Gaseous Phosphorus Compounds. IV. Thermodynamic Study of Gallium Monophosphide with a Mass Spectrometer and Dissociation Energy of Aluminum Diphosphide

Abstract

Gaseous equilibria involving the GaP molecule have been studied by means of the Knudsen-effusion technique in combination with mass spectrometric analysis of the vapor phase. In order to obtain the desired phosphorus and gallium partial pressures of comparable magnitude, mixtures of gallium and either WP or Th3P4 which were evaporated from a graphite Knudsen cell were employed. From the experimentally determined partial pressures the equilibrium constants and the enthalpies of the reactions Ga(g)+0.5 P2(g)=GaP(g), Ga(g)+0.5 P4(g)=GaP(g)+0.5 P2(g), 0.5 Ga2(g)+0.5 P2(g)=GaP(g), and GaAg(g)+0.5 P2(g)=GaP(g)+0.5 Ag2(g) have been determined. These reaction enthalpies have been combined with appropriate literature data to yield the dissociation energy, D0° = 54.0 ± 3.0 kcal mol−1, the heat of sublimation, ΔHs,298° = 111.2 ± 5.0 kcal mol−1, and the heat of formation ΔHf,298° = 90.3 ± 4.1 kcal mol−1, of gaseous gallium monophosphide. The molecule AlP2 has been observed over the congruently vaporizing AlP(s). The enthalphy of the reaction AlP2(g)=Al(g)+P2(g) was obtained as ΔH0° = 33 ± 5 kcal mol−1. From this value the heat of atomization of AlP2, D0° = 149 ± 6 kcal mol−1, was calculated.