Studies on TTA complexes with metal ions—II

J. Inorg.Nucl. Chem., 1963,Vol.25, pp. 1011to 1019. PergamonPressLtd. Printedin NorthernIreland

STUDIES ON TTA COMPLEXES WITH METAL IONS--II INVESTIGATIONS

OF SOME TETRAVALENT IN THE SOLID STATE*

COMPLEXES

Y. BASKIN a n d N. S. KRISHNA PRASAD~" Metallurgy Division, Argonne National Laboratory, Argonne, Illinois (Received 12 November 1962; in revised form 4 February 1963) Abstract--Single crystals of TTA chelates of tetravalent uranium, thorium, hafnium, zirconium, cerium and plutonium have been prepared. X-ray diffraction data revealed that the compounds are isostructural, with only slight variations in their orthorhombic cell dimensions. Studies were also conducted of the optical properties, infrared absorption spectra and differential thermal analysis curves of the compounds. THENOYLTRIFLUOROACETONE ( T T A ) I ~ s ~ - - C O - - C H ~ - - C O - - C F

a forms complexes

with v a r i o u s metal ions which can be extracted f r o m aqueous solutions by using n o n p o l a r solvents such as benzene o r c a r b o n tetrachloride, a-7~ M u c h o f the w o r k r e p o r t e d in the literature pertains to the solvent extraction purifications a n d analytical a p p l i c a t i o n s involving the c o m p l e x i n g agent, b u t little w o r k has been c o n d u c t e d on the n a t u r e o f the m e t a l - T T A complexes in the solid state. In an earlier p a p e r (Part I) la~, u r a n y l - T T A c o m p l e x in an a q u e o u s - a l c o h o l i c m e d i u m was investigated b y s p e c t r o p h o t o m e t r i c a n d p o t e n t i o m e t r i c methods. This p a p e r deals with crystallographic, optical, infra-red spectra a n d differential t h e r m a l anaysis studies o f T T A chelates o f tetravalent u r a n i u m , t h o r i u m , h a f n i u m , zirconium, cerium a n d p l u t o n i u m . EXPERIMENTAL Preparation of the complexes UT4. To a solution of tetravalent uranium, obtained by photochemical reduction of uranyl nitrate in 50 per cent alcohol, was added an alcoholic solution of TTA++ of equivalent strength. The amount added was slightly less than that required to complex all of the uranium, thus leaving a slight excess of metal ions in solution. The precipitate formed was filtered, washed first with water and then with alcohol (UT4 is far less soluble in alcohol than is UO2T2). The crystals were finally dried and stored in a desiccator at room temperature. * Work performed under the auspices of the U.S. Atomic Energy Commission. 1"DR. BASKINis an Associate Ceramist, Metallurgy Division, Argonne National Laboratory, Argonne, Illinois. At the time the study was conducted, N. S. KRISHNAPRASADwas a participant in the International Institute of Nuclear Science & Engineering at Argonne National Laboratory. He is now at the Chemistry Division, Atomic Energy Establishment, Trombay, India. +*Thenoyltrifluoroacetone (TTA) manufactured by Columbia Organic Chemical Company, Inc., Columbia, South Carolina. m M. CALVINand E. L. ZEBROSKI,U.S.A.E.C. Report BC-42 (1947). t~ W. M. LA'rIMER,U.S.A.E.C. Report BC-61 (1947). ta~H. A. LEW and A. BROIDO,U.S.A.E.C. Report CNL-37 (1948). m E. H. HUFFMANand L. J. BEAFAtT,J. Amer. Chem. Soc. 70, 3179 (1948). ~5~R. A. BOLOMEYand L. WISH, U.S.A.E.C. Report ORNL-136 (1948). ~GIE. K. HYDE, Proceedings of the International Conference on the Peaceful Uses of Atomic Energy, Geneva, 1955, Vol. 7, p. 282, United Nations (1956). 171F. L. CtrLL~R,Proceedings of the International Conference on the Peaceful Uses of Atomic Energy, Geneva, 1955, Vol. 9, pp. 468, 471,473. United Nations (1956). ts~ K. M. ABUBACKERand N. S. KRISHNAPRASAD,J. Inorg. Nucl. Chem. 16, 296 (1961). 1011

1012

Y. BASKIN and N. S. KRISHNA PRASAD TABLE I.--X-RAY POWDER ThT4

UT4 I/1 vis.

d (obs.)

