Heat capacities and thermodynamic properties of Cr3Te4 and Ni3Te4 from 298 to 950 K. Structural order-disorder transitions

J. Chem. Thermodynamics 1973, 5, 545-551

Heat capacities and thermodynamic properties of Cr3Te4 and Ni3Te4 from 298 to 950 K. Structural order-disorder transitions FREDRIK GRONVOLD

Department of Chemistry, University of Oslo, Blindern, Oslo 3, Norway (Received 20 November 1972) The heat capacities of Cr3Te4 and Ni3Te4 have been determined in the range 298 to about 950K by adiabatic-shield calorimetry with intermittent electrical energy inputs and temperature equilibration between inputs. Values of heat capacity, enthalpy, and entropy are tabulated for selected temperatures after inclusion of earlier low-temperature results. For Cr3Te~ a ?~-typetransition occurs with heat capacity maximum at 903 K, presumably connected with disordering of vacancies in the grossly defective structure. The estimated order-disorder entropy change is 2.2 J K-1 tool- 1 for Cro.7~Te, which compares reasonably well with the assumed disordering of kmol of Cr and vacancies (AS = 2.88 J K-1 mol-~). For NisTe4 no transition occurs, except for a slightly enhanced heat capacity in the region 350 to 500 K, indicating only a slight tendency for vacancyordering under the conditions in question. The vacancies thus probably remain largely disordered in every other metal layer parallel to 001 down to the lowest temperatures, and the zero-point configurationat entropy is accordingly approximately 1.5 J K- 1tool- x for {Ni3Te~.

1. Introduction The presence of extended solid solutions is characteristic of many 3d transition-metal chalcogenides at higher temperatures. Thus, neither Cr3Te 4 nor Ni3Te 4 are true compounds, but members of a solid-solubility series extending from about Meo.83Te to Meo.67Te in the chromium+tellurium system for samples annealed at 875K, (x) and from Meo.92Te to Meo.5oTe in the nickel + tellurium system for samples annealed at 725K and cooled at room temperature over a period of 2d. (2) The structures of the phases are of NiAs-like type with vacancies in the Me-lattice. X-ray and neutron diffraction studies have shown that in CraTe 4 these vacancies are located in every other chromium layer in an ordered way. (3' 4) The ordering is reflected in the lowsymmetry (monoclinic) structure of the compound. For Ni3Te 4 no such structural deformation has been observed, and it seems reasonable to interpret the presence of a hexagonal instead of monoclinic structure to the absence of ordering of vacancies within every other nickel layer parallel to 001. For CraTe 4 the presence of a structural order-disorder transition should then be expected, possibly in the temperature range accessible with our heat-capacity calorimeter. Measurements have therefore been carried out on a carefully prepared sample with stoichiometric composition Cr3Te 4. For Ni3Te 4 the possible absence of a thermal anomaly might indicate the presence of structural disorder at low temperatures, since

546

F. GRONVOLD

no termal anomaly has been observed (s) in the range 5 to 350 K for the related samples N i T e l l o and NiTel.so. Determination of the high-temperature thermodynamic properties of Cr3Te4 and Ni3Te4 is of further interest since their thermophysical behaviour is unknown, except for a drop-calorimetric investigation on the related compound NiTe in the range 288 to 700K. (6)

2. Experimental SAMPLES The compounds were prepared from high-purity transition metals and tellurium. The hydrogen-treated electrolytic flake chromium was a gift from Dr W. DeSorbo and the Research Laboratory of General Electric Co., Schenectady, U.S.A. According to the information provided, it was 99.97 mass per cent pure with impurities (p.p.m. by mass): Fe < 70, O < 60, and all other metals < 100. The 99.998 mass per cent nickel was supplied by Koch-Light Laboratories Ltd., Colnbrook, U.K. in the form of 5 mm diameter rods. The tellurium used was a "Special high-purity semi-conductor grade Te", 99.999 mass per cent pure, from the American Smelting and Refining Co., 120 Broadway, New York, N.Y. 10005, U.S.A. The chromium +tellurium mixture was heated in an evacuated and sealed silicaglass tube, placed inside a second tube for oxidation protection in case of breakage due to solid-state transformation. It was heated to 1275 K over a period of 6 d, kept at this temperature for 3 d, and slowly cooled to room temperature. The sample was finely crushed and then tempered at 1075 K for 6 d in another silica-glass tube; t73.722g of the sample was finally transferred to a calorimetric ampoule and tempered at 775 K for 2 months. The nickel+tellurium mixture was heated to fusion in an evacuated and sealed silica-glass tube. After 2 h at 1275 K the sample was cooled to room temperature, finely crushed, and tempered at 775 K for 25 d; 205.288 g of sample was then transferred to a calorimetric ampoule. X-ray photographs were taken of the samples in Guinier-type cameras with 8.0 cm diameter. Cu(K0q) radiation was used, with KC1 as a calibration substance with a(293 K) = 0.629 19 nm. (7) The structure of Cr3Te 4 was found to be monoclinic with lattice constants a = 0.6888nm, b = 0.3946nm, c = 1.236nm, p = 91.12 °, in excellent agreement with earlier findings. For Ni3Te 4 the hexagonal structure was confirmed with a = 0.3916 nm, e = 0.5361 nm, which fitted very well with earlier data (2) for adjacent compositions. CALORIMETRIC TECHNIQUE The calorimetric apparatus and measurement technique have been described in detail (a) along with results obtained for the heat capacity of a standard sample of 0~-A1203. The calorimeter is operated with adiabatic shields and intermittent energy inputs with temperature equilibration between each input. The 50 cm a sample container of fused silica has a well for the heater and platinum resistance thermometer, axially

