Standard enthalpies of formation of some praseodymium alloys by high-temperature direct synthesis calorimetry
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I.
INTRODUCTION
THE thermodynamic properties of the alloys between early and late transition metals are important both from a theoretical and a technological point of view. In the past several years, we have pursued a systematic study of the thermochemistry of these alloys. A review of our work on equiatomic alloys of Ti, Zr, and H f with late transition metals was given by Topor and Kleppa. m Recently, Selhaoui and Kleppa have reported new standard enthalpy of formation data for alloys of Sc + Me, t2] Y + Me, E31 La + Me, E41 Ce + Me, and Lu + Me, tS] where Me stands for an element from group VIII in the periodic table. In the present article, this study is extended to alloys of Pr with Ni, Ru, Rh, Pd, Ir, and Pt. New standard enthalpies of formation are obtained by high-temperature direct-synthesis calorimetry for eight intermetallic compounds in these six binary systems. The results are compared with the data in the literature and with the predicted values from the model of Miedema e t al.[6] Successful determinations of the standard enthalpies of formation of intermetallic compounds of Sc, Y, La, Ce, and Lu with elements from the Fe, Pd, and Pt groups indicate that direct-synthesis calorimetry is applicable to alloys between the rare earth elements and the elements of group VIII. A survey of the available phase diagrams for the binary systems of Pr with the previously mentioned elements shows that PrNis, PrRh2, PrIr2, and PrPt2 all melt congruently, while PrRu2 is an incongruently melting compound. There is no phase diagram for the Pr-Pd system. However, we believe it is reasonable to assume the existence of the intermediate compounds PrPd2 and PrPd in analogy with the Pr-Pt system in which PrPt exists as a congruently melting compound. On the other hand, the existence of PrRh is questionable, and there is no evidence for the compounds PrRu and PrIr. Hence, we decided to determine the standard enthalpies of formation of PrNis, PrRu2, PrRh2, PrPd2, PrPd, Prlr2, PrPt2, and PrPt. QITI GUO, Professor, is with the Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, Guizhou Province 550002, People's Republic of China. O.J. KLEPPA, Emeritus Professor, is with James Franck Institute, University of Chicago, Chicago, IL 60637. Manuscript submitted May 4, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS B
II. EXPERIMENTAL AND STARTING MATERIALS The experiments were carried out at 1473 _ 2 K in a single-unit differential microcalorimeter which was described earlier, tT] All experiments were conducted under a protective atmosphere of Ar. This gas was purified by passing it through a silica tube full of Ti grains maintained at approximately 900 ~ to eliminate possible trace amounts of O and N. The actual synthesis reactions were carried out in BeO crucibles. Calibration of the calorimeter was performed by dropping pieces of 2-ram-diameter high-purity Cu wire of a known mass from room temperature into the calorimeter at 1473 -+ 2 K. The enthalpy of pure Cu at 1473 K, 46,465 J/mol, was taken from Hultgren e t al.
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