Thermodynamic properties of tungsten ditelluride (WTe2) II. Standard molar enthalpy of formation at the temperature 298.15 K

Thermodynamic properties of tungsten ditelluride (WTe2) II. Standard molar enthalpy of formation at the temperature 298.15 K

M-2723 J. Chem. Thermodynamics 1992, 24, 639-647 Thermodynamic properties of tungsten ditelluride (WTe2) II. Standard molar enthalpy of formation at...

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M-2723

J. Chem. Thermodynamics 1992, 24, 639-647

Thermodynamic properties of tungsten ditelluride (WTe2) II. Standard molar enthalpy of formation at the temperature 298.15 K P. A. G. O ' H A R E

Chemical Kinetics and Thermodynamics Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, U.S.A. and G. A. HOPE

Griffith University, Nathan, Queensland, Australia 4111 (Received 11 October 1991) The standard molar enthalpies of formation of WTe2(cr) and TeF6(g) have been determined by combustion calorimetry in high-pressure fluorine: AfH~(WTez, cr, 298.15 K) = -(38 + 5) kJ. mol- 1, and AfH~(TeF6, g, 298,15 K) = -- (1380.7 + 1.3) kJ. mol 1. Two hightemperature investigations of the vaporization of WTe2 give derived enthalpies of formation that agree with this result, but which have * rather large uncertainties ' ' that arise from estimated thermodynamic properties used in the calculations. The enthalpy of formation of TeF6 replaces an earlier determination, now thought to be in error.

I. Introduction This investigation represents the final p a r t of an effort to d e t e r m i n e a c c u r a t e values for the s t a n d a r d m o l a r enthalpies of f o r m a t i o n AfH2, of the d i c h a l c o g e n i d e s (excluding oxides) of m o l y b d e n u m a n d tungsten b y the technique of fluorinec o m b u s t i o n calorimetry. Similar p r e v i o u s studies h a v e yielded enthalpies of f o r m a t i o n at T = 298.15 K for MoS2, (a) MoSe2, (2) M o T e 2 , (3) WS2, (4) a n d WSe2 .(5) A l t h o u g h p r a c t i c a l a p p l i c a t i o n s of W T e 2 have been d e s c r i b e d in c o n n e c t i o n with a n u m b e r of m o d e r n technologies including, for example, as a c a t h o d e c o m p o n e n t of p h o t o r e c h a r g e a b l e batteries, (6) very little reliable i n f o r m a t i o n is available c o n c e r n i n g its t h e r m o d y n a m i c properties. In a review article (7) p u b l i s h e d a b o u t five years ago, we were able to list only two t h e r m o d y n a m i c investigations of W T e 2. Since t h a t time, W e i r a n d C a l l a n a n (8) have m e a s u r e d the l o w - t e m p e r a t u r e heat c a p a c i t y of the same s a m p l e of telluride used in the present study, a n d they also r e p o r t e d the usual derived t h e r m o d y n a m i c p r o p e r t i e s to T = 360 K. A p a r t from t h a t work, no o t h e r research on W T e 2 of a t h e r m o d y n a m i c n a t u r e has been s u b s e q u e n t l y described in the literature. 0021 9614/92/060639+09 $02.00/0

© 1992 Academic Press Limited

640

P . A . G . O ' H A R E A N D G. A. H O P E

Previous fluorine-combustion calorimetric studies of the molybdenum and tungsten dichalcogenides were performed at Argonne National Laboratory. Those activities have since been terminated, but a new similar effort has been underway at the National Institute of Standards and Technology (NIST) since early 1990. This paper reports the first results of the NIST investigations. The standard molar enthalpy of formation AfH~(WTe2, cr, 298.15 K) determined in the present study is based upon the standard molar energy of combustion at T = 298.15 K of WTe 2 in F 2 according to the reaction: WTez(cr) + 9Fz(g) = WF6(g) + 2TeF6(g),

(1)

combined with the standard molar enthalpies of formation at T = 2 9 8 . 1 5 K of WF6(g ) and TeF6(g). Upon completion of the measurements of the energy of combustion of WTe z, we found the calculated result for AfH°(WTe2) to be surprisingly small when compared with similar values for the other Mo and W dichalcogenides. There were no apparent difficulties with the determination of the energy of combustion, and AfH°(WF6) has been reported by several investigators(9 l a) with concordant results. Therefore, it was suspected that our published value ~12) for AfHm(TeF6, g, 298.15 K) might be in error. This suspicion has now been confirmed by our current measurements of the energy of combustion of Te in F2, according to: Te(cr) + 3Fz(g) = TeF6(g).

