Raman study of phonon modes in bismuth pyrochlores

Abstract

The Raman spectra of the cubic bismuth pyrochlores Bi3/2Zn0.92Nb3/2O6.92, Bi3/2ZnTa3/2O7, Bi3/2MgNb3/2O7, and Bi3/2MgTa3/2O7 have been measured at room temperature. The frequencies of the Raman modes, obtained from first-principles calculations, for Bi2Ti2O7 are presented for comparison. The spectra of the four samples are similar and agree well with the first-principles calculations. Each bismuth pyrochlore shows more than the six modes expected for the ideal pyrochlore structure. The analysis shows that many of the additional modes could be explained as the relaxation of the selection rules due to the displacive disorder. The Raman modes are assigned by reference to spectra of other pyrochlore materials, comparison to infrared data, and the ab initio calculations.