Infrared and Raman studies of the dead grain-boundary layers in SrTiO3 fine-grain ceramics

Infrared reflectivity, terahertz and Raman spectroscopy studies were carried out with two types of SrTiO3 ceramic of mean grain size 1500 and 150 nm, and the results were compared with single-crystal data. Strong ferroelectric soft-mode stiffening was detected in the infrared data upon the decrease of the grain size, fully compatible with the reduction of the low-frequency permittivity in the whole temperature range 300-10 K. Weaker stiffening of this mode observed in the Raman spectra is explained by the higher phonon wavevector active in the Raman response and by a different geometry of its propagation. Raman data also confirmed a smaller tetragonality below the structural phase transition as compared to single crystals. This is explained by a special topology of tetragonal domains along the grain boundaries, in which the tetragonal axes are oriented perpendicularly to them. Broad low- frequency and low-temperature features in the Raman spectra are assigned to the smeared first-order ferroelectric soft-mode response from the polarized regions (similar to 3 nmthick) with reduced permittivity attached to the very narrow grain boundaries. Such an effect is caused by a local frozen polarization of mean value similar to 15 mu C cm(-2) tending perpendicular to the grain boundary. To account for the full permittivity size effect, the grain boundary itself is modelled by a very thin (similar to 1 nm) dead layer with a strongly reduced local permittivity (similar to 8). The microscopic origin of the narrow dead layer is apparently a strong oxygen deficit, as suggested from recent local experiments and first-principles calculations by other authors. Coupling between the E-components of the ferroelectric and structural soft-mode doublets observed in the Raman spectra is due to the propagation of the E-symmetry polaritons through the polar grain boundaries, whereas the noncoupled A-symmetry polaritons are localized inside the non-polar grain bulk. Since the structurally disordered grain boundaries are very narrow, taking up only a very small volume fraction, their effect on the relaxation of the phonon momentum conservation can be neglected.