The room-temperature Raman spectra of Sr 1−x Ca x TiO 3 (SCT) for x = 0.06 (SCT06), 0.12 (SCT12) and 0.30 (SCT30) are shown to contain first-order Raman lines corresponding to E g and B 1g modes, which appear below 105 K in pure SrTiO 3 . The E g line shows slight asymmetry, indicating the orthorhombic structure of SCT for x 0.06. The low-temperature Raman spectra of SCT06 and SCT12 contain Raman-active modes of symmetry B 2g and E g , predicted by group theory but not observed in SrTiO 3 or SCT0.7 (x = 0.007) by previous workers. The polar hard modes TO 2 (175 cm −1 ) and TO 4 (550 cm −1 ) are present even at room temperature for SCT06 and SCT12 due to Ca 2+ -centred ferroelectric microregions. The intensity of these modes starts increasing below 55 and 80 K for SCT06 and SCT12 respectively, which are nearly 2 T c , where T c is the temperature of the smeared dielectric peak. The presence of such an extended precursor region is akin to relaxor behaviour. For SCT30, two new lines at 79 and 128 cm −1 appear below the paraelectric to antiferroelectric phase transition temperature of 230 K. It is shown that Raman lines characteristic of both ferroelectric and antiferroelectric phases are present in SCT12 below 100 K, confirming the frustration model for the highly smeared dielectric response for this composition.

This paper deals with the structural Raman-active and the disorder-induced TO and LO Raman modes in Sr 1−x Ca x TiO 3 (SCT) ceramics. Spectra were recorded at ambient pressure in a wide range of temperatures from 8 K up to ≈500 K. The chemical composition (x) was chosen so as to investigate the phase boundaries which have been previously determined in the x-T phase diagram. An eigenvector analysis is given for all the known phases. The Raman data confirm the occurrence of a double-tilt system of Imma space group lying between the tetragonal I4/mcm and the orthorhombic Pbnm structures for x ½ 0.06. Higher-order transformation of the antiferroelectric phase (0.20 Ä x Ä 0.40) at low temperature is also suggested. The main interesting disorder-induced effects originate from the temperature behaviour of the hard polar modes TO 2 and TO 4 , which appear in the Raman spectra of the paraelectric domain. This behaviour allows one to discuss the precursor range of ferroelectricity in the SCT system in terms of ferroelectric-or antiferroelectric-type dipolar interactions.

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The phase transition behaviour of two 'nearly cubic' compositions, x = 0.40 and 0.43, of Sr(1-x)Ca(x)TiO(3), representative of Pbnm and Pbcm space group structures, has been investigated using temperature dependent dielectric, x-ray diffraction and Raman scattering techniques. For x = 0.40, a first order antiferroelectric phase transition occurs around 364 K followed by an antiferrodistortive phase transition involving the R point ([Formula: see text]) instability at ∼800 K. For x = 0.43, on the other hand, there is no antiferroelectric phase transition. Instead, two antiferrodistortive transitions around 463 and 850 K linked with the M ([Formula: see text]) and R ([Formula: see text]) point instabilities, respectively, are observed. Both the compositions also exhibit qualitatively different types of phase transitions below room temperature. Evidence for a new phase transition at low temperatures is presented for x = 0.43.