HEAT CAPACITIES OF Cr3Te4AND NiaTe4

547

located in the cylindrical silver calorimeter. The calorimeter+sample assembly is suspended inside a double-walled silver shield system with enclosed heaters. Outside the shields is a heated guard system, also of silver. The whole assembly is placed in a vertical tube furnace. The temperature differences between corresponding parts of calorimeter and shield are measured by means of Pt-to-(Pt + 10 mass per cent Rh) thermopiles. The amplified signals are recorded and also used for automatic control of the shield heaters to maintain quasi-adiabatic conditions during input and drift periods. The temperature of the guard body is kept automatically 0.4 K below that of the shield, while the temperature of the furnace core is kept 10 K lower to secure satisfactory operation of the control units. Heat-capacity measurements of the empty calorimeter were carried out in a separate series of experiments. They represent from 57 to 64 per cent of the total outside the transition regions. Small corrections were applied for differences in mass of the empty and full silica containers, and for "zero" drift of the calorimeter. The temperature excursions of the shields from the calorimeter temperature were of negligible importance. The thermometer resistance was measured with a Mueller bridge (Leeds & Northrup Model 8072), automated locally with stepping motors and a gated nulldetector, operated by a computer (Hewlett-Packard Model 2114 B) in the more recent series of experiments. The derived temperatures are judged to correspond with the IPTS-68 to within 0.1 K at 900 K. Precision is considerably better, and the temperature increments are probably accurate to 0.002 K. The computer-operated energy inputs from a constant-current supply (John Fluke Current Calibrator Model 382 A in most of the experiments) were measured with an integrating voltmeter (Hewlett-Packard Model 2401 C). The accuracy of the energy inputs is about 0.025 per cent. Both Mueller-bridge and (potential xtime) readings are automatically transferred to punch cards (IBM 545) together with time and other relevant information, and then processed by a digital computer.

3. Results and discussion The results of the heat capacity determinations on Cr3Te4 and Ni3Te, are presented in table 1 in chronological order for +Me3Te4. The approximate temperature increments can usually be inferred from the adjacent mean temperature in the table. The curves of heat capacity against temperature are shown in figure 1. For Cr3Te4 a decrease in heat capacity is observed just above room temperature as result of a magnetic transition in the sample. Above 500K the heat capacity increases steadily towards a maximum of 150.33 J K - 1 tool- 1 at 902.39 K over a temperature increment of 1.33K. It then decreases again to a minimum of 30.16JK -~ tool -1 at 928.63 K. For Ni3Te 4 the heat capacity increases more uniformly with temperature. A slightly enhanced increase is observed in the region 340 to 370 K, followed by a less pronounced increase in the region up to 600 K. No signs of a ~-type transition are observed in the range up to 920 K. Values of Cp, {H°(T)-H°(O)}, and {S°(T)-S°(O)} are listed in table 2 after inclusion of earlier low-temperature results for Cr3Te4, (~) and interpolated values (s)

TABLE 1. Molar heat capacities of ~Cr3Te4 and ~Ni3Te4 T K

C~0rCrsTe4) JK-lmo1-1

T K

C~(kCr3Te4) J K - l m o l -x

T K

Cp(~CrsTe4) jK-tmol-X

M(kCrsTe4)=95.21 gmo1-1 Series ! 678.88 690.30 701.61 712.89 724.25 735.55

29.09 29.17 29.24 29.75 29.64 30.04

Series II 736.54 29.70 748.65 29.89 760.40 30.34 772.07 30.76 783.66 30.68 795.21 31.30 806.74 31.53 818.23 31.71 829.68 32.06 Series III 844.31 33.10 851.86 34.07 868.94 34.62 880.93 35.83 892.72 39.32 T K