(2)

This new result, which is based on a combustion technique that is rather different, and better defined, than that used previously, is believed to be more reliable, and leads to a much more reasonable value for AfH°(WTe2).

2. Experimental Tungsten ditelluride was prepared at Griffith University by direct combination of stoichiometric amounts of the elements. Fine details of the method used have been given in reference 8. The tellurium was part of the same batch that had been used earlier ~3) for the synthesis of MoTe2. Powdered tungsten, of nominal purity 99.99 mass per cent, was purchased from Electronic Space Products, Inc.t This powder was heated to redness in a tungsten boat in a high vacuum in order to remove any oxide that might have been present. A two-stage crystal-growth technique was used: first, the elements were combined in a sealed tube; then the resulting crystals, still in the tube, were shaken vigorously and distributed along the tube bottom to grow the final sparkling crystalline product by vapor transport. The transport agent was 0.3 mass per cent of TeC14. The tube was broken under water, and approxima]ely 20 g of crystals was gathered on a filter and dried in a desiccator. This material was sealed in a glass tube and shipped, by way of NIST, to R. D. Weir at the Royal Military College of Canada, Kingston, Canada. t Certain commercial equipment, instruments, or materials are identified to describe the experimental conditions completely, but in no way should this be construed as an endorsement by the National Institute of Standards and Technology.

THERMODYNAMIC PROPERTIES OF WTe2

641

Subsequent treatment of the WTe z, including removal of about 80 mg of water by pumping in a high vacuum, as well as the measurement of the heat capacity, has been described by Weir and Callanan38) After completion of those measurements, the calorimetric specimen was returned to NIST for the fluorine-bomb experiments. A portion of the WTe 2 was examined in a scanning tunnelling microscope; there was no evidence for the presence of an oxide layer on the surface of the material, nor was there any indication of adsorbed oxygen. The positions of the atoms fitted precisely the published ~13) X-ray structure for the 001 plane. In trial experiments, it soon became clear that, similar to MoTe2, ~3) neat W T e / d i d not react to completion in fluorine. It was necessary to use a tungsten saucer as an auxiliary combustion aid, in conjunction with a sulfur fuse, to bring about complete conversion of WTe 2 to W F 6 and 2TeF 6 according to reaction (1). High-purity tungsten sheet, of thickness 0.025mm, was purchased from Schwarzkopf Development and formed into saucers approximately 4 cm in diameter. The sulfur fuse was part of U.S. Bureau of Mines sample U S B M - P l b . High-purity tellurium, taken from a fragmented zone-refined ingot (catalog no. 12607), was purchased from Johnson-Matthey (Aesar/Alfa). Fluorine of 99.99 mass per cent purity, prepared at Argonne National Laboratory by distillation in a low-temperature still, was shipped in 0.25 kg masses to NIST. As we have pointed out earlier in this paper, a new laboratory for fluorinecombustion calorimetry has been set up at NIST. It is quite similar to the previous arrangement at Argonne. A manifold for handling high-pressure fluorine was constructed from 316-stainless-steel lines and valves (Autoclave Engineers, Inc.) and was kept under vacuum with rotary oil pumps (Alcatel). Waste fluorine and gaseous fluorides were passed through a copl0er column filled with activated alumina spheres (Alcoa). The glovebox and associated purification unit were purchased from T. M. Vacuum Products, Inc. Both the oxygen and water compositions of the glovebox nitrogen atmosphere were continually monitored. Typically, mass fractions of I I0 6 and 5" 10 --6 w e r e observed for HzO and O z, respectively. All operations in which the combustion bomb was open were performed in the glovebox. Calorimetric specimens were also permanently stored there. An automatic electronic balance (Sartorius R160P), capable of weighing calorimetric samples with an accuracy of approximately 3' 10 -5 g, had been installed in the glovebox. The (bomb + fluorine storage tank) reaction vessel, calorimeter (ANL-R-2), and quartz-crystal based temperature-measuring system were the same ~s those used in earlier work (3) at Argonne. The temperature of the calorimetric laboratory was maintained at approximately 293 K. The calorimetric system was calibrated by combustion in oxygen of standard reference material NIST SRM 39i benzoic acid, which has a certified specific energy of combustion of - ( 2 6 4 3 4 + 3 ) J . g 1 under prescribed conditions. This value becomes - 2 6 4 1 3 J . g 1 under standard conditions at 298.15 K. As with MoTe/, (3) complete combustion of W T e / c o u l d be achieved only with the assistance of tungsten as an auxiliary substance. We have mentioned that there was reason to suspect an error in our earlier calorimetric study (tz) of reaction (2). In the course of that investigation, we observed that tellurium, unlike its chemical analogs

