Partial Pressures in Equilibrium with Group IV Tellurides. I. Optical Absorption Method and Results for PbTe

An optical-absorption method has been developed for measuring the partial pressures of Te2(g) and MTe(g) in equilibrium with MTe(c), where M is Pb, Sn, or Ge. The partial optical densities of Te2(g) at 4360 Å and of MTe(g) at 3650 or 3100 Å are obtained by a Beer's-law analysis of the data, which use optical constants for Te2(g) and MTe(g) found in calibration experiments. Published vapor-pressure data are then used to calculate the partial pressures of Te2(g) and MTe(g) from their respective partial optical densities. This paper reports results obtained by the optical absorption method for the Pb–Te system. Between 725° and 924°C, the maximum melting point of PbTe, the partial pressure of PbTe(g) in equilibrium with solid PbTe is given by logp(Torr) = —11,430/T°K+10.612, corresponding to a heat of sublimation of 52.3±1.0 kcal/mole. The PbTe(g) partial pressure is independent of the composition of PbTe(c) within the limits of experimental error. The partial pressure of Te2(g) varies strongly with the composition of the solid phase, as shown by data for Pb-saturated and Te-saturated PbTe. The maximum partial pressure of Te2(g) in equilibrium with PbTe(c) is 12.5 Torr, which is attained between 796° and 832°C for Te-saturated PbTe. At these temperatures the partial pressures of Te2(g) in equilibrium with Pb-saturated Te are about two orders of magnitude smaller. The partial pressures of PbTe(g) and Te2(g) in equilibrium with PbTe(c) at its maximum melting point are 11.8 and 2.5 Torr, respectively.