ZrF 6. The X-ray photographs of single crystals of Cs3ZrF7 always showed some trace of C%ZrFs, even though considerable effort was made to exclude moisture. The reciprocal lattice of Cs~ZrF e was always axially related to that of CssZrFT, and twinning was evident. These relation.s suggest that CsaZrF 7 decomposes into CszZrF 6 and CsF at an equilibrium temperature well below the temperature ranges encountered in this study and that this reaction is greatly accelerated in the presence of water vapour. CsF is not identifiable in the reaction product because of its extreme hygroscopicity. The compound CsaZrF ~ was observed to vary in composition as shown in the phase diagram, accommodating slightly more than 20 per cent CsF into its lattice at the solidus. Refractive indices of the solid solution were observed to vary in this composition range from 1.470 at the stoichiometric composition to 1.480 near the CsF-rich terminus of the solid solution. Solid solution of an alkali fluoride in 3:1 compounds has not been observed previously in any alkali fluoride-MF4 system. That cation sites in CsaZrF7 are completely occupied in an ordered arrangement by Cs ~ and Zr 4+ ions excludes the possibility that this solid solution is substitutional, but the solution mechanism has not been identified. One of the purposes of CsaZrF 7 single-crystal studies was to learn the nature of the anionic configuration in the solid phase. This goal was precluded by the occurrence of the disordered anionic lattice. In an attempt to obtain information about ionic association in the molten state, we measured the effect of ZrF 4 on CsF freezing-point depression from 0 to 10 mole % ZrF 4. On the assumption that the activity of CsF
Phase equilibria in the system C s F - Z r F 4
563
TABLE 2 . - - C s F - Z r F . PHASE TRANSITION TEMPERATURE DATA Composition (mole ~ ZrF~) 0 2.0 4"0 5"35 6-0 6.20 7.0 8-05 9.1 10.1 !1.2 13 14 15 17 20 21 22.2 23 25 26 30 32 33.3 34"5 36 37 38 39 40 41 42 43 44 45 46 47 50 51 52 53 54 55.6 60 61 62.3 63 63-7 65 66.7 68 69-2 75
Liquidus temp. (°C) TA* TGQ'[" VO~ 702 693 683 675 672 670 666 659 652 643 640 683 705 720~4§
646
754
761
530 486 464 473
784 t: 2
625
644£4 634 656 628
734:]::4 778 784 777 746 714 645 566 563 557
Other transition temperatures (°C) TA TGQ
646 638 645
720
726 754::[:4
Solidus temp. (°C) TA TGQ VO
700:[:4 708 i 4
780
700
517 444 :L 2
523 502 526 525 504 421 401 411 425 417 425
530~2
530 526~3
427 413 :~ 2
464 530, 452, 320 450, 329 463, 331
462 464 :t: 464i2
425 416 i 2
317 33O
416 :L 2 471 487 501 498 510 506
499:[:2 512:]:2 515::[:2
426 414 425 399
317 329
518 518 513 505 488 492
547
485 449 461 467 465 465 477
443 425,328 325
559 ! 2 550
475 572 473 623
658 664
476 465
326
741
* Thermal analysis of cooling curve data. t Thermal gradient quenching data. ~: Phase transition temperature determined by direct visual observation of melt during freezing, § Temperature limits indicate temperature interval between segments of annealing tube.
564
G. D. ROBBINS, R. E. THOMA and H. INSLEY TABLE 3.--OPTICAL AND X-RAY DIFFRACTION DATA FOR C s F - Z r F , COMPOUNDS CssZrF~ Equilibrium stability range: <784°C Optical character: isotropic Refractive index: 1.471 X-Ray diffraction data Symmetry: Space group: Density: d(A), obs. d(A),
fcc, a = 9.70 A* F~[3m, F432, or Fm3m 4.53 g/cm (calculated) talc. Lobs. hkl
3"43
3'43
vs
2-92
2"92
vw
220 311
2.803
2.779
vw
222
2"424
2"424
s
400
1"980
1.989
s
422
1"715
1'714
m
440
1.535
1"533
ms
620
1.297
1.296
ms
642
fl-Cs~ZrFe Equilibrium stability range: <465°C Optical character: uniaxial ( - ) Refractive indices: No, = 1.482, Ne = 1-460 X-ray diffraction data d(A), obs.
