Competition and coexistence of polar and non-polar states in Sr1−x Ca x TiO3: an investigation using pressure dependent Raman spectroscopy

The competition and cooperation between ferroelectric and anti-ferro-distortion (AFD) instabilities are studied using pressure dependent Raman spectroscopy on polycrystalline powder samples of Sr1−x Ca x TiO3(x = 0.0, 0.06, 0.25, 0.35). For x = 0.0 composition, a broad polar mode is detected in the Raman spectra above 6 GPa, while for x = 0.06 composition, the polar modes appear well above 9 GPa where the AFD modes showed strong suppression. In x = 0.25 and 0.35 composition, the application of small pressure resulted in the appearance of strong AFD modes suppressing the polar modes. At elevated pressures, re-entrant polar modes are observed along with the broad AFD modes and some new peaks are also observed, signifying the lowering of local symmetry. The reappearance of polar modes is found to be related to pressure induced symmetry disorder at local level, suggesting its electronic origin. The re-entrant polar modes observed at higher pressure values are found to be significantly broad and asymmetric in nature, signifying the development of ferroelectric micro regions/nano domains coexisting with AFD. The lower symmetry at local length scale provides a conducive atmosphere for coexisting AFD and FE instabilities.