Raman study of phonon modes in bismuth pyrochlores

The Raman spectra of the cubic bismuth pyrochlores ${\text{Bi}}_{3/2}{\text{Zn}}_{0.92}{\text{Nb}}_{3/2}{\text{O}}_{6.92}$, ${\text{Bi}}_{3/2}{\text{ZnTa}}_{3/2}{\text{O}}_7$, ${\text{Bi}}_{3/2}{\text{MgNb}}_{3/2}{\text{O}}_7$, and ${\text{Bi}}_{3/2}{\text{MgTa}}_{3/2}{\text{O}}_7$ have been measured at room temperature. The frequencies of the Raman modes, obtained from first-principles calculations, for ${\text{Bi}}_2{\text{Ti}}_2{\text{O}}_7$ 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.

Figure 1

(Color online) Raman spectra of BMN, BZN, BMT, and BZT. Their intensities have been scaled (by 1, 1.14, 1.64, and 2.46, respectively) and shifted for ease of comparison.

Figure 2

(Color online) Raman spectra corrected by a Bose factor for our four samples (left to right: BMN, BMT, BZN, and BZT). The obtained imaginary part of the Raman susceptibility was fitted to a sum of Lorentzian functions: $\sum A_j/\left[1+4{\left(\omega -{\omega }_{oj}\right)}^2/{\gamma }_j^2\right]$. The figure shows the data (black), the fitting (red), and the individual functions (blue) from the fit.

Figure 3

(Color online) Gaussian fit to the imaginary part of the Raman susceptibility (left to right: BMN, BMT, BZN, and BZT). The figure shows the data (black), the fitting (red), and the individual functions (green) from the fit.