Symmetry: hexagonal, P~ml,t a = 6-41 A, c = 5.01 A d(A), calc.:~ /, obs. hkl d(A), obs. d(A), calc.~ /, obs. 5"551
100
2.098
210
5"01
110
1.974
112
4'11 3'73
hkl
m 3'720
3.54
vs
101
1-932
1"935
m--
211
1"857
1"860
m--
202
1"850
300
1.670
w+
3-20
3-205
m+
110
1.667
w÷
003
2-771
2"776
w+
200
1-601
w~
103
2.687
2-700
w+
111
}-480
w÷
2.501
2"505
w~-
002
1.336
w
2.430
2"428
m-I-
201
1"308
w--
2.286
2"284
m --
102
1.236
w--
Phase equilibria in the system C s F - Z r F 4 Table 3
(contd.) ~-CsZrF 5 Equilibrium stability range: 518-330°C Optical character: biaxial ( + ) N~ -- 1.464 N~ -- 1.476 2V = 75 ° X-Ray diffraction data d(A), obs,
1
5.80
m
4-59
m
3.88
vs
3.78
s
3-68
m
3.50
vs
3.28
s
2.417
m
2.097
m
fl-CsZrF5 Equilibrium stability range: < 3 3 0 ° C Optical character: uniaxial ( - ) , 2 V variable, N,o = 1"476, N~ = 1"464 X-ray diffraction data d(A), obs.
I
d(A), obs.
I
6.65 6.15 4.33 4"27 3.98 3-88 3-81 3-70 3.59 3.50 3"42 3-23 3"15 3"097 2.996
s w w+ w w+ m -m-vs s w÷ m~ vs w÷ m vw
2.680 2.572 2.513 2'320 2.258 2.247 2'210 2,179 2.159 2'074 2.000 1.850 1'738 1'466
vw vw w+ w w w w w w w w w vw vw
* G. D. RoaatNs and J. H. BURNS, X-Ray Diffraction Study ofCsaZrFT, O R N L T M - 3 1 0 (July 7, 1962). t V. H. BODE and G. TEUFER, Z. anorg. Chem. 283, 18 (1956). :~ Calculated on basis of parameters given in above Refe'rence.
565
G. D. ROBBINS, R. E. THOMAand H. INSLEY
566
can be deduced from the expression
acs F = [(Nc,+) / (Ntot, l catton)] [( NF-) / (Ntotal ,nion) ] where a -- activity and N = number of moles, ideal liquidus curves can be generated for each ZrF~-' complex assumed to be the predominant species in the melt. For 7~o
i
700 690 u
68o
W
e:: 670 ::D w
660
\
tad
~- 650
]
640
[
\
-"
\
\
i
630
!EXR
\ \
I
CURVE
ZrF84 -
\ \ ZrF95\ \ ZrF~o ~-
620 CsF
2
4 5 6 7 8 ZrF4 CONCENTRATION (mole %)
9
~0
~4
FIG. 2.--Comparison of calculated and observed liquidus curves in the system CsF-ZrF~ example, assuming that Zr 4-" is the complex species of zirconium present in the melt, the equation In acsF = In (1 -- 4Xzrr,)(l -- ZXzrF, ) = [ - - A H # ( r -1 - - To-1) +
@dt
dt]/R
can be used to calculate a corresponding ideal liquidus curve. Although ACp for CsF is not known experimentally, ACp for the alkali fluorides decreases approximately linearly within the alkali fluoride series. Extrapolation of these values indicated that ACpcsF is approximately zero. Using this approximation, ideal liquidus curves were constructed for several ZrF~ -x complexes assumed to be the stable complex in the melt. Figure 2 shows these results and the experimentally determined liquidus. The data presented here indicate a coordination number near eight for the tetravalent ion in molten C s F - Z r F 4 mixtures. In similar determinations CANTOR(6) inferred that zirconium is also 8-coordinated in molten N a F - Z r F 4 mixtures. Further evidence of 8coordination by zirconium is found in the investigation of the K~ZrF 6 structure by BODE and TEUFER(Tj and in the complete determination of the structure of the compound LiaBeF4ZrFs by SEARS and BURNS.(8) (6) S. CANTOR, J. Phys. Chem. 65, 2208 (1961). ('~ V. H. BoDe and G. TEUFER, Aeta Cryst. 9, 929 (1956). is} D. R. SEARS and J. H. BURNS, J. Chem. Phys. 41, 3478 0964)
Phase eqmlibria in the systemCsF-ZrF4
567
The subsystem CssZrF7-CsZrF5 Two invariant points are located in the subsystem Cs3ZrF7-CsZrFs: the peritectic at 37 mole % ZrF 4 and at 530°C and the eutectic at 42 mole % ZrF 4 and at 425°C. In quenched specimens, a progressive diminution in the ambunt of quench growth below the liquidus at 26 mole ~/o ZrF4 from high to low temperatures was observed along with a corresponding increase in the index of refraction of the primary phase. These data show that a small region of CssZrF7 solid solution exists, although the persistence of quench growths, observed in small amounts at temperatures just above the solidus, indicates that the composition of the solid solution area does not extend to 26 mole % ZrF 4. Crystals of the incongruently melting compound CszZrF6 obtained from slowly cooled or from rapidly quenched specimens were routinely found to be hexagonal with lattice constants, a = 6.41 and e = 5.41 A, identical with those of the phase reported by BoDE and TEUrER.