Infrared activity in the Aurivillius layered ferroelectricSrBi2Ta2O9

Abstract

Experimental studies were carried out on infrared-active phonons in the Aurivillius ferroelectric ${\mathrm{SrBi}}_2{\mathrm{Ta}}_2{\mathrm{O}}_9$ (SBT), using reflectivity measurements (down to $200{\mathrm{cm}}^{\mathrm{-}1}$) and transmission measurements (down to $100{\mathrm{cm}}^{\mathrm{-}1}$) on crystals, pellets, and thin films. An analysis was done of the contrasting consequences of the competing orthorhombic and pseudotetragonal symmetries in SBT. Reflectivity results for light polarized in the $\mathrm{ab}$ plane show that the anisotropy in this plane is small in the frequency range from 300 to $1200{\mathrm{cm}}^{\mathrm{-}1},$ indicating the influence of the approximate tetragonal symmetry. Dielectric dispersion properties were derived for this polarization $\left(E\perp c\right)$ in this frequency range. The transverse-optical (TO) and longitudinal-optical (LO) frequencies corresponding to the dominant $E\perp c$ band are 613 and $773{\mathrm{cm}}^{\mathrm{-}1},$ respectively. This strong, high-frequency band arises from a mode dominated by the motion of the oxygen sublattice; its TO and LO frequencies yield an oxygen-ion Szigeti effective charge of $-1.5e.$ Frequency estimates for the TO (LO) pairs of other strong bands are 188(330) and $334\left(451\right)\mathrm{}{\mathrm{cm}}^{\mathrm{-}1}$ for $E\perp c,$ and 610(675) and $780\left(815\right)\mathrm{}{\mathrm{cm}}^{\mathrm{-}1}$ for $E\parallel c.\mathrm{}$ In addition to the main infrared bands, the main Raman bands of SBT are also reported.