5 10 100 1 10 10 15

10-72 9-36 8"80 6-86 5.72 5.37 5.12

60 20 2 25 5 1 20 2 5 15 2 2 15 2 15 10 5 5

d (calc.)

10"74 9'51 8"86 6-831 5"755 5"368 5'113 (4"616 4"60 t4.590 (4"479 4"45 { 4"467 t 4'428 4.20 4"188 ~4'093 4.08 t4.063 ~3'899 3.88 t3.864 3.82 3.822 3.71 3.702 3-53 3.525 3-42 {3"416 3"415 3'36 3"347 3"201 3'20 3"196 2.94 2"932 2"84 2-840 2-792 2'80 2.783 2"577 2"58b* 2'568 2"46 2"38 2.30

~2.456 t2.450 2.376 2"307

15

{ 2-300 [2"295 (2.240 2.23b { 2.233

10

2.09

I0

2'04

* b=broad

hkl 001 011 200,120 121 031 002 320 321 122,202 212 141 240 222401 420 312 340 051 322 013 402 242 440,520 351 441 323 442 333 403 433 124,204 560 353 324 453 334 404 424 562

[ 2.226 154 ~2.097 354 {2.094524,444 t 2.086 125,205 [ 2.037 812 { 2.032 1"10"0 [2-031 760,840

I/I vis.

HfT 4

d (obs.)

d (calc.)

hkl

2 10

10.53 9.46

100 5 5 2 10 2

8-84 8.34 7.38 6-46 5.70 5.36

15 1

5.14 4.85

60

4-58

001 011 200 120 111 021 211 221 002 040 320 231 122 202 141 212 240 222 420

30

4.46

1 40

4"17 4.08

10.70 9'50 ~8.93 t8.83 8-38 7.43 6-503 5.708 5.349 ( 5.157 ~ 5.152 4-854 ~4.589 t4.588 ~ 4-496 { 4.478 t 4-469 4-192 4.094

5

3.85b

{ 3.8963.850

051340

5

3.713 3.71 t 3.711 3.41 3.427 3-376 3.36 3.373 3-372 3"231 3.21 3'208 ~3-073 3-05b t3.037 2.935 2.92 t2.915

042 322 242 160 440 520 351 260 261 531 540 451

20

2'85

~2.857 t2.842

620 071

2

2"77

~2"795 t 2-785

333 403

I

2.71

~2.723 t 2.707 ~2.578 t2.577

460 271 080 640

30 10 20 5 5

10

2.58

10

2.56

10

2.47

5

2"37

5

2"32

5

2"29

20

2'24

2

2-20

10

2"09

10

2-04

1/I vis.

d (obs.)

10 40

10"78 9"51

560 711 353 044, 324 642 453 334 404 173 722 054 344 205,125 444,524 1.I0"0 760, 812 840

hkl

10

001 011 120 200 021 121 211 031 131 002 320 231 122 202 141 4.42 {4"454 4"389 240 4-18 4'175 222 3'97 3"973 420 3"82 3'814 340,051 3'70 3-705 042 3"51 3'489 251 3'41 3"406 242 3"30 3-294 440 3-17 3"180 351 3"04 3"035 261 ~2.940 043 2"93 t2.922 323 2.87 2'858 451 ~2"796 522 2"79b t 2-780 601 2-67 2'675 271 2.579 124 2.57 2.576 204 2'560 551,433 2-53 2'526 542 2"42 2.422 560 2'387 711 2.38 2'385 044 2.375 324 f 2'288 453,404 2.28 { 2.272 642 363 t 2"270 [2"233 613 2'23 t2.231 154

15

2"20

15

2.09

2

2.03

100 5 5 20 5 15 20 5 70 30 15 50 20 30 2 5 30 15 1 2 10 20 10 20 1 10 5 15

2-562124,204 (2'476 {2.462 [ 2-457 2"374 [2"321 t 2.309 [2.299 t2-294 2"253 2-246 2.244 2-205 ~2.096 t2.081 2'049 2-048 2.047

d (calc.)

10-79 9"54 ~8'78 8-67b t8.63 7.31 7.412 6-81 6.811 6.39 6.400 5-70 5.752 (5.457 5'44 t 5.397 5-01 5.010 4.80 4'786 4-598 4-59 4'576

~ 2-203 { 2.195 [ 2.190 [ 2"096 |2.094 {2.087 12.082 t2.081 2.020

344 722 480 354, 125 205 444 215 524 831

Studies on T T A complexes with metal i o n s - - I I DIFFRACTION

DATA

ZrT4 I/I vis.

d (obs.)

