SUPERSTRUCTURE AND THERMAL HYSTERESIS EFFECTS AT T-CURIE IN CD2NB2O7 PYROCHLORE

Calorimetric and dielectric studies of nominally 'pure' Cd2Nb2O7 singl e 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 'overheat ed' ferroelectric state and is found to decrease with decreasing a sca nning rate. At room temperature, the X-ray diffraction topographic ana lysis shows an existence of a spatial cellular-like dislocation supers tructure, with the cells being of a few microns. Below T-c, the relati vely slow time-dependent processes are due to clamping the microscopic and tiny-scale domains by the microscopic dislocation superlattice an d impurities. The processes developing on a long-time scale are ascrib ed to quasi-nonergodicity of the system long before a glassy state app ears (below T-g approximate to 20 K).