Evolution of phase transitions in SrTiO$_3$–BiFeO$_3$ solid solutions

The dielectric and acoustic properties of $(1-x)$SrTiO$_3$–$x$BiFeO$_3$ solid solutions (0 $\le x\le$ 1) have been studied in the temperature range from 4.2 to 850 K. Evolution of the antiferrodistortive transition and its suppression in the concentration range 0.1 $< x <$ 0.2 have been revealed. X-ray diffraction data obtained at room temperature, as well as the observed acoustic anomalies combined with dielectric measurements, have been used to estimate the concentration range of the existence of the relaxor state, as well as of the formation and coexistence of the antiferromagnetic and ferroelectric phases at high BiFeO$_3$ concentrations.