642

P. A. G. O'HARE AND G. A. HOPE

sulfur and selenium, did not combine readily, or completely, with high-pressure fluorine, and we therefore employed a disk of boron carbide B4C as a thermal insulator to promote essentially complete reaction to TeF 6. U p o n reconsideration of that work, we now believe that the most likely source of error was the correction used to allow for the combustion of approximately 1 mg masses of the B4C sample support. Consequently, we decided upon tungsten as an auxiliary material for the present experiments with tellurium, and deliberately avoided the use of B4C. It turns out, then, that essentially identical techniques were used for the experiments with WTe2 and Te: the material to be reacted was placed on an approximately 0.7 g saucer made from tungsten sheet, a small quantity of sulfur was added as fuse, and the assemblage was placed on top of a 25.8 g prefluorinated nickel crucible whose sides were perforated with 0.5 cm holes (to allow rapid circulation of the fluorine around the sample as combustion proceeded) and which rested on the head of the bomb. A similar arrangement was used to determine the energy of combustion of tungsten, sulfur being used here also as a fuse. In order to improve the combustions of tellurium, a ~ 6 g quartz disk was interposed between the crucible and the b o m b head; the disk, almost completely protected from direct exposure to hot fluorine, did not react chemically to any significant extent. The fluorine storage tank was charged ( T ~ 293 K) to a pressure of 1000 k P a for the experiments with WTe 2 and to 1500 k P a for the experiments with Te and W. The prefluorinated (bomb + tank) assembly was placed in the water bucket of the calorimeter and, after completion of the fore-rating period, the tank valve was opened remotely. U p o n entry of the fluorine into the bomb, the sulfur fuse ignited spontaneously, and that, in turn, initiated the reaction of the tungsten and the materials resting upon it. Combustions proceeded as expected: of WTe2 to W F 6 and 2TeF 6, and of W to WF6; no solids were observed in the crucible. However, after each combustion of Te a small, white-colored substance, ~ 2 m m 2 in area and of approximate mass 1 mg, was found in the crucible; these residues were too small to recover for X-ray diffraction analysis and were assumed to consist of TeF 4. This assumption is based on previous experience with the combustion calorimetry of tellurium-containing compounds, and our experimental observations that, in the reaction vessel with initial fluorine pressures of approximately 1 MPa, lower fluorides of W do not form.

