Doping and temperature-dependent optical properties of oxygen-reducedBaTiO3−δ

Abstract

We report on optical properties of reduced ${\text{BaTiO}}_{3\ensuremath{-}\ensuremath{\delta}}$ at different doping levels including insulating and metallic samples. In all the samples, including metallic one, we observe structural phase transitions from the changes in the infrared-active phonon modes. Metallic ground state is confirmed by the Drude-type low-frequency optical reflectance. Similar to ${\text{SrTiO}}_{3\ensuremath{-}\ensuremath{\delta}}$ we find that the midinfrared-absorption band in ${\text{BaTiO}}_{3\ensuremath{-}\ensuremath{\delta}}$ appears and grows with an increase in the oxygen-vacancy concentration. Upon decrease in temperature from 300 K, the midinfrared band shifts slightly to higher frequency and evolves into two bands: the existing band and a new and smaller band at lower frequency. The appearance of the new and smaller band seems to be correlated with the structural phase transitions.