RAMAN-SCATTERING OF FERROELECTRIC SR1-XCAXTIO3, X=0.007

First-order Raman Scattering of A(1g) and E(g) modes originating from the antiferrodistortive F-2u soft mode confirms the cubic-tetragonal (O-h-D-4h) phase transition in Sr0.993Ca0.007TiO3 at T-0 = 125 K. Within the cubic and the tetragonal phase first-order Raman scattering of the soft ferroelectric F-1u mode (TO1) and of the hard TO2 and TO4 optic modes evidences the existence of Ca2+-centred polar microregions. Their size increases rapidly on cooling towards the ferroelectric ordering temperature, T-c = 18 K. Splitting and incomplete softening of the F-1u components is consistent with an order-disorder transition into an orthorhombic low-temperature phase with C-2v symmetry.