Unusual B1-B2 transition in PbSe under high pressure: evidence for two intermediate phases; transport, structural, and optical properties

We report the results of a study of the thermoelectric power (Seebeck effect), the electrical resistivity, X-ray diffraction, and Raman spectroscopy under high pressure (P) to 15 GPa for binary and tin-doped lead selenide (PbSe and Pb 1-x Sn x Se (x = 0.125)). The gathered data were inconsistent with previous models suggesting only one intermediate phase between the ambient NaCI (Bl) lattice and a high-pressure metal CsCl (B2) lattice. Furthermore, the data hint at the existence of two competing intermediate phases. From the thermopower we established that a phase that is characterized by n-type conductivity is dominating, while the other one - of p-type conductivity is probably optional. The electrical conductivity in both of the phases is semiconducting. In the X-ray diffraction studies in one of the three investigated samples of Pb 0.875 Sn 0.125 Se we could refine a pattern of the intermediate phase taken at ∼9.5 GPa in the Cmcm space group. This corresponds to the symmetry of earlier proposed structural types of TlI and CrB. The patterns of the intermediate phase in the two other samples show additional strong reflexes that are not related to either the NaCI or the CsCl lattices. This presents evidence of the presence of one more undetermined phase. The Raman spectra of the intermediate phase showed a dependence on a pressure-transmitting medium (methanol-ethanol or KCl). The pressure-driven NaCl → CsCl transition and a pressure dependence of the semiconductor energy gap of PbSe are compared with those of other lead salts, namely, PbTe and PbS (galena). It is surmised that there is no interrelation between the closure of the energy gap at the NaCl phase and the phase transition to the intermediate phase.