Composition Stability Limits of PbTe. II

Small single crystals of Cu-doped, Fe-doped, and undoped PbTe have been saturated with either Pb or Te from an external phase between 400° and 884°C, quenched, and their Hall constants and conductivities have been measured. Small single crystals of 3×1018 cm—3, p-type PbTe pulled from nearly stoichiometric melts have been vacuum-annealed between 395° and 800°C for short times, quenched, and measured. The composition at the invariant melting point is at least 50.012 at. % Te (7×1018 holes/cm3 at room temperature) on the basis of the simplest applicable model. The Pb-rich solidus line crosses the stoichiometric line near 860°C. The range of stability is a maximum near 800°C and corresponds to 0.015 at. % (1.5×1018 electrons/cm3 for Pb-saturated PbTe and at least 7.7×1018 holes/cm3 for Te-saturated PbTe). Te-saturated PbTe precipitates excess Te with great rapidity above 600°C, the resulting precipitates behaving like a bulk Te-rich phase. Cu is a donor in PbTe and Fe is tentatively assumed to be a low-lying donor which is un-ionized at room temperature in n-type PbTe. Our previous results for PbTe saturated with Pb at 800°C and below, including the room-temperature instability, are shown to be in error due to a subsequently detected Cu impurity. Our new results suggest that the predominant atomic point defects in PbTe are Pb vacancies and Te vacancies.