Raman spectra of the ferroelectric phase of SrTi18O3:: Symmetry and domains below Tc and the origin of the phase transition

Polarized Raman spectra of isotope substituted (SrTiO3)-O-18 (STO18) crystals were studied in the ferroelectric phase below T-c=24.5 K. Samples were confirmed to be a single domain in the tetragonal phase (T-c < T < T-0=108 K). Raman spectra in various phonon propagation directions K-p were found to satisfy the selection rules for orthorhombic C-2v symmetry with the spontaneous polarization P parallel to the tetragonal axes, either [110](c) or [110](c). Below T-c, STO18 is inhomogeneous and consists of a number of small ferroelectric domains as well as remnants of the tetragonal structure. Soft TO modes originated from the lowest Gamma(15) mode in cubic phase were observed at 20 and 12.5 cm(-1). On heating, these modes soften but do not freeze at T-c. Near T-c, a quasielastic scattering component appears and its intensity critically increases. Although for temperatures far from T-c, the transition is promoted by the softening of the TO modes, the appearance of P at T-c cannot be attributed to the freezing of Slater mode since P is not parallel to the cubic axis [100](c). Origin of the relaxational mode near T-c and the direction of P are discussed in terms of the orientational ordering of locally polarized clusters which already exist above T-c. These results indicate that the phase transition in STO18 is a peculiar transition and has both order-disorder and displacive nature.