Infrared-Absorption Studies on Barium Titanate and Related Materials

The infrared-absorption spectrum of BaTi${\mathrm{O}}_3$ has been measured for thin single crystals and for powder samples dispersed in pressed KBr disks. Absorption bands for single-crystal samples occur at 495 ${\mathrm{cm}}^{-1\mathrm{}}$ and at ca 340 ${\mathrm{cm}}^{-1\mathrm{}}$, arising from normal vibrations of the Ti${\mathrm{O}}_3$ group. A third vibration, a motion of Ba against the Ti${\mathrm{O}}_3$ group, occurs below the experimentally accessible range. A frequency of about 225 ${\mathrm{cm}}^{-1\mathrm{}}$ is expected for this vibration on the basis of a comparison of the specific heat contributions of the observed bands with the measured low-temperature specific heat. Measurements were made on the 495-${\mathrm{cm}}^{-1\mathrm{}}$ band over a wide temperature range. As the crystal changes from the cubic to the tetragonal, orthorhombic, and rhombohedral structures, there occurs band splitting which can be related to the change of crystal symmetry. The spectra of the perovskite titanates, SrTi${\mathrm{O}}_3$, PbTi${\mathrm{O}}_3$, and CaTi${\mathrm{O}}_3$, and the perovskite niobates, KNb${\mathrm{O}}_3$ and NaNb${\mathrm{O}}_3$, have been found to be similar, in general features, to that of BaTi${\mathrm{O}}_3$. The slight differences in band frequency and structure can be related to differences in unit-cell size and symmetry. Integrated band intensities have been found to be in reasonable agreement with measurements on other oxide systems that have vibrations in this spectral region.