Dynamic spectral response of solid solutions of the bismuth-strontium ferrite Bi$_{1-x}$Sr$_x$FeO$_{3-\delta}$ in the frequency range 0.3–200 THz

Ceramic samples of Bi$_{1-x}$Sr$_x$FeO$_{3-\delta}$ have been investigated using X-ray diffraction analysis, Raman spectroscopy, and infrared spectroscopy. It has been shown that the spectra of these solid solutions should be considered as a superposition of resonant phonon responses of the cubic perovskite and a relaxation response. No relaxators are observed in the pure compounds BiFeO$_3$ and SrFeO$_{3-\delta}$. It has been found that two concentration phase transitions occurs with the symmetry changes in the range 0.1 $<x<$ 0.2 and in the range 0.8 $<x<$ 1.0.