C,(~-Ni3Te~) JK-lmo1-1

902.46 912.80

80.90 31.12

Series IV 880.63 36.05 888.27 38.38 893.24 39.49 897.95 48.52 900.96 121.91 902.39 150.33 904.33 54.73 907.16 34.76 915.38 30.57 928.63 30.16 94114 30.28 Series V 312.81 323.94 335.18 346.55 357.95 369.37 380.81 392.28 T

28.92 28.53 27.11 26.97 26.89 26.76 26.93 26.87

Cp(~NisTe4) JK-lmo1-1

Ser~s VI 403.78 26.77 415.32 26.79 426.88 26.78 438.48 26.97 450.12 26.83 461.80 26.86 479.40 26.94 503.00 26.98 520.77 26.99 532.67 27.11 544.58 27.44 556.50 27.46 568.45 27.74 580.45 27.60 592.50 27.99 604.60 27.89 616.75 27.94 Series VII 628.95 28.06 641.18 28.33 653.47 28.53 665.82 28.67 678.22 28.82 T K

Cn(~-Ni3Te4) JK-lmo1-1

M(kNi3Te4)=98.07gmo1-1 Series I 304.72 316.04 327.96 339.72 351.37 362.92

25.76 25.77 25.99 26.22 26.68 26.90

Series II 570.71 581.65 592.58 603.50 614.41 625.31

28.25 28.31 28.34 28.42 28.65 28.82

Series III 641.38 28.91 654.01 29.08 666.62 29.21 679.21 29.25 Series IV 489,58 27.77 500.68 27.85 511.81 27.84

522.96 534.12 545.30 556.51

28.00 28.16 28.24 28.36

Series V 674.82 29.25 686.44 29.54 698.09 29.80 709.80 29.80 721.56 30.18 733.41 30.27 745.30 30.41 757.33 30.56 769.15 30.74 780.98 30.88 792.78 31.20 Series VI 804.43 31.40 815.88 31.82 827.16 32.04 838.37 32.25 849.63 32.41 861.04 32.66 872.62 32.82

884.44 . 896.53 908.68 920.89

33.01 33.28 33.62 34.10

Series VII 311,.71 25.93 322.61 25.70 333.46 26.14 344.26 26.26 355.02 26.79 365.71 27.13 376.38 27,28 387.06 27.38 397.77 27.32 408.50 27.44 419.25 27.51 430.02 27.58 440.79 27.70 451.58 27.78 462.40 27.82 473.25 27,87 484.13 27.93 495.05 28.01 506.02 28.18

HEAT CAPACITIES

O F CraTe~ A N D NiaTea

549

50 45 40

5'

35:

25 "-~ 300 '

400 '

500 '

6;0~

7; 0

800 '

9; 0

T/K

FIGURE 1. Molar heat capacity C~ of Cr3Tea (upper part) and of Ni3Te4 (lower part). --©--, experimental results; . . . . , estimated non-transitional heat capacity. for NiaTe 4. The present results for CraTe 4 are slightly higher than the previous ones in the transition region around 300 K. The total transitional enthalpies and entropies are presumably the same for the two samples and the present results are therefore joined with the previous ones at 350K. An estimate of the entropy and enthalpy of the 903 K transition in Cr3Te 4 is derived by subtracting the non-transitional contribution as defined by a linearly increasing heat capacity from 27.4 J K - a reel - 1 at 550 K to 30.2 J K - 1 mol - 1 at 940 K. The resulting transitional entropy and enthalpy values for ~-Cr3Te 4 are here 1.08 J K - 1 reel - 1 and 975 J m o l - 1, respectively. In agreement with earlier findings (9) for F%Se4 the transition is related to disorder of chromium atoms and vacancies in the structure. I f the disorder affects 0.25 Cr atoms and 0.25 vacancies of Cro.75 Do.zsTe the entropy increment is ideally: AS = - (R/2)(0.5 In 0.5+0.5 In 0.5) = 2 . 8 8 J K -1 mo1-1. This value is somewhat higher than the observed one, 2.22 J K - 1 reel- 1 for Cre.75Te. It should be remarked, however, that the estimate is based upon the assumption of the transition being complete already at 940 K. This is probably not the case and the transitional entropy is accordingly somewhat underestimated. The assumption of disorder among all iron atoms and vacancies leads to a much higher transitional entropy: AS = 4.67 J K - 1 mol - 1, and is therefore rejected as improbable. For Ni3Te 4 no similar transition is encountered. It would be expected to occur at a lower temperature than for Cr3T % in compliance with the less ionic nature of the