3. Results Because the calorimeter has been relocated, the quartz-crystal thermometer reset, and other minor alterations made, detailed numerical particulars of the ~alibration experiments are set out in table 1. The standard deviation of ~(calor), _+0.00005.e(calor), is similar to previous such results for this calorimetric system obtained at Argonne National Laboratory. Details of the individual quantities used to derive the energies of combustion of tungsten, tungsten ditelluride, and tellurium are given in tables 2, 3, and 4. M a n y of the symbols are c o m m o n to those tables and to table 1, and have been defined by Hubbard. u4) Masses of material placed in the b o m b are denoted by m; they were

THERMODYNAMIC

PROPERTIES

OF WTe 2

643

T A B L E 1. Results for benzoic acid c a l i b r a t i o n of the c a l o r i m e t e r ~

m'/g m"/g A0~o~JK {m'A¢ Um/M}/J AU(fuse)/J AU(cont)J AU(ign)/J A0~/K ~(calor)/(J - K 1)

1.00060 0.00277 0.01256 --26450.5 --47.8 36.9 -0.5 1.90129 13917.9

0.99962 0.99772 0.00315 0.00345 0.01050 0.01245 --26424.6 --26374.3 -54.4 -59.6 36.9 36.9 -0.5 -0.5 1.89961 1.89682 13920.0 13916.7

0.99970 0.99971 0.00331 0.00308 0.01743 0.01613 --26426.7 --26426.9 -57.1 -53.2 36.9 36.9 -0.5 -0.5 1.90008 1.89994 13919.1 13918.2

0.99994 1.00007 0.00269 0.00285 0.01381 0.01456 --26433.0 --26435.9 -46.4 --49.2 36.9 36.9 -0.5 -0.5 1.90014 1.90078 13916.3 13914.7

0.99990 0.00281 0.01246 --26432.0 -48.5 36.9 -0.5 1.89964 13920.6

(e(calor)) = (13917.9 -t-0.7) J. K -a

" The s y m b o l s in the table have the following meanings: m' is the m a s s of benzoic acid; m" is the m a s s of thread; 0 ~, Of, a n d A0co~r are the initial a n d final t e m p e r a t u r e s of the e x p e r i m e n t a n d the c o r r e c t i o n to the observed t e m p e r a t u r e increase, respectively; rn'A~ Um/Mis the energy of c o m b u s t i o n of the benzoic acid sample; AU(fuse) is the e n e r g y of c o m b u s t i o n of the thread; AU(cont) is the c o r r e c t i o n for the contents of the b o m b (crucible, electrodes, water); A U(ign) is the c o n t r i b u t i o n of the electrical i g n i t i o n energy; A0¢ = Of - 0 i - A0~or~; a n d g(calor) is the energy e q u i v a l e n t of the c a l o r i m e t r i c system w i t h a n u n c e r t a i n t y given as the s t a n d a r d d e v i a t i o n of the mean.

T A B L E 2. E n e r g y of c o m b u s t i o n of t u n g s t e n in fluorine ( T = 298.15 K, p° = 101.325 k P a ) m(W)/g m(S)/g A0c/K ~(calor)( A0c)/J AU(S)/J AU(cont)/J AU(gas)/J AU(blank)/J {A~ U~/M(sample)}/(J.g - 1)

1.16278 0.00212 0.78880 -10978.4 80.4 -11.8 0.0 - 7.5 - 9389.0

1.26812 0.00253 0.86098 -11983.0 96.0 - 1~2.9 0.0 - 7.5 - 9389.8

1.18091 0.00471 0.80791 -11244.4 178.6 -12.1 0.0 - 7.5 -- 9387.2

1.15353 0.00133 0.77996 -10855.4 50.4 -11.7 0.0 - 7.5 -- 9383.5

1.16751 0.00109 0.78872 -10977.3 41.3 -11.8 0.0 - 7.5 - 9383.5

1.24741 0.00107 0.84253 -11726.2 40.6 -12.6 0.0 - 7.5 - 9384.0

({A¢ U2jM(sample)}) = --(9386.2 + 1.2) J . g - 1

T A B L E 3. S u m m a r y of c o m b u s t i o n results for W T e 2 ( T = 298.15 K, p° = 101.325 k P a ) m(WTez)/g m(W)/g m(S)/g A0c/K e ( c a l o r ) ( - A0c)/J AU(cont)/J AU(gas)/J AU(blank)/J AU(W)/J AU(S)/J {Ac U~lM(sample))/(J.g 1)

