Effect of Particle Size and Shape on the Infrared Absorption Spectra of Barium Titanate and Strontium Titanate Powders

Abstract

Effects of particle size and particle shape have been studied in the infrared absorption spectra of BaTiO3 and SrTiO3 powders to determine whether previously developed methods to treat such effects are applicable to crystals having high dielectric constant and ``soft'' lattice modes. The spectrum of both materials exhibits three absorption bands (BaTiO3: 545, 400, and 180 cm−1; SrTiO3: 605, 400, and 175 cm−1) that are considerably removed from the TO-mode frequencies computed from bulk single-crystal reflection data (BaTiO3: 510, 183, and 33.8 cm−1; SrTiO3: 546, 178, and 87.5 cm−1). These frequencies were successfully correlated with the bulk-crystal frequencies by use of classical electromagnetic scattering theory and an extension of Fröhlich's method. The low-frequency ``soft'' modes of both crystals were found to have an unusually large dependence on particle shape with the consequence that the powders absorb radiation at the corresponding LO-mode frequency.