Superstructure and thermal hysteresis effects at T_curie in Cd₂Nb₂O₇ pyrochlore

Abstract

Calorimetric and dielectric studies of nominally ‘pure’ Cd₂Nb₂O₇ single crystals for a few thermal scanning rates in the temperature region 100–300 K are presented. The thermal hysteresis near the second-order phase transitions (PT) at T_s=205 K and T_c=196 K is due to ‘overheated’ ferroelectric state and is found to decrease with decrease a scanning rate. At room temperature, the X-ray diffraction topographic analysis shows an existence of a spatial cellular-like dislocation superstructure, with the cells being of a few microns. Below T_c, the relatively slow time-dependent processes are due to clamping the microscopic and tiny-scale domains by the microscopic dislocation superlattice and impurities. The processes developing on a long-time scale are ascribed to quasi-nonergodicity of the system long before a glassy state appears (below T_g ≈20 K).