High-temperature degradation of the polar phase of bismuth tellurite

Powders and crystals formed in the Bi2O3-TeO2 system near the Bi2TeO5 compo sition were studied in the subsolidus region at 600-900 degrees C by therma l analysis, second optical harmonic generation, and conductometry technique s. The occurrence of the reversible polar phase-centrally symmetrical phase transition at 780-820 degrees C accompanied by a diffuse endothermic effec t was substantiated by X-ray data. Taking into account the special features of the crystal structure of Bi2TeO5, the conclusion was drawn that redistr ibution of oxygen over vacant sites of the oxygen-deficient fluorite-like c rystal lattice in this temperature region was responsible for intense diele ctric relaxation and high conductivity. The structural mechanism of the tra nsition was shown to be different from ferroelectric. An alternative physic ochemical mechanism was suggested. This mechanism was based on the assumpti ons of, first, high anionic conductivity in the centrally symmetrical Bi2Te O5 phase disordered with respect to oxygen and, second, oxygen atom displac ements by distances of the order of interatomic distances during the phase transition.