Thermochemistry of binary alloys of transition metals: The systems Me-Gd, Me-Ho, and Me-Lu (Me = Cu, Ag, and Au)
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I.
INTRODUCTION
DURING recent years, systematic experimental studies of the standard enthalpies of formation of binary metallic compounds formed between the noble metals and transition metals have been pursued by the present authors. The enthalpies of formation of the considered alloys have been determined by high-temperature direct synthesis drop calorimetry at 1473 K. Our investigations were facilitated by the development by Kleppa and Topor[1] during 1988 and 1989 of a new reaction calorimeter which may be operated continuously at temperatures up to about 1500 K. Using this calorimeter, we first studied the solid and liquid alloys of silver and gold with Sc, Y, and La,[2] with Ti, Zr, and Hf,[3] and with V, Nb, and Ta.[4] More recently, our investigations were extended to the alloys of the noble metals with the lanthanide metals. So far we have reported new information on the enthalpies of formation of solid and liquid alloys of Cu, Ag, and Au with Ce[5] and with Pr and Nd.[6] In the present work, we are studying the binary alloys formed between the noble metals and the lanthanide metals Gd, Ho, and Lu. We have determined the standard enthalpies of formation of a total of 18 congruently melting compounds in these nine binary systems. For some compositions, the corresponding heats of mixing of the liquid alloys have also been obtained. Our values for the enthalpies of formation of the solid alloys are compared with predicted values from the semiempirical model of Niessen et al.[7] Also, by comparison of the new results with values reported in our previous investigations, we are able to establish a systematic trend for the standard enthalpies of formation for two or three characteristic compounds through the lanthanide series of elements. II.
EXPERIMENTAL
A. General The direct synthesis method was used for all the intermetallic compounds as well as for the liquid alloys. All the
K. FITZNER, Associate Professor, is with the Institute of Metallurgy and Materials Science, Polish Academy of Sciences, 30-059 Krakow, Poland. O.J. KLEPPA, Emeritus Professor, is with the James Franck Institute, University of Chicago, Chicago, IL 60637. Manuscript submitted December 5, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
calorimetric measurements were carried out at 1473 5 2 K in our modified SETARAM-type calorimeter. Details of the construction of this calorimeter as well as of the ‘‘liner’’ assembly were given in the original publication.[1] All the experiments were performed under a protective atmosphere of argon gas, which was purified by passing it over titanium chips maintained at about 1123 K. The samples were contained in crucibles made from BN or from BeO. For our calorimetric experiments, we pressed small, cylindrical pellets of the correct composition from a mixture of the two metal powders; these pellets were then dropped from room temperature into the calorimeter at 1473 5 2 K. At this temperature, a chemical reaction between the two metals occurs. The heat effects actually measured in the calorimeter are the
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