Chemistry Department, Moscow State University,
Moscow B-234, 119899, Russia
(Received 10 December 1993)
Direct syntheses of indium tellurides InTe and In0.976Te were carried out in a calorimetric bomb. The energies of formation of these compounds were found not to be significantly different. The standard molar enthalpy of formation of InTe was found to be -(71.2+0.4) kJ/mol.
1. Introduction
Indium tellurides belong to the group of materials which are useful for electronics so that research into the thermodynamic behaviour of these substances is a rather practical problem. The thermodynamic quantities and particularly the enthalpies of formation of some tellurides (as InTe, for example) had been established only with large uncertainty or even unknown at all (as in the case of In9Te7, for example). The purpose of this investigation was to get a more precise value of the enthalpy of formation of InTe.
The available information on the enthalpy of formation of indium telluride, InTe, is rather discrepant. Hahn et al. /1/ obtained DfH0m(InTe,cr,298.15K)=-(96+13) kJ/mol by burning InTe and a mixture of equal amounts of In and Te in a microcalorimetric bomb in oxygen. Gerasimov et al. /2/ determined the enthalpy of formation of InTe from In(l) and Te(cr) at T=673 K by an electromotive-force method and obtained the value -(23+8) kJ/mol. Robinson et al. /3/ and Jena et al. /4/ used liquid-metal solution calorimetry for the determination of the enthalpy of formation of indium telluride at T=273 K. The results published /3,4/ are -(71.6+1.0) kJ/mol and -71.8 kJ/mol, respectively. The critical analysis of references 1 to 4 indicates that the most reliable from the above-mentioned values is -(71.6+1.0) kJ/mol, but the uncertainty of this value seems to be rather doubtful: it was estimated as simply a maximum deviation from the mean of four results. The proper statistical treatment of results /3/ is impossible because of the absence of experimental facts /3/; nevertheless, in accordance with the statistical analysis, taking the criterion of Student into account, the real uncertainty should be much greater. The fundamental handbook /5/ offers the value for the standard molar enthalpy of formation of InTe(cr,298.15 K) based on the result reported by Robinson et al. /3/, namely -(72.0+2.1) kJ/mol, the uncertainty being reasonably doubled in comparison with the value given by the authors /3/.
It is obvious from the aforesaid that the re-investigation of the enthalpy of formation of InTe is desirable and we decided to carry out the direct synthesis of InTe in a calorimetric bomb with an electrical microfurnace.
2. Experimental
The In and Te used for synthesis of InTe were of semiconductor grade and have nominal purities of 99.999 mass per cent.
The reaction was carried out in a sealed evacuated quartz ampoule. The ampoule was filled with indium and tellurium according to the following procedure. The required amount of melted indium was poured in vacuum into a preliminarily weighed (weighing accuracy was 2*10-5 g) ampoule (indium was dropped through a quartz funnel supplied with a capillary to remove oxides from the indium surface); after cooling, the ampoule with indium was weighed, filled with tellurium, and sealed under vacuum. Two batches with different mole fractions of Te (0.500 and 0.506) were used in the calorimetric experiments. The variation of the mole fraction of Te was due to the information that the maximum on the plot of dependence of melting temperature on composition in case of the InTe-phase corresponds to a mole fraction of Te equal to 0.508 /6/, or (0.505+0.001) /7/. The batch containing mole fraction 0.506 of Te (the mean from cited values) was used to decide whether there was some specificity in the enthalpy of formation.
Use was made of anisoperibol calorimeter with electrical microfurnace employed earlier /8/. The temperature of the water jacket was 302 K, the main period of the calorimetric run began at T=298 K and lasted 3 h (due to the large thermal inertia of the quartz ampoule with reaction products); durations of the initial and final periods were 2 h and 1 h, respectively (it was shown experimentally that reduction of the duration of the final period to 1 h had not decreased the accuracy of the thermal measurements).
