Activity coefficients and partial molar volumes of boron in platinum-rhodium alloys

Activity coefficients of B in the α phase of Pt and of three PtRh alloys with 3, 6, and 10 wt.% Rh were obtained by equilibration of B 2O 3 with H 2-H 2O mixtures between 750 and 1000 °C and subsequent determination of the B mole fraction x B ⩽ 0.06 by the mass gain of the samples or by photometric analysis. Additions of Rh to Pt have rather large but opposing effects on the partial molar enthalpies and entropies of B, resulting in partial molar excess Gibbs energies within the modest range between −40 and −55 kJ mol −1. The change in the lattice contraction from −13.5 to −3.2 pm/ x B in the α phase of Pt and of Pt-10Rh can be fully described in terms of a substitutional solution model with strictly constant partial molar volumes of 9.09 cm 3 mol −1 and 8.28 cm 3 mol −1 for Pt and Rh respectively and a partial molar volume for B that increases linearly from 8.2 to 8.9 cm 3 mol −1 between pure Pt and Pt-10Rh.