Solute–solute and solute–solvent interactions in transition metal alloys: Pt–Ti system

The activity of Ti in solid Pt has been measured as a function of composition at 1573 K using a metal oxide–gas equilibration technique under controlled oxygen partial pressures. Thin foils of Pt were equilibrated with TiO_x at constant oxygen chemical potentials. Oxygen partial pressures were established using Ar–H₂–H₂O gas mixtures of controlled composition. A solid state cell, based on yttria doped thoria as the solid electrolyte, independently determined the chemical potential of oxygen in the gas phase. The concentration of Ti in solid Pt was determined using spectrophotometric methods. The activities of Ti were computed from the oxygen potentials established by the gas phase coupled with independent data on the thermodynamic properties of titanium oxides. The excess chemical potential of titanium in solid Pt at 1573 K in J mol^-1 can be represented as Δμ^ETi = —83 940 — 214 140 (1— X_Ti)² with an error of ±2800 J mol^-1. The activity coefficient of Ti at infinite dilution determined from this study and that of other elements of the first transition series in solid Pt obtained from previous work confirm the trend predicted by both Miedema’s model and Engel–Brewer theory. The attractive interaction between the solute and the solvent (Pt) increases with decreasing atomic number of the solute. The self-interaction parameters of the first transition series elements in solid Pt indicate an increase in solute–solute repulsion with decrease in 3d electron concentration of the solute. The standard Gibbs energy of formation of TiPt₃ is —282·57 ± 4 kJ mol^-1 at 1573 K. The large negative values of the Gibbs energy of formation of phases in the system Pt–Ti indicate that Pt is not phase compatible with nitrides and carbides of Ti at high temperatures