The quartz ampoule containing indium and tellurium was placed into the furnace in the calorimetric bomb so arranged that solidified indium in the ampoule was found above tellurium to make easier the mixing of components during the melting. Then the bomb was scavenged with pure argon and filled with argon to a pressure of 1.013*105 Pa.
The furnace in the calorimetric runs had been switched on during 8 min to 10 min (it corresponds to an input of about 24 kJ to 30 kJ into the calorimeter). Under these conditions the temperature of the ampoule is sure to be above the melting temperature of InTe (968 K). Completeness of reaction between indium and tellurium in experiments on direct synthesis of InTe was controlled by the following procedure. After the end of a calorimetric experiment concerned with synthesis, the bomb was not disassembled but was used for determination of the energy equivalent of the calorimetric system. The energy equivalent was determined twice (or more than twice). Eighteen measurements were taken to find the mean value of the energy equivalent (the spread of heat capacity of ampoules due to the variation of their masses was negligible - not more than 0.5 J/K or 5 J/Ohm for a 25 Ohm copper thermometer - and was not taken into account at all). Then a new series of seven runs was carried out to find the energy equivalent for the calorimetric system in which the ampoule containing the reaction products was placed not into the furnace but on the bottom of the bomb. The mean results of the two series as well as uncertainties were not significantly different. This is evidence for the completeness of synthesis of InTe in the calorimetric bomb. The results of both the series were treated all together; the mean value of energy equivalent e = (119384+26) J/Ohm was found after 25 runs. This value was used for the calculation of the results of runs on the synthesis of InTe (here and below the uncertainties are given for a 95 per cent confidence interval).
X-ray examination of the reaction products was carried out when a calorimetric run was over. The lines of InTe only were observed on the X-ray diffraction pattern, the lattice parameters of specimens containing mole fractions 0.500 and 0.506 of Te being a=0.84296 nm, b=0.84296 nm, c=0.71328 nm, and a=0.84160 nm, b=0.84160 nm, c=0.71372 nm, respectively, the volume of the unit cell being 0.50685 nm3 and 0.50552 nm3, respectively.
The results of calorimetric runs on the synthesis of InTe in which use was made of the batches containing mole fractions 0.500 and 0.506 of Te are listed in tables 1 and 2, respectively. In these tables mIn, and mTe, are the masses of indium and tellurium, A is the mole fraction of Te in the batch, qel is the electric energy supplied to heat the furnace in the bomb, DR is the corrected change of resistance (corresponding to the temperature rise) in the main period of the calorimetric experiment, e*DR is the total energy change in the calorimetric system, DuIn and DuTe are the energy changes in synthesis of InTe divided by the mass of indium and tellurium, respectively, calculated using the formulas:
DuIn = (e*DR-qel)/mIn, DuTe = (e*DR-qel)/mTe.
The significant difference of the values of DuIn obtained in runs with different mole fractions of Te is obvious from the comparison of results listed in tables 1 and 2, while the values of DuTe are practically the same in both tables. This phenomenon is probably due to the formation of vacant sites in the indium sublattice of In0.976Te (mole fraction 0.506 of Te) accompanying the formation of an equivalent amount of indium atoms with higher degree of oxidation, whose additional bonds should compensate the bonds which had disappeared because of the formation of vacant sites. The difference of energies of both bonds should not be large /3/ and the concentration of vacant sites in In0.976Te is rather low, so no difference of enthalpy of formation between the two specimens of indium tellurides, which have been investigated, was revealed within the limit of the uncertainty of calorimetric measurements.
With this in mind we decided to calculate the enthalpy of formation of indium telluride on the basis of the mean value of DuTe obtained as a result of combined treatment of all the results: DuTe=(558.0+2.0) J/g or (-558.0+3.3) J/g when the uncertainty of the energy equivalent was taken into consideration. Correction for impurities was not made because of the negligible content of impurities in the starting materials. We have finally:
DfH0m(InTe,cr,298.15 K) = -(71.2+0.4) kJ/mol.
This value is in a good agreement with published results /3,4/, but the extent of its uncertainty is substantially lower.
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