10 20

10.55 9-44

100 2 5 2 60 30 20 50 15 15 5 15 5 1 5 5 2 5 5 1

10

5 10

10

1013

d (calc.)

10.78 9-53 8-79 8.69b t8.64 6.77 6.812 5-69 5.760 ,f 5.105 5-04b t 5.015 f4.595 4.57 14.572 (4.462 4.45 t4.458 4'18 4.173 3-97 3.977 3.82 3-819 3"707 3 " 6 8 b 3.672 3"42 3"407 3.29 3"291 3.185 3.17 3.178 3.167 3'03 3-038 2.86 2-861 (2.782 2.77 { 2.770 t 2.762 2.68 2-677 2.59 {2.577 2.573 f2.446 2.44 t 2-443 f2.383 2-38 t 2.374 '2.288 2.28 2.286 2-273 ,2.271 '2.249 2'24 2'233 2'230 '2.202 2-20 2"197 2.191 2.095 2.094 2.092 2"09 2-086 2.081 2-080

CeT4 hkl

HI vis.

001 011

5 10

200120 121 031 040 320 122 202 212 141 222 420 340, 051 042 322 242 440 351 033 260 261 451 333 620 403 271 124 204 163 343 711,044 324 453 404 642 363 054 613 154 344 480 722 354 125 205 444 215 524

100 2 2 5 2 10 100 15 30 30 15 10 15 1 20 5 5 15 5 10 10 10 10

15

10 10 15 1 15

10

d (obs.) 10'46 9.51

d (calc,)

PuT4 hkl

10-71 001 9-51 011 f8'92 120 8.76b t 8.77 200 6.76 6.853 121 6.35 6.448 211 5.70 5.801 031 5.37 5.353 002 5"07 5'089 320 4"590 321, 122 4"54b 4-569 202 4.505 141 4"18 4"180 222 4.05 4.058 401 (4.038 420 4.00 3-984 411 340 3.86 (3.875 3-867 051 3.73 3.722 042 3.68 3.689 322 3.41 3.426 242 {3"347 440 3.33 3.323 520 3.314 123 3-224 351 3.20 3.194 441 2.90 2.900 451 ( 2.823 522 2.81 ] 2.813 620 t 2-794 611 2"71 2-714 271,460 2'60 2"599 362 2"55 (2'564 371, 124 L 2-561 204 2"461 560 2"46 2-454 353 044 2'378 324 2"37 2.370 2-357 144 642 ' 2'298 2.285 404 2.29b 2'284 363 ~2-278 073 2.255 740 2.26 2"248 613 2.243 173 2.236 562 2"23 2-231 660 2'230 480 2'216 722 2.20 2.203 344 2'15 2"148 633 524 ,2.085 2-082 125 205 2.08 2-080 2.072 643 215 • 2-070 2.028 802,760 2"02 { 2-021 1-01-1 2.018 840

1/.I vis.

d (obs.)

5 10

10"65 9-66

25

10.71 9-51 ~8"92 8.92b t 8.77 7.32 7.45 6.43 6.45 5.71 5.80 5.06 5.09 ~ 4"59 4"55 ~ 4"57 t 4"50 ~4"46 4"45 t4.46 4.17 4.18 {4.04 4.01 3.98

001 011 120 200 021 211 031 320 321,122 202 141 212 240 222 420 411

5 25

3.86 3.68

15

2'46b {2.46 2.45 2'37 {2-38 2.37 ( 2"30 ~2"28 2"29b | 2.28 t 2.28 2"26 2.25 ~ 2'25 t 2.24 2-20 (2.22 2.20 2"15 2.15 2.08 2"08 2"08 2'08 2.07 2-07 2"05 (2"06 2"05 2.03 2"02 2"02 2.02

340 051 322 013 431 242 440 520 123 033 360 522 620 611 271,460 124, 371 204 560 353 044 324 642 404 363 073 740 613 173 722 344 633 524 125 205 643 215 482 831 802,760 1.10-1 840

100 I0 10 15 15 80 15 2

d (calc.)

