High-Temperature Behavior and Polymorphism in Novel Members of the Perovskite Family Pb 2 LnSbO 6 (Ln = Ho, Er, Yb, Lu)

Abstract

The synthesis, crystal structure, and dielectric properties of four novel members of the family of double perovskites Pb2LnSbO6 are described. The roomtemperature crystal structures were refined from neutron powder diffraction (NPD) data in the monoclinic C2/c (No. 15) space group. They contain a completely ordered array of alternating LnO6 and SbO6 octahedra sharing corners, tilted in antiphase along the three pseudocubic axes, with a abb tilting scheme, which is very unusual in the crystallochemistry of perovskites. The lead atoms occupy highly asymmetric voids with 8-fold coordination due to the stereoactivity of the Pb2þ electron lone-pair. Several trends are observed for the entire family of compounds upon heating. The Ln = Lu, Yb, and Er oxides display three successive phase transitions in a narrow temperature range, as shown by differential scanning calorimetry (DSC) data, while the Ln = Ho shows only two transitions. Different crystal structure evolutions have been found from temperature-dependent NPD and DSC, following the space-group sequence C2/c f P21/n f R3 f Fm3mfor Ln = Lu and Yb, the sequence C2/c f unknown f P21/n f Fm3mfor Ln = Er, and C2/c f P21/n f Fm3mfor Ln = Ho. The Ln/Sb long-range ordering is preserved across the consecutive phase transitions. Dielectric permittivity measurements indicate the presence of a paraelectric/antiferroelectric transition (associated with the last structural transition), as suggested by the negative Curie temperature from the CurieWeiss fit of the reciprocal permittivity.