Mass spectrometric study of the thermodynamic properties of solid Au−Pt alloys

The ratios of vapor pressures of gold in equilibrium with liquid gold and with each of eleven Au−Pt alloys have been measured over the approximate temperature range of 1340° to 1520°K by means of a dual-chamber effusion cell and a mass-spectrometric detection system. From these ratios, thermodynamic activities and related integral and partial molar thermodynamic quantities have been evaluated at 1423°K. When the thermodynamic quantities are referred to a hypothetical standard state of superheated gold at 1423°K, the results indicate both substantial positive deviations from ideal behavior and positive molar heats of mixing as is expected for miscibility-gap systems. The integral moalr quantities are adequately predicted by either the subregular or the Lumsden model. Observed integral entropies are discussed in terms of thermal, electronic, and configurational contributions, whereas the positive integral molar free energies are tentatively attributed to strain effects arising from ion-core repulsions. R. W. Jones formerly Graduate Student, Northwestern University, Evanston, Ill. This research was supported by the Advanced Research Projects Agency through the Northwestern University Materials Research Center. Partial support for the mass spectrometer facility was provided by the U. S. Atomic Energy Commission, Document Code COO-1147-30.