Infrared-active phonons in RTiO3 perovskites (R=La,Ce,Pr,Nd,Sm,Gd).

Abstract

The room-temperature reflectance (100--1000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) 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 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, 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 ${\mathrm{BaTiO}}_{3}$.