CRYSTAL STRUCTURES OF SrBi_2Ta_2O_9 AND Sr_(0.8)Bi_(2.2)Ta_2O_9 FERROELECTRIC MATERIALS

摘要

Structural and ferroelectric properties of stoichiometric SrBi_2Ta_2O_9 and Sr-deficient-and-Bi-excess Sr_(0.8)Bi_(2.2)Ta_2O_9 bulk ceramics materials are studied. The ferroelectric Curie temperatures for SrBi_2Ta_2O_9 and Sr_(0.8)Bi_(2.2)Ta_2O_9 are 300 and 400°C, respectively. Structure analysis by neutron powder diffraction reveals that Bi_2O_2 layer and TaO_6 octahedra are considerably distorted and that atomic displacement along the a-axis causes ferroelectric spontaneous polarization. In Sr_(0.8)Bi_(2.2)Ta_2O_9, both Bi-substitution and cation-vacancies exist at the Sr-site. The calculated polarization of Sr_(0.8)Bi_(2.2)Ta_2O_9 is higher than that of the stoichiometric sample, which is consistent with observations of remanent polarization in thin-film capacitors. The Bi-substitution and the cation-vacancies at the Sr site enhance structural distortion in the TaO_6 octahedron and lead to the higher Curie temperature and the higher ferroelectric spontaneous polarization.