(3"883.87 3.69 (3.52 1 3.51 , 3.50 103.41 /3.43 ' 3.35 20 3.32 3-32 t 3-31 1 3.18 3.17 2 2.98 2.97 ~ 2.82 15 2.82b ] 2.81 I, 2.79 2 2"71 2"71 2.56 15 2'56b 2'56

10

20 15 15 5 20

2 5

hkl

1014

Y. BASKINand N. S. KRISHNAPRASAD

ThT~, CeT4, ZrT4 and HfT4. Thorium nitrate solution (purified by solvent extraction with TBP in xylene), zirconium nitrate (E. Merck, reagent grade), ceric ammonium nitrate (E. Merck, reagent grade) and hafnium nitrate solution (prepared from spectroscopically pure hafnium metal by dissolving in hydrofluoric acid, precipitating as hydroxide and finally dissolving in nitric acid) were used for the preparation of these compounds following the same general procedure described for UTa. In these cases, however, it was necessary to precipitate the complexes by dilution with water after mixing the alcoholic solutions of the reagents. Most of the precipitates were recrystallized in benzene to yield crystals of suitable size for single-crystal X-ray analysis. PUT4. High purity plutonium metal was dissolved in hydrochloric acid and oxidized to the tetravalent state by addition of several drops of nitric acid. The solution was taken up in alcohol and treated with slightly less than the calculated amount of TTA (also in a water-alcohol solution) to give PUT4. The precipitate was filtered and washed with distilled water and alcohol and finally recrystallized from benzene.

Analysis With the exception of PUT4, all the compounds were analysed by careful ignition and weighing of the resulting oxides. Analysis in all cases corresponded to the formula MT4, where M stands for the metal and T for the ionized TTA. On the basis of its X-ray powder pattern, which is very similar to those of the other compounds, the TTA chelate of Pu is presumed to have a comparable formula (PUT4).

Apparatus Single-crystal X-ray studies were made on UT~, ThT4, HfT4, ZrT4 and CeT4 using the Buerger precession camera and MoK~ radiation (0.7107 A). Precession photograph measurements were corrected for film shrinkage. Zero and upper level phorographs were taken of CeT4 using a Weissenberg camera. Powder photographs ~ere taken of all the compounds with CuKa radiation (1.5418/~). The powder patterns were not corrected forfilm shrinkage owing to the absence of visible reflexions in the back-reflexion region. Infra-red spectra were recorded on a Perkin-Elmer Model 421 infra-red Spectrophotometer employing the KBr pellet technique. Nujol mulls were not used. However, there was no change in the spectra using the KBr method, indicating that no exchange of Br- took place with T-. Differential thermal analysis curves were obtained with a R. L. Stone Model DS-2 Differential Thermal Analyser using a dynamic oxygen atmosphere and a heating rate of 10°C/minute. The number of molecules per unit cell was determined from the observed density values. These were obtained by precisely suspending single crystals in an aqueous solution of thallium formate and determining the density of the solution pycnometrically. RESULTS

X-ray crystallographic data Single crystal studies established the i s o m o r p h i s m o f the five T T A chelates investigated. U s i n g d a t a o b t a i n e d f r o m the single-crystal p h o t o g r a p h s , p o w d e r p h o t o g r a p h s o f all the c o m p o u n d s were indexed. Calculated i n t e r p l a n a r spacings were o n l y given for those (hkl) reflexions which a p p e a r e d on the Weissenberg a n d precession p h o t o g r a p h s . P o w d e r diffraction d a t a for the six c o m p o u n d s are given in Table 1 a n d clearly reveal the i s o m o r p h i s m o f the c o m p o u n d s . X - r a y c r y s t a l l o g r a p h i c d a t a on these c o m p o u n d s are listed in T a b l e 2. W i t h the exception o f P u T 4, the values were derived f r o m single-crystal p h o t o g r a p h s . Lattice constants o f P u T 4 were p r e s u m e d to be essentially identical to those o f CeT 4, because o f the virtual equivalence o f the two diffraction p a t t e r n s (see Table 1). The length o f the c-axis changes very little in the series o f six c o m p o u n d s . The length o f the b-axis varies s o m e w h a t more, a n d t h a t o f the a-axis most. A l t h o u g h the unit cell volumes fall within a relatively n a r r o w range, there a p p e a r s to be a direct

1015

Studies on T T A complexes with metal i o n s - - I I TABLE 2.--X-RAY CRYSTALLOGRAPHICDATA

Orthorhombic; Z = 4(C32F12Hl~MO,S4)

Compound

a(A)

UT4 ThT4 HfTa ZrT4 CeT4 Pub

17"701 17"842 17"258 17.271 17.532 17.53

b(A) 20.446 20"630 20'393 20.422 20.719 20.72

c(A)

Unit cell Volume (A3)

Density Calc. (g/cm3)

Density Obs. (g/cm3)

10"736 10"698 10'794 10"779 10.710 10.71

3885"5 3937'7 3798"9 3801-8 3890.4 3890.

