X-ray-absorption fine-structure studies of PbSxTe1-x alloys: Ferroelectric phase transitions induced by off-center ions.

Abstract

Studies of ferroelectric phase transitions (FPT's) induced by off-center ions in highly polarizable systems are important for understanding cooperative phenomena in electric dipole systems. Using the x-ray-absorption fine-structure technique, we have investigated the local structure of disordered ${\mathrm{PbS}}_{\mathit{x}}$${\mathrm{Te}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ (x=0.10, 0.18, and 0.30) alloys at temperatures ranging from 10 to 300 K. The sulfur ions are found to be off center in both the high- and low-temperature phases for all three samples. The corresponding displacement of the ions off site center depends strongly on sulfur concentration. We also find strong evidence that the Pb sublattice and Te sublattice shift relative to one another below the transition temperature. Our results suggest that the ${\mathrm{PbS}}_{\mathit{x}}$${\mathrm{Te}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ alloys may undergo a complex phase transition similar to that in ${\mathrm{Ge}}_{\mathit{x}}$${\mathrm{Pb}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$Te: a coupled orientational order-disorder and displacive FPT, where the long-range ordering of dipoles created by off-center sulfur ions may induce the bulk displacive FPT.