Standard enthalpies of formation of Ptti, Ptzr, and Pthf
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I,
INTRODUCTION
THE compounds formed between early transition metals (Groups III and IV) and the platinum group metals are very stable compared to most intermetallic compounds. It is known, for example, that the combination of hafnium with platinum gives rise to a very large release of heat so that the reaction may proceed at an explosive rate. t~'EjDue to the experimental difficulties and the inadequacy of the available calorimetric methods, reliable thermochemical data for such compounds are very scarce. Brewer and Wengert established the trend in the stability of a large number of intermetallic compounds of zirconium with late transition metals. They determined the limits of the Gibbs free energies of formation by equilibrating the intermetallic compound with graphite and the appropriate carbide.LZ] Later the thermodynamic activities of 25 related binary alloys were determined by the same method, by equilibrating the alloy with nitride and nitrogen, or by measuring the partial vapor pressures.I31 Quite recently values or limits for the Gibbs free energies of formation of 84 intermetallic compounds involving transition elements, lanthanides, and actinides were presented.[4] During the past year we have initiated a systematic investigation of thermodynamic properties of intermetallic compounds formed between early and late transition metals. In these studies we have made use of a new calorimetric method, high temperature mixing calorimetry, first developed for thermochemical studies of refractory materials such as borides t5'6'71 and silicides, t8'9'1~ This method differs from traditional ones such as combustion, solution, and direct reaction calorimetry. It involves dropping mixtures of solids from 298 K into a high temperature calorimeter in which they generate a homogeneous liquid mixture. This multicomponent liquid mixture is formed either (a) from several solid elements or (b) from a solid mixture of the same overall composition as under (a), but which now includes the compound which is being studied. The difference between the two heat effects associated with (a) and (b) yields the standard enthalpy of formation. So far, enthalpies of formation have been reported for PdTi, IHI PdZr and PdHf, I~21RhTi, RhZr, and RhHf, I131and RuTi, RuZr, RuHf. 1141To establish the general trend in the
thermodynamics properties of the compounds between early and late transition metals it is necessary to have also enthalpy data for the compounds of Group IV metals with Pt, Ir, and Os. In the present communication our study is extended to PtTi, PtZr, and PtHf, and the standard enthalpies of formation of these compounds is reported. The new results will be compared with other values recently obtained by direct high-temperature reaction calorimetry, with predicted values, and with our previously published results for the compounds of Ti, Zr, and Hf with Pd, Rh, Ru.
II.
EXPERIMENTAL
A. Method The calorimetric method used in the present study has been described.iS'6] The choice of the liquid alloy composition generated in the hi
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