1.03766 0.80387 0'00317 1.30245 18127.4 - 18.3 0.9 6.7 a 7545.3 120.3 - 10092.4 ({Ac

1.01460 0.78757 0.00236 1.27225 - 17707.0 - 17.8 0.9 6.7 a 7392.3 89.5 - 10088.1

0.51312 0.69208 0-00371 ,0.84878 - 11813.2 - 10.1 0.9 6.7 a 6496.0 140.8 -- 10093.0

U~lM(sample)})=

0.50540 0.76782 0.00169 0.88844 - 12365.2 - 13.3 2.2 5.4 b 7206.9 64.1 10090.8

-(10090.4__+ 1.1) J . g - 1 Ac U~(WTe2) - - (4430.2 _ 1.1) k J- m o l - 1 ACHm(WT%) = --(4445.1 +_ 1.1) kJ" m o l 1

a E x p e r i m e n t s performed with a t a n k pressure of 1000 k P a at T,,~ 293.5 K. b E x p e r i m e n t s p e r f o r m e d with a t a n k pressure of 1500 k P a at T ~ 293.5 K.

0.72988 0.76297 0.00181 1.04791 - 14584.7 - 15.8 2.2 5.4 b 7161.4 68.7 -- 10087.7

P. A. G. O ' H A R E A N D G. A. H O P E

644

T A B L E 4. S u m m a r y of c o m b u s t i o n results for Te (T = 298.15 K, p° = 101.325 k P a ) m(Te)/g m(W)/g m(S)/g A0dK 8 ( c a l o r ) ( - A0~)/J A U(cont)/J AU(gas)/J AU(blank)/J AU(W)/J AU(S)/J A U(TeF4)/J {A~ U~/M(sample)}/(J. g - ~)

0.61741 0.73296 0.00233 0.97804 - 13612.3 - 19.2 2.2 5.4 6879.7 88.4 - 1.8 -- 10783.1

({A~

0.62720 0.82839 0.00166 1.04815 - 14588.0 - 20.6 2.3 5.4 7775.4 63.0 - 2.5 -- 10786.0

0.61932 0.85586 0.00173 1.06050 - 14759.9 - 20.8 2.2 5.4 8033.3 65.6 - 0.7 -- 10777.8

0.62723 0.71411 0.00067 0.96822 - 13475.6 - 19.0 2.2 5.4 6702.8 25.4 - 2.9 -- 10780.3

0.56079 0.75797 0.00083 0.94689 -- 13178.7 - 18.6 2.3 5.4 7114.5 31.5 - 0.8 -- 10778.4

U~JM(sample)})=

-(10781.1 _+ 1.5) J.g-~ A~ Um(Te ) = - ( 1 3 7 5 . 7 __+0.6) kJ" m o l AcHm(Te ) = Af H°(TeF6, cr, 298.15 K) = --(1380.7_+ 1.3) k J . mo1-1

T A B L E 5. A u x i l i a r y q u a n t i t i e s (T = 298.15 K) used in the calculations

p/(g. cm 3) c~/(J. K 1 . g - l )

W 19.25 ~ 0.132 c

Te 6.23" 0.202 d

WTe 2 7.5 b 0.18 e

Ni 8.9" 0.443 c

C~,m/(j. K

F2 22.99 ~

WE 6 119.03 c

TeF 6 109.23 y

SF 6 88.66 C

1 . m o 1 1)

S 2.07 ° 0.707 c

SiO 2 2.66" 0.742 c

" Crystal Data Determinative Tables, 3rd Edition, Volume II: Inorganic Compounds, D o n n a y , J. D. H.; O n d i k , H . M . : editors. U.S. N a t i o n a l B u r e a u of S t a n d a r d s and the J o i n t C o m m i t t e e on P o w d e r Diffraction Standards, U.S.A., 1973. b Reference 13. c Reference 28. d W a g m a n , D. D.; Evans, W. H.; Parker, V.B.; S c h u m m , R. H.; H a l o w , I.; Bailey, S. M.; Churney, K. L.; N u t t a l l , R. L. J. Phys. Chem. Ref Data 1982, 11. S u p p l e m e n t no. 2. e Reference 8. s Reference 20.