1"919 1'883 1'859 1.705 1'749 1.92

1"87 1"89 1"80 1.67 1.76 --

correlation between the metal covalent radius and either the cell volume or the a-axis length. Thorium and uranium have somewhat larger covalent radii than either hafnium or zirconium (°,1°) and this appears to be reflected in the cell volumes of their respective T T A chelates. Buerger precession photographs were taken of crystals of each compound along the three major crystallographic axes. All corresponding patterns exhibited identical symmetry and showed the same systematic extinctions. A CeT 4 crystal, representative of the other compounds, was further investigated with a Weissenberg camera. Oscillation and Weissenberg patterns were obtained with the a-axis of the crystal oriented parallel to the camera oscillation axis. Equi-inclination photographs were taken of eight higher levels. The precession photographs taken along the three major axes all showed two fold rotations around the centre together with a mirror reflexion. All of the photographs exhibited the same systematic absences, namely: reflexions of the type (h01) with h odd, (hkO) with k odd, (h00) with h odd, and (0k0) with k odd. The probable space groups are either D2a-P21212 or C2v4-Pma2. More investigation is needed before an unequivocal assignment can be made to one of these two space groups.

Optical examination Optical examination of the crystals of the various compounds confirmed the fact that they were orthorhombic. ZrT 4 and H f T 4 were colourless, ThT 4 pale yellow, U T 4 and PuT4 brown, and CeT 4 deep reddish brown. The observed colours agree with those reported for ThT 4 and ZrT4, but not with the green colour reported for UT4 .m) The UT 4 crystals show pronounced pleochroism, with colours ranging from deep brown to straw yellow. The pinacoidal and prismatic faces were best developed in all of the crystals. No signs of twinning were observed. Even the coloured crystals were sufficiently translucent to permit determination of their refractive indices. Optical data on the compounds are listed in Table 3. The c8~L. PAULING,Nature of the Chemical Bond, Comell University Press, Ithaca, New York (1939). ~10~A. F. WELLS,Structural Inorganic Chemistry, (3rd Ed.) Oxford (1961). ~11~E. L. ZEBROSKI,U.S.A.E.C. Report BC-63,(1947).

1016

Y. BASK1N and N. S. KRISHNA PRASAD

~

IlL

~'1

Ill

~2

Ill

-H.~

° 0 0

~2 N

-H.~

~ --

~ 0

o 6 6

-H-~

~z ~

.d o

.

I,-,

0 e4

o 0

~

0

0

~

~

~.~ °~

0

0 o

.o_d°~o 0~<0

1017

Studies on TTA complexes with metal ions--II

average refractive index varies relatively little in the series, demonstrating the equalizing influence of the predominant T T A component. The average refractive index of pure T T A is about 1.51. WAVELENGTH(MICRONS) 6 L

7 i

e i

'

9 i

~ t

12 ~

6

r~ I

7 i

WAVELENGTH a

MICRONS) ~o

12

15

"rhT,

TTA

i

18oo 17oo

i 1600

i 1500

1400

1300

FRE(~JENCY

I 1200

i 1I00

(GM-')

i ;IDO0

i 900

i go0

i 700

. . ~ , " ~

i 600 FREQUENCY

( C M ~)

FIG. 1.--Infra-red absorption spectra.

