Infrared Studies of Perovskite Titanates

The infrared reflection and transmission spectra of CaTi${\mathrm{O}}_3$, SrTi${\mathrm{O}}_3$, and PbTi${\mathrm{O}}_3$ have been measured from 4000 to 30 ${\mathrm{cm}}^{-1\mathrm{}}$ at room temperature. The reflection data have been analyzed by means of a Kramers-Kronig method, from which the frequencies of the normal optically active vibrations were derived in agreement with data obtained from transmission spectra. Three infrared active vibrations were observed for cubic SrTi${\mathrm{O}}_3$ as required by group theory. However, in the case of CaTi${\mathrm{O}}_3$ and PbTi${\mathrm{O}}_3$, which crystallize as slightly distorted perovskite structures to become orthorhombic and tetragonal, respectively, additional bands were found which had not been observed previously. These distortions from ideal cubic symmetry would split degenerate modes to possibly give additional frequencies. The results are compared with those of other workers. From the presented data two features emerge: (1) Two normal modes ${\nu }_1$ and ${\nu }_2$ are essentially constant for these titanates independent of the cation. (2) The vibration of lowest frequency, ${\nu }_3$, which is responsible for the ferroelectric behavior, appears to decrease with increasing cation mass. This suggests a mode assignment as proposed by Last.