INFRARED-ACTIVE PHONONS IN RTIO3 PEROVSKITES (R=LA, CE, PR, ND, SM, GD)

The room-temperature reflectance (100-1000 cm-1) of the rare-earth titanites (RTiO3 R=La,Ce,Pr,Nd,Sm,Gd) has been measured. The spectrum for LaTiO3, CeTiO3, and PrTiO3 consists of the three main perovskite vibrational bands. The central band, near 340 cm-1, consists of several barely resolved modes which begin to separate in NdTiO3 and SmTiO3 and which split into two clusters of modes in GdTiO3. These features are correlated with an increasing tetragonal distortion of the TiO6 octahedron as the rare-earth ionic size decreases. Using the LO-TO splitting, the effective ionic charges are estimated and the RTiO3 perovskites are shown to be less ionic than SrTiO3 or BaTiO3.