550

F. GRONVOLD

f o r m e r c o m p o u n d . Thus, the e n h a n c e d heat c a p a c i t y a r o u n d 400 K might indicate the d i s a p p e a r a n c e o f m i n o r configurational order. It is a n t i c i p a t e d t h a t p r e p a r a t i o n o f Ni3Te 4 u n d e r the conditions closer to equilibrium at a m b i e n t t e m p e r a t u r e s might result in o r d e r e d d i s t r i b u t i o n o f the vacancies. This will n o t be the case under o r d i n a r y conditions, h o w e v e r , a n d z e r o - p o i n t configurational e n t r o p y is therefore expected in TABLE 2. Molar thermodynamic properties of ~-Cr3Te~ and +Ni3Te4 T

C~,

JK-lmo1-1

- - H°(0) Jmol-1

H°(T)

S°(0) jK-lmol-1

S°(T) --

298.15 350 400 450 500 550 600 650 700 750 800 820 840 860 880 900 920 940 950

kCraTe4: 28.31 26.75 26.85 26.83 27.00 27.38 27.89 28.40 29.16 30.11 31.32 32.00 32.96 34.17 35.95 58.0 30.40 30.17 30.22

M = 9 5 . 2 1 gmo1-1 5894 6228 7571 8913 10257 11616 12998 14404 15843 17323 18858 19491 20140 20811 21509 22318 23317 23921 24223

41.87 46.26 49.83 52.99 55.82 58.41 60.81 63.07 65.20 67.24 69.22 70.00 70.78 71.57 72.38 73.28 74.39 75.04 75.36

298.15 350 400 450 500 550 600 650 700 750 800 850 900 950

+Ni3Te4: 25.55 26.83 27.44 27.73 27.93 28.18 28.53 29.03 29.67 30.47 31.39 32.40 33.47 34.56

M = 9 8 . 0 7 gmol -x 5565 6911 8269 9649 11040 12443 13860 15298 16765 18268 19814 21408 23055 24756

40.10 44.26 47.89 51.14 54.07 56.75 59.21 61.51 63.69 65.76 67.76 69.69 71.57 73.41

Ni3Te 4. T h e o r d e r - d i s o r d e r e n t r o p y o f nickel a t o m s a n d vacancies in every other metal layer parallel to 001 in the present sample is e s t i m a t e d b y fitting a h e a t - c a p a c i t y curve to the i n t e r p o l a t e d values for Ni3Te 4 in the region 220 to 300 K a n d the present results f r o m 460 to 600 K. The resulting t r a n s i t i o n a l e n t r o p y value is 0.14 J K - 1 m o l - 1 for ~-Ni3Te 4. The absolute e n t r o p y values for use in chemical t h e r m o d y n a m i c calcula-

HEAT CAPACITIES OF CraTe4 AND NiaTe4

551

tions for -~Ni3Te 4 should therefore most probably be increased by about 1.5 J K -~ t o o l - 1 over those given in table 2. Further confirmation o f this will have to await the results o f detailed equilibrium studies. The continued support by Norges almenvitenskapelige forskningsrfid is gratefully acknowledged. Bjorn L y n g Nielsen kindly assisted with the preparations and measurements.

REFERENCES 1. Gronvold, F.; Westrum, E. F., Jr. Z. Anorg. Allg. Chem. 1964, 328, 272. 2. Barstad, J.; Gronvold, F. ; Rost, E. ; Vestersjo, E. Acta Chem. Scand. 1966, 20, 2865. 3. Chevreton, M.; Bertaut, E. F. ; Jellinek, F. Acta Crystallogr. 1963, 16, 43•. 4, Andresen, A. F. Acta Chem. Scand. 1970, 24, 3495, 5. Westrum, E. F., Chou, C. ; Machol, R. E. ; Gronvold, F. J. Chem. Phys. 1958, 28, 497. 6. Tilden, W. A. Phil. Trans. Roy Soc. London, Ser. A 1904, 203, 139. 7. HamMing, P. G. Acta Cryst. 1953, 6, 98. 8. Gronvold, F. Acta Chem. Scand. 1967, 21, 1695. 9, Gr~nvoid, F. Acta Chem. Scaled. 1968, 22, 1219.