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

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, $D_0\text{°\hspace{0.17em}=\hspace{0.17em}54.0\hspace{0.17em}±\hspace{0.17em}3.0}{\mathrm{kcal\; mol}}^{\text{−1}}$ , the heat of sublimation, ${\mathrm{\Delta H}}_{\text{s,298}}\text{°\hspace{0.17em}=\hspace{0.17em}111.2\hspace{0.17em}±\hspace{0.17em}5.0}{\mathrm{kcal\; mol}}^{\text{−1}}$ , and the heat of formation ${\mathrm{\Delta H}}_{\text{f,298}}\text{°\hspace{0.17em}=\hspace{0.17em}90.3\hspace{0.17em}±\hspace{0.17em}4.1}{\mathrm{kcal\; mol}}^{\text{−1}}$ , 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 ${\mathrm{\Delta H}}_0\text{°\hspace{0.17em}=\hspace{0.17em}33\hspace{0.17em}±\hspace{0.17em}5}{\mathrm{kcal\; mol}}^{\text{−1}}$ . From this value the heat of atomization of AlP₂, $D_0\text{°\hspace{0.17em}=\hspace{0.17em}149\hspace{0.17em}±\hspace{0.17em}6}{\mathrm{kcal\; mol}}^{\text{−1}}$ , was calculated.