Softening of lattice modes in the region of structural phase transitions in a composite Bi$_2$(Sn$_{0.7}$Fe$_{0.3}$)$_2$O$_7$/Bi$_2$Fe$_4$O$_9$

The composite compound Bi$_2$(Sn$_{0.7}$Fe$_{0.3}$)$_2$O$_7$/Bi$_2$Fe$_4$O$_9$ with a ratio of 91%/9% was obtained by solid-phase synthesis. The mutual influence of two different crystal structures on structural transitions was studied using X-ray diffraction, IR spectroscopy and sound attenuation coefficient. IR absorption spectra were studied in the temperature range 80–500 K and the frequency range 350–7000 cm$^{-1}$. The types of vibrations of phonon modes are determined. The influence of the Bi$_2$(Sn$_{0.7}$Fe$_{0.3}$)$_2$O$_7$ matrix on the phonon vibration modes of mullite Bi$_2$Fe$_4$O$_9$ was established in the form of a frequency shift $w$ = 634 cm$^{-1}$, splitting of the mode $w$ = 574 cm$^{-1}$, and disappearance of the mode at the frequency $w$ = 812 cm$^{-1}$. In the regions of phase transitions Bi$_2$(Sn$_{0.7}$Fe$_{0.3}$)$_2$O$_7$ and Bi$_2$Fe$_4$O$_9$, a softening of the IR spectra modes was found. The features of the temperature dependence of the sound attenuation.