Optical phonon characteristics of incommensurate and commensurate modulated phases of Bi_3NbO_7 ceramics

摘要

Raman and infrared-reflectivity spectroscopies were used to investigate the optical phonon modes of the cubic-incommensurate and tetragonal-commensurate phases of Bi_3NbO_7 ceramic samples. For both structures, the observed features could be well described by applying the factor-group method to average oxygen-deficient fluorite subcells; i.e., it was possible to predict the correct number and the activities of the observed phonon modes. The spectra showed rather broad bands, which are mainly for the cubic-incommensurate phase, probably because of the disorder of the oxygen vacancies and the folding of the Brillouin zone due to the modulation of the atomic positions. This can partially explain the higher quality factor and the potential interest in the tetragonal phase of this material for applications in microwave devices.