Polar grain boundaries in undoped SrTiO3 ceramics

Raman and infra-red spectroscopic investigations of nominally pure SrTiO3 c eramics have revealed a clear presence of polar phase whose manifestation s teeply increases on cooling from room temperature. The Raman strengths of t he Raman-forbidden infra-red modes are proportional to omega (-alpha)(TOI) (alpha approximate to 1.5) where omega (TOI) is the polar soft mode frequen cy. Our explanation is based on an assumption of a temperature independent polarization Pf fixed at grain boundaries. The effective polar soft mode is slightly shifted upwards at low temperatures, becomes broader and strongly coupled with the E-g component of the structural soft mode which shows tha t P-f is perpendicular to the tetragonal c-axis. (C) 2001 Elsevier Science Ltd. All rights reserved.