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 BaTiO 3 +0.04BaO+ x Cr 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 µm (sintering temperature 1400 °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.