Strong solute-solvent interactions in the systems Mg-Pd and Al-Pd: manifestation of charge transfer and s-p-d hybridization

Abstract

The activities of Mg and Al in solid Pd have been measured in the temperature range 1425 to 1725 K using metal-oxide equilibration technique under controlled oxygen partial pressures. Thin foils of Pd were sandwiched between pellets of either MgO or Al₂O₃. Oxygen partial pressures were established using H₂ + CO₂ gas mixtures and measured using a solid state cell based on yttria-doped thoria as the electrolyte. The concentration of Mg or Al in solid Pd was determined using inductively coupled plasma - atomic emission spectrometer (ICP-AES) and electron probe microanalysis (EPMA). The activity of Mg or Al was computed from the standard Gibbs energy of formation of MgO or A1203 and the oxygen potential established by the gas phase. The very small values for the activity coefficients (2.3. 10^-7 < γMgo < 7.4. 10^-5 and 2.2.10^-10 < γAlo < 3.6.10^-8) indicate unusually strong interaction between the solute and the solvent, caused primarily by charge transfer from the solute to the solvent, with a smaller contribution from hybridization and covalent bonding involving s, p and d electrons.