An optical, EPR and electrical conductivity study of blue barium titanate, BaTiO3−δ

The electronic UV–VIS–NIR absorption spectra of single crystalline BaTiO_3−δ (BTO) are studied in the temperature range of 102–1173 K in pure oxygen and at conditions of moderate and strong reduction of the material. The strongly reduced crystals are of deep blue colour. The optical spectra of blue BTO are characterised by a strong absorption in the NIR region at around 7000 cm⁻¹, which is attributed to polaronic defects associated with the formation of Ti³⁺ in the material. This assumption is supported by fits of the spectra using polaronic line shape functions appropriate for disordered systems and also by the electrical conductivity of blue BTO which, in agreement with results from the optical spectra, exhibits an activation energy of 0.20 eV. The EPR spectra of moderately reduced BTO powders show an anisotropic g-factor, which is compatible with the optical spectrum. The temperature dependence of the band gap energy of BTO was found to be given as dE_g/dT = −7.21 × 10⁻⁴ eV/K.