Partial Pressures in Equilibrium with Group IV Tellurides. III. Germanium Telluride

Abstract

The partial pressures of Te₂(g) and GeTe(g) over Ge–Te condensed phases between 1 and 52 at. % Te have been determined between 525° and 940°C by measuring the optical density of the vapor. Partial optical densities due to each species are calculated and converted to partial pressures by assuming Beer's law and published vapor pressures. Over Te‐saturated GeTe(c), pT_e2 reaches a maximum value of 2.5±0.2 Torr between 644° and 679°C. In the same temperature interval pT_e2 over Ge‐saturated GeTe(c) is about 20 times smaller. At the maximum melting point of GeTe(c), 725°C, pT_e2 is 0.6±0.3 Torr and pG_eT_e is 17 Torr. Values of pG_eT_e and pT_e2 over GeTe‐saturated Ge(c) reach maximum values of respectively 125 and 2.2 Torr near 890°C. The value of pG_eT_e is the same over Te‐ and Ge‐saturated GeTe(c) and up to 725°C is given by logp (Torr)= —(10.1±0.2)10³/T+8.84. The standard Gibbs free energy of formation of GeTe(c) according to Ge(g)+ ½Te₂(g)=GeTe(c) is ΔG°=—(117.8±3)+41.3(10^—3)T kcal/mole. The heat of fusion of GeTe(c) is 11.3±2 kcal/mole. For the dissociation of GeTe(g) into Ge(g) and Te₂(g), ΔG°=(71.5±2)—(14±1)(10^—3)T kcal/mole between 637° and 940°C. This leads to a value of 95±4 kcal/mole for the dissociation energy, D₀°, of GeTe(g) at 0°K in close agreement with the spectroscopic value of 95±9 kcal/mole. Our previous data for the standard enthalpies of formation of PbTe(g) and SnTe(g) lead to dissociation energies of PbTe(g) and SnTe(g) at 0°K of 52.7 and 79.3 kcal/mole, respectively.