Grain boundary effects on dielectric, infrared and Raman response of SrTiO3 nanograin ceramics

Dielectric, infrared reflectivity and Raman measurements were carried out on dense undoped SrTiO3 ceramic with the average grain size of 150 nm processed by spark-plasma sintering. The results were compared with our earlier data on conventional ceramics (grain size 1500 nm), see Petzelt et al., Phys. Rev. B 2001, 64, 184111, and on single crystals. Permittivity is dramatically reduced at low temperatures, the soft mode correspondingly stiffened and in Raman spectra the forbidden infrared modes are more pronounced compared to conventional ceramics. All the effects can be accounted for by the existence of polar dead layers with smaller permittivity at grain boundaries. Two models are suggested to explain the data quantitatively. The first model assumes that the dead layer is caused by grain-boundary dipole moment, which penetrates into the grain bulk with the polarization correlation length. Its estimate, using the soft-phonon branch curvature from inelastic neutron data and Landau theory, yields 1.2 nm at 120 K and 5.6 nm at 5 K. Fitting the data with brick-wall model to this dead-layer thickness required also temperature dependence of its permittivity (ɛ ∼12 at 120 K and ∼56 at 5 K). Comparably good fit of both ceramics can be obtained with temperature-independent dead-layer parameters. The latter model is supported by strongly reduced local refractive index at room temperature in the grain-boundary region (von Benthem et al., Phys. Rev. Lett. 2004, 93, 227201).