Infrared-active phonons inRTiO3perovskites (R=La,Ce,Pr,Nd,Sm,Gd)

The room-temperature reflectance (100–1000 ${\mathrm{cm}}^{\mathrm{-}1}$) of the rare-earth titanites (R${\mathrm{TiO}}_3$ R=La,Ce,Pr,Nd,Sm,Gd) has been measured. The spectrum for ${\mathrm{LaTiO}}_3$, ${\mathrm{CeTiO}}_3$, and ${\mathrm{PrTiO}}_3$ consists of the three main perovskite vibrational bands. The central band, near 340 ${\mathrm{cm}}^{\mathrm{-}1}$, consists of several barely resolved modes which begin to separate in ${\mathrm{NdTiO}}_3$ and ${\mathrm{SmTiO}}_3$ and which split into two clusters of modes in ${\mathrm{GdTiO}}_3$. These features are correlated with an increasing tetragonal distortion of the ${\mathrm{TiO}}_6$ octahedron as the rare-earth ionic size decreases. Using the LO-TO splitting, the effective ionic charges are estimated and the R${\mathrm{TiO}}_3$ perovskites are shown to be less ionic than ${\mathrm{SrTiO}}_3$ or ${\mathrm{BaTiO}}_3$.