Temperature and pressure induced phase transition in IV-VI compounds

The crystal structure of SnS has been investigated as a function of temperature by elastic neutron scattering from 293 K to 1 000 K. SnS undergoes a second order displacive phase transition from the α-phase (B16, Pbnm) to the high temperature β-phase (B33, Cmcm) at 887 K. The mean square displacement of the Sn atom which is isotropic at room temperature becomes increasingly anisotropic and anharmonic near Tc. Energy dispersive X-ray diffraction experiments have been performed on IV-VI semiconductors PbS, PbSe, PbTe, GeS, GeSe, SnS and SnSe as a function of hydrostatic pressure to 340 kbar. PbS, PbSe and PbTe undergo transition from the NaCl type (B1) phase to the orthorhombic phases (B33 or B16) at about 22, 45 and 60 kbar, respectively. The orthorhombic phases of PbS and PbSe have been found to be of TlI type (B33, Cmcm) and that in PbTe has been found to be GeS type (B16, Pbnm). PbS, PbSe and PbTe undergo further phase transitions to the cubic CsCl type (B2) phase at about 215, 160 and 130 kbar, respectively. The orthorhombic IV-VI compounds GeS, GeSe, SnS and SnSe do not undergo any phase transition up to 340 kbar.