Structural and optical properties of chromium-doped hexagonal barium titanateceramics

Abstract

The influence of chromium on the crystallographic phase andthe microstructure of ceramics with the nominal compositionBaTiO3+0.04BaO+xCr2O3 (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 microstructureexhibits exaggerated, plate-like grains with a mean grain size≥100 µm (sintering temperature1400 °C). In the hexagonalphase the EPR-active CrTi3+ ions substitute both for Ti(1) (corner-sharing octahedron) and Ti(2) (face-sharingoctahedron) sites. In air-sintered ceramics chromium is incorporated with valence states3+ and 4+, whereas for reduced samples the valence state3+ predominates. Optical transmission both of air-sintered and reduced samples doped withnominally 5.0 mol% Cr was measured in the visible light region. The absorption spectraexhibit distinct absorption bands. Their assignment to chromium defects with differentvalence states is discussed. The Jahn–Teller distortion caused by the electron configurationd2 (CrTi4+) is proposed as the driving force for the high-temperature phase transition cubic hexagonal.