Incorporation of chromium into hexagonal barium titanate:: an electron paramagnetic resonance study

The EPR spectra of BaTiO3 + 0.04 BaO + 0.01 Cr2O3 ceramics are studied in three frequency bands (9, 34 and 94 GHz). Three different, axially symmetric EPR spectra assigned to Cr3+ ions substituted at Ti sites corresponding to the different distorted octahedra in tetragonal and hexagonal crystal surroundings are observed at room temperature. The temperature dependence of the EPR,spectra allowed the unambiguous assignment of the chromium ions to the lattice site in tetragonal and hexagonal modifications. The concentration measurements revealed that only 30% of the chromium is incorporated in the grains as Cr3+ ions whereas the remnant is in the EPR-silent tetravalent charge state or in Cr-rich secondary phases segregated at grain boundaries, pores and triple points. The intensity ratio of the two Cr3+ spectra in hexagonal BaTiO3 depends on the preparation conditions.