Structural and optical properties of chromium-doped hexagonal barium titanate ceramics

The influence of chromium on the crystallographic phase and the microstructure of ceramics with the nominal composition BaTiO3 + 0.04BaO + xCr(2)O(3) (0 <= x <= 0.025) was investigated by systematic studies of x-ray diffraction (XRD) and electron paramagnetic resonance (EPR). At Cr concentrations <= 0.1 mol% a hexagonal phase appears (room temperature). For nominal concentrations >= 1.0 mol% the material is 100% hexagonal and its microstructure exhibits exaggerated, plate-like grains with a mean grain size >= 100 mu m (sintering temperature 1400 degrees C). In the hexagonal phase the EPR-active Cr-Ti(3+) ions substitute both for Ti(1) (corner-sharing octahedron) and Ti(2) (face-sharing octahedron) sites. In air-sintered ceramics chromium is incorporated with valence states 3+ and 4+, whereas for reduced samples the valence state 3+ predominates. Optical transmission both of air-sintered and reduced samples doped with nominally 5.0 mol% Cr was measured in the visible light region. The absorption spectra exhibit distinct absorption bands. Their assignment to chromium defects with different valence states is discussed. The Jahn-Teller distortion caused by the electron configuration d(2) (Cr-Ti(4+)) is proposed as the driving force for the high-temperature phase transition cubic -> hexagonal.