converted from apparent masses by means of the densities given in table 5. In table 1, e(calor) applies to the empty bomb. Thus, A U(cont) in tables 2, 3, and 4 is a correction that allows for the contents of the bomb, such as sample supports, fluorine, and fluoride gases formed during the experiments; values of the heat capacities used to calculate AU(cont) are to be found in table 5. Hubbard (14) has also defined AU(gas) and described its calculation; for that purpose, we have used intermolecular-force constants from the literature for Fz(g), ~15) WF6(g), (16) and TeF6(g);(iv) the small amounts of SF 6 present were ignored. The quantity AU(blank), measured in separate experiments by expansion of Fz(g) into an evacuated bombs was calculated as outlined in an earlier publication. (18) Contributions to the overall measured energy of the reaction from the energy of combustion of tungsten and sulfur are denoted by AU(W) and AU(S), respectively; put simply, AU(W)/J = -9386.2-m(W)/g (see table 2), and AU(S)/J = - 3 7 9 4 4 . m(S)/g. ~19) Finally, AU(TeF4) is the correction for the (hypothetical) conversion of the TeF 4 residue to TeF6; it is based on the standard molar enthalpy of formation of TeF4, (2°) and is given by

THERMODYNAMIC PROPERTIES OF WTez

645

AU(TeF4)/J = --1.8.m(TeF4)/mg. For the conversion of specific energies of combustion to molar energies of combustion, we have used molar masses of 439.05 g. mol- ~ for WTe2 and 127.60 g. mol- 1 for Te. 4.

Discussion

The present result for AfHm(TeF6, g, 298.15 K), -(1380.7___ 1.3) kJ" tool 1, is given in table 4. It is 8.9 kJ. tool-1 more negative than the value of -(1371.8 + 1.8) kJ. mol-1 recommended by us in reference 3. Bousquet et a/. (21) reported AfHm(TeF6) = - (1381 _+25) k J" m o l - 1 which, because of the large uncertainty, agrees with both values. In our earlier study, a disk of B4C was used to support the calorimetric specimens of tellurium in order to promote complete combustion. After each of six combustion experiments, it was observed that the mass of the disk had decreased (by between 2.12 mg and 0.69 mg). Consequently, the measured overall energy of each reaction was reduced, on the basis of the assumption that B4C had reacted with fluorine to yield 4BF3(g) and CF4(g). In retrospect, such an assumption might not have been valid. It is possible that the loss in mass of the disk could have been due, in part, to the evaporation of a volatile impurity or to the combustion of binders used to enhance the mechanical strength of the BeG. Unfortunately, we are unable now to check the validity of those hypotheses. Nevertheless, because of the improved technique used for the present determination and the lack of ambiguity in the interpretation of the reaction products, we are confident that the new value for AfHm(TeF6) is reliable. The standard molar enthalpy of formation of WTe z was obtained from the enthalpy of reaction ArH m by means 'of the equation: AfH~(WTe2, cr, 298.15 K) = AfH~(WF 6, g, 298.15 K ) + 2AfH~(TeF6, g, 298.15 K ) - A r H ~.

(3)

The AfH~ values for W F 6 and TeF 6 were taken to be -(1721.7 + 1.7)kJ" tool a,¢9) and -(1380.7__l.3)kJ'mo1-1, and ArH ~ is given in table 3. Thus, we calculate AfHm(WTe 2, cr, 298.15 K) = - ( 3 8 +__5)k J" mo1-1, for the formation reaction: W(cr) + 2Te(cr) = WTez(cr).

