TEMPERATURE-DEPENDENCE OF THE OPTICAL-ABSORPTION EDGE IN HEXAGONAL BATIO3

Optical absorption spectra in hexagonal BaTiO3 have been measured between room temperature and 620 K for light polarized parallel c(parallel to) and perpendicular c(perpendicular to) to the hexagonal c-axis. The absorption coefficient increases exponentially at least two orders of magnitude with increasing photon energy around 2.9 eV similar to 3.3 eV at 300 K; two absorption edges for c(parallel to) and c(perpendicular to) are very nearly parallel and separated by Delta E=E(c(parallel to))-E(c(perpendicular to))=-0.04 eV. Urbach-tail broadening is observed at all temperatures and the parameter values are estimated. Some comparisons on the absorption spectra are made between the hexagonal and perovskite BaTiO3.