~9~ The thermal data as well as the optical data obtained in our experiments show that Cs2ZrF 6 undergoes a solid state transition at 465°C, the hexagonal form occurring as the equilibrium phase at the lower temperature. Crystals of the high-temperature modification were not retained even in 15- to 20-mg specimens which were rapidly quenched. As noted by BODEand TEtrFER,RbzZrF6 is isomorphous with the hexagonal form of CszZrF6. The hexagonal form of RbzZrF6 is also the lower-temperature modification of the phase. Even though crystals of the high-temperature form of CszZrF6 could not be retained during the quenching act, determinations of liquidus transitions in the primary phase field of the compound could be made with the polarizing light microscope. As the primary phase, the crystals appeared in a polycrystalline and highly twinned form, as though they were relic structures of crystals which had inverted via a solid-state transition. That the relic structures were those of hexagonal Cs2ZrF 6 was confirmed by X-ray diffraction analyses. Inversion of Cs2ZrF 6 on cooling appears to be accompanied by such a small energy change that detection of the thermal effect accompanying the inversion was noted in 50-g melts only in the presence of several mole per cent of the liquid phase, and the inversion was not detected either at the compound composition or in the two-solidphase region Cs3ZrFT-CS2ZrF6. Quenched specimens at 34.5 mole % ZrF 4 were observed to contain crystalline Cs2ZrF 6 and quench growths, indicative of the presence of rapidly cooled liquid. If only the thermal data were considered, it could be inferred that the high-temperature form of CszZrF6 can accommodate a small amount of dissolved ZrF 4 and that the inversion temperature of the pure phase is depressed by solid solution resulting in a eutectoid located at app.roximately 36 mole ~/o ZrF4 and at 465°C. Optical and X-ray diffraction data obtained from quenching experiments were identical regardless of the composition from which the crystals were obtained. The presence of quench growths in quenched specimens at 34-5 mole % ZrF 4 (see Table 3) and the uniformity of identification data indicate the improbability of eutectoid formation. The compound CsZrF 5 exhibits a solid-state transition as does Cs2ZrF 6. The optical data obtained from the quenching studies indicated that the high-temperature modification was nearly as difficult to retain in cooled melts as was the high-temperature form of Cs~ZrF6, and crystals sufficiently well formed for optical measurements ~g~ V. H. BODE a n d G. TEUFER, Z. anorg. Chem. 283, 18 (1956). 6
568
G. D. ROBBINS,R. E. THOMAand H. INSLEY
were not obtained. Specimens quenched from temperatures above 330°C produced X-ray diffraction patterns containing a group of poorly resolved lines, differentiable, however, from those of the low-temperature form (see Table 3). Optical determination of the liquidus in the primary phase field of the compound with the use of the quenching method was not successful because of the rapid inversion of CsZrFs, for relic structures were not produced on inversion of the solid phase. The heat of fusion for the compound was sufficiently large that the liquidus transition could be detected by thermal means, and liquid-solid transitions involving CsZrF 5 as the primary phase were determined by this means. Visual determination of the melting point of CsZrF 5 using optical-grade CsF gave the value of 518°C, somewhat lower than that obtained from either the direct determination using less-pure CsF or from extrapolation of liquidus points obtained with the less-pure material. If formed from very pure components, the compound CsZrF 5 tends to supercool somewhat more than if prepared from impure components.
The subsystem CsZrFs-ZrF 4 Equilibrium crystallization reactions take place in CsF-ZrF 4 mixtures containing more than 50 mole % ZrF 4 simply by precipitation of ~-CsZrF s or monoclinic ZrF 4 from the liquid phase. Final freezing occurs at the eutectic invariant point at 55 mole % ZrF 4 and at 465°C. Experimental determinations of equilibrium transitions in this subsystem are difficult because of several factors: 1. Minute amounts of contaminant oxides or oxyfluorides sharply depress the apparent liquidus in the ZrF 4 primary phase field. 2. In the presence of even very small concentrations of oxide contamination a heretofore unrecognized metastable face-centred cubic form of ZrF a may be stabilized. The refractive index of the cubic form of ZrF 4 is 1.560, in contrast with the refractive indices of the monoclinic form, 1.560, 1:598 and 1.606. The lattice constanL a 0, offcc ZrF 4 is 7-88 A. The unit cell contains eight molecules of ZrF 4 and has a calculated density of 4.539 cm-3; ~1°~ 3. At atmospheric pressure pure ZrF 4 sublimes at ,~905°C. Thus the relatively high vapour pressure of CsZrFs-ZrF 4 mixtures further complicates high-temperature measurements with these materials. Although at atmospheric pressure ZrF 4 sublimes rather than melts, it vaporizes congruently. The condensed subsystem CsZrFs-ZrF 4 is therefore quasibinary.
Acknowledgements--The authors are pleased to acknowledge the assistance of J. H. BURNS,who contributed much to the determination of crystal structures of Cs3ZrF~ and cubic ZrF4. We are also indebted to T. N. MCVAYfor assistingin the measurementsof the optical properties of CsF-ZrF4 phases, and to E. H. GUINN and G. M. HEBERTfor assisting in some of the experimental investigations. ~x0~j. H. BURNS,ORNL, unpublished work.