(4)

There has been no previous direct determination of AfH~(WTez), but there have been two investigations from which this value may be estimated. Opalovskii et al. ~zz) used a quartz-membrane null manometer to measure the equilibrium vapor pressure of Te2(g) over WTe2 in the temperature range 1080 K to 1162 K. No details were given of the method of calculation nor of the constants of the vapor-pressure equation. However, for the reaction: W(s) + Tez(g) = WWez(s),

(5)

Opalovskii et al. reported ArH m =-(196.6_+2.1) kJ. tool -1 which, we assume, may be applied to the approximate mean temperature of l l 0 0 K . The enthalpy increments {H~,(T)--H~(298.15 K)} are well established for W(s) and Tez(g), but there is no such information for WTe 2. We know from published results that

646

P.A.G. O'HARE AND G. A. HOPE

enthalpy increments at the same temperature are approximately equal for the pairs of analogs MoS2 and WS2 ( ~ 5 9 k J . m o l 1)(23,24) and MoSe2 and WSe 2 (~65 kJ. mol 1).(24,25) Equations are also available to represent the estimated heat capacities of MoSe 2 and MoTe 2. Brewer and Lamoreaux (26) obtained them by extrapolating the low-temperature heat-capacity results of Kiwia and Westrum; (27) integration of those equations gives polynomial expressions for the enthalpy values. Thus, for MoSe 2 at T = l l 0 0 K , the enthalpy increment is estimated to be 64kJ.mo1-1, close to the experimental result (24) of 64.9kJ.mo1-1. This good agreement gives us confidence that the estimated enthalpy increment for MoTe2 at 1100 K, 66 kJ. mol 1, is probably also quite reasonable, and we assume that a value of (65 _+3) kJ. mol-~ for WTe2, the other member of the pair, should not be seriously in error. Combination of this estimate with the enthalpy increments for W, 21.0kJ.mol 1(28) and Tez(g), 33.1kJ.mol 1,(29) yields A{H~(ll00 K) --H~(298.15 K)} = ( l l + 3 ) k J . m o l 1 for reaction (5). From ArH~(ll00 K) given earlier, and AfHm(Tez, g, 298.15K ) = (163.2___0.5)kJ.mol 1,(29) we deduce AfHm(WTe2, s, 298.15 K) = -(44_+ 4)k J" mol 1, which is in quite good agreement with the present determination. The dissociation pressure of WTe2 to Te2(g) as a function of temperature has also been measured by Obolonchik and Nesterovskaya. (3°) They studied the decomposition of WTe 2 in a flowing stream of argon between 973 K and 1173 K and for the reaction: WTez(cr ) = W(cr) + Tez(g),

(6)

lg(p/p °) = - 6503.3(K/T) + 4.67.

(7)

gave the relation: At T = l l00K, a convenient intermediate temperature, equation (7) implies ArG~(1100 K) = 26.2kJ'mol 1. At this point in the calculation, in the absence of high-temperature thermodynamic properties for WTe z, the quantity o T o ), where T ' = 298.15 K, has to be estimated. An approximate value: (AoT S m-AT,Hm/T 162.2 J . K - 1 . m o l 1, is obtained for MoTe 2 from the Brewer and Lamoreaux (26) polynomial. We believe that the corresponding quantity for W T e 2 should be somewhat larger and accordingly estimate ( 1 7 0 + 1 0 ) J . K 1.mol 1. From the literature, we take values of 47.3 J . K -1. tool -1, and 281.4 J. K -1. mo1-1, for W , (28) and Te2(g),(29) respectively. By means of a third-law calculation: ArH~(298.15 K) = -- (200.8 + 11) k J" mol- 1 for reaction (6) and, thence, with AfHm(Te2, g, 298.15 K) = (163.2+0.5)kJ'mol-1, (29) we obtain ArH~(WTe2, cr, 298.15K ) = --(38±11) k J ' m o l -t. This result, just as that deduced from the work of Opalovskii et al., (22) agrees quite well with our present determination. It is clear that the uncertainties in the enthalpy-of-formation values calculated from the high-temperature studies would be significantly reduced if precise experimental enthalpy increments were to become available for WTe 2. We hope to be in a position to provide such information in the very near future. We are grateful to Stanley Abramowitz for his help in getting this program started, to A1 Ledford and Russell Ryan for their assistance in assembling the laboratory, and to Gerald K. Johnson for providing the high-purity fluorine.

THERMODYNAMIC PROPERTIES OF WTe 2

647

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