Effect of the Pb2+ lone electron pair in the structure and properties of the double perovskites Pb2Sc(Ti0.5Te0.5)O6 and Pb2Sc(Sc0.33Te0.66)O6: relaxor state due to intrinsic partial disorder

Abstract

We describe the preparation, the crystal structure refined from neutron powder diffraction (NPD) data, and study of the permittivity of two related double perovskites, Pb₂Sc(Ti_0.5Te_0.5)O₆ and Pb₂Sc(Sc_0.33Te_0.66)O₆. These compounds were synthesized by standard ceramic procedures; Rietveld refinements from room temperature NPD data show that the crystal structures are well defined in a cubic unit cell (space group Fmm) with double parameter, a = 2a₀≈ 8 Å. They contain a completely ordered array of ScO₆ and (B,Te)O₆ (B = Sc, Ti) octahedra sharing corners; the PbO₁₂ polyhedra present an off-center displacement of the lead atoms along the [1 1 1] directions, due to the electrostatic repulsion between the Pb²⁺ 6 s electron lone-pair and the Pb–O bonds of the cuboctahedron. Both compounds present a low temperature, highly dispersive maximum in permittivity, the position of which follows the Vogel–Fulcher relation with freezing temperatures of 156 and 99 K for Pb₂Sc(Ti_0.5Te_0.5)O₆ and Pb₂Sc(Sc_0.33Te_0.66)O₆, respectively, exhibiting a typical phenomenology of relaxors.