Standard enthalpies of formation of PdZr and PdHf
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I.
INTRODUCTION
T H E R E is a great scarcity of reliable themodynamic data for intermetallic compounds, undoubtedly due to the experimental difficulties associated with their determination. Special interest is attached to compounds formed between two transition metals. Because of the reactive character of some of these metals (particularly groups 111 and IV) and the highly refractory properties of many others, such compounds usually cannot be studied by conventional calorimetric methods. Recently Gachon and Hertz have attempted to determine the enthalpies of formation of intermetallic compounds of early transition metals with Pd and Pt by direct reaction calorimetry at very high temperatures. However, due to the many uncertainties associated with their measurements, the authors consider their results to be "tentative" or "indicative" rather than definitive. Since experimental data in this field are lacking, many attempts have been made to predict the thermodynamic properties of intermetallic compounds semi-empirically or theoretically. About 20 years ago Brewer, 2'3'4 adopting and extending theoretical ideas previously advanced by Pauling and Engel, predicted that very stable compounds should be formed when early transition metals are alloyed with late transition metals. The high stability results from the strong Lewis acid-base type interaction between d-electron deficient and d-electron rich metals. At about the same time Kaufman, 5 attempting to calculate equilibrium phase diagrams from thermodynamic data, made rough estimates of the enthalpies of formation of many intermetallic compounds. More recently Miedema and co-workers,6'7 using a semi-empirical model, have predicted the enthalpies of formation for a wide range of binary alloys from the difference in the work functions of the two metals and the chargedensity mismatch at the Wigner-Seitz boundary. Watson and Bennett, 8 on the basis of energy band theory, using a semiempirical interpolation scheme for the d-bands, predicted the enthalpies of formation of most binary equiatomic alloys formed among the transition metals. A more recent publication by the same authors 9 was "precipitated" by new experimental data, some of which were considered to be incompatible with data used in their earlier publication, s These changes had a very significant impact on the adopted LETITIA TOPOR is Senior Research Associate, James Franck Institute, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, and O.J. KLEPPA is Professor of Chemistry and Geophysical Sciences, University of Chicago. Manuscript submitted December 17, 1986. METALLURGICALTRANSACTIONS A
input parameters, the most important of which was the width of the d-band. We will illustrate the effect of the new data on the values given for the compounds considered in our study, PdTi, PdZr, and PdHf. In 1981 Watson and Bennett predicted the enthalpies of formation of these three compounds to be - 7 3 , - 102, and - 109 kJ g atom -~ , respectively; in 1984 the corresponding values were - 6 4 , - 7 7 ,
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