Infra-red spectra

The infra-red spectra of certain fl-diketones and some of their metallic chelates, particularly the acetyl-acetonates, have been investigated previously. (12-1s) The infra-red spectra of T T A and its chelates with tetravalent uranium, thorium, zirconium hafnium and cerium in the region 1800-550 cm -1 are given in Fig. 1 and the frequencies at which the absorption peaks occur are listed in Table 4. The infrared spectra of the compounds investigated show strong similarities with little variation in the frequencies of their absorption bands. As was reported by FERRARO and HEALY (xg), who investigated the infra-red spectra of ThT 4 and UO~T z, the spectra are complex in nature and the strong absorption in the area 1650-1550 cm -1 is due to the carbonyl vibration. Whereas for pure T T A this absorption occurs as a single band at 1650 cm -~ in the case of the five metallic chelates investigated there is a splitting of the band as well as a shift to lower frequencies. This shift to lower frequencies is in agreement with the results previously reported for the metallic chelates with acetylacetone and is attributed to the formation of M - - O bands. (12)R. RASMUSSEN,D. D. TLrNNICLIFFand R. R. ]3RATTAIN,J. Amer. Chem. Soc. 71, 1068 (1949). (la) j. LECOMTE,Di3c. Faraday Soc. 9, 125 0950). (141C. DUVAL,R. FREYMANNand J. LECOMTE,Bull. Soc. Chim. (France), 19, 106, (1952). t15)H. W. MORGAN,U.S.A.E.C. Document 2659 (1949). (is) L. J. BELLAMYand R. F. BRANCH,J. Chem. Soc. 4491 (1954). (tT) R. L. BELFORD, A. E. MARTELL and M. J. CALVIN, Or. Inorg. Nucl. Chem. 2, 11 (1956). (is) R. WESTand R. RILEY,J. Inorg. Nucl. Chem. 5, 295 (1958). (19)j. R. FERRAROand T. V. HEALEY,J. Inorff. Nucl. Chem. In press (1962).

1018

Y . BASKIN a n d N . S. KRISHNA PRASAD

TABLE 4.--INFRA-RED ABSORPTION PEAKS TTA

UT4

(cm-0

I

ThT4

HfT4

ZrT4

CeT4

(cm-0

I

( c m -1) I

( c m -1) I

( c m -1) I

(cm -1) I

1620 1568 1535 1508

m s m m

1620 m 1570 vs 1540 s 1505 m

1633 s 1578 vs 1540 s 1510 s

1632 m 1579 vs 1545 m 1512 m

1600m 1569 s 1534 s 1508 m

1405 s 1355 m

1404 s 1355 rn

1409 s 1358 s

1410 s 1358 rn

1408 s 1355 m

1315 vs 1254 s 1230 m 1196 s

1308 v s 1252 m 1230 m 1194 s

1327 vs 1252 s 1233 s 1198 s

1327 v s 1255 s 1234 m 1200 s

1309 vs 1252 s 1230 m 1196 s

1136

1136

1140

1140

1135

1650 m

1450 1414 1358 1343

m s m m

1255 s

1180

vs

1115 s 1085 s

1082 w 1063 m 1012 w

972 m 932 w 848 w

930 860 796 730 684 636 580

736 s 672 w 629 m

vs ~

vs

very strong;

w w w w w w w

vs

1080 w 1060 m 1010 w 928 855 796 730 680 632 578

s = strong;

vs

1088 m 1066 m 1015 m

w w m m w w w

937 860 796 730 687 640 580

m = medium;

m w m m w w w

vs

1088 w 1068 w 1020 w 938 862 792 732 690 640 588

w w w w w w w

w = weak.

TABLE 5 . - - M E L T I N G POINTS OBTAINED FROM DTA CURVES Compound

Melting point (°C)*

L i t e r a t u r e v a l u e (°C)

TTA UT4 ThTa HfT4 ZrT4 CeT4

50 250 235 240 240 190

38-42t -2 2 4 t11~ -2 2 9 c11J --

* Values are accurate to ±10°C. t Value given by manufacturer.

vs

1084 w 1062 m 1015 w 930 858 796 730 680 645 578

w w w m w w w

Studies on TTA complexes with metal ions--II

u'r.

I

HfT4

I

I

I

I

L

L

I

ThT4

I

F

10t9

cer,

I

1

I

I

ZrT 4

TTA

1 REFERENCESAMPLETEMPERATURE('C) FIO. 2.--DTA curves.

Differential thermal analysis Differential thermal analysis curves for the various compounds and pure TTA are shown in Fig. 2. The endothermic peaks are assumed to represent melting, since the peaks for ThT 4 and ZrT4 occur at temperatures close to those reported for the two compounds (ix). Melting points of the compounds, based on the DTA studies, are listed in 'Table 5 along with a few literature values. No reasons are apparent for the relatively low melting temperature of CeT~. The exothermic peaks at around 300°C are thought to be associated with oxidation of sulphur, whereas those at 400-450°C are assumed to be caused by oxidation of CO and possibly carbon.

Acknowledgement The

authors gratefully acknowledge the assistance of K. M. Abubacker, Chemistry Division, Atomic Energy Establishment, Trombay, Bombay, India, in the preparation of UT~, ThT4, ZrT4 and CeT4.