ARaman scattering study of the phase transitions in SrTiO3and in the mixed system (Sr1-xCax)TiO3 atambient pressure from T = 300 K down to 8 K

Abstract

The room-temperature Raman spectra of Sr1-xCaxTiO3 (SCT) for x = 0.06 (SCT06), 0.12 (SCT12)and 0.30 (SCT30) are shown to contain first-order Raman lines corresponding to Eg and B1g modes, which appear below105 K in pure SrTiO3. The Eg line shows slightasymmetry, indicating the orthorhombic structure of SCT for x⩾0.06. The low-temperature Raman spectra of SCT06 andSCT12 contain Raman-active modes of symmetry B2g andEg, predicted by group theory but not observed in SrTiO3 orSCT0.7 (x = 0.007) by previous workers. The polar hard modesTO2 (175 cm-1) and TO4 (550 cm-1) arepresent even at room temperature for SCT06 and SCT12 due toCa2+-centred ferroelectric microregions. The intensityof these modes starts increasing below 55 and 80 K for SCT06and SCT12 respectively, which are nearly 2 Tc, where Tc is thetemperature of the smeared dielectric peak. The presence ofsuch an extended precursor region is akin to relaxor behaviour.For SCT30, two new lines at 79 and 128 cm-1 appear belowthe paraelectric to antiferroelectric phase transition temperature of 230 K. It is shown that Raman lines characteristicof both ferroelectric and antiferroelectric phases are presentin SCT12 below 100 K, confirming the frustration model for thehighly smeared dielectric response for this composition.