Paramagnetic resonance and local position of Cr3+ in ferroelectric BaTiO3

EPR spectra of Cr3+, S=(3/2), substituting for Ti4+ are reported as a function of temperature T in all four phases of BaTiO3. In the three ferroelectric phases (FEPs), the principal axis of the Hamiltonian is always along the polar axis. There are two crystal-field terms, one proportional to the square of the polarization and a large one linear in T. The latter is the same in all FEPs. The existence of the first term shows that the Cr3+ remains centered in the octahedral cell. The existence of the latter, not observed for Fe3+, points to large thermal fluctuations of the Cr3+. These are ascribed to the absence of antibonding, repelling eg electrons directed towards the oxygen atoms which are present for Fe3+. Saturation of the b20(T) term for low T is accounted for by a Debye model for Cr3+ with an energy of only 236.6 K, proving independently a flat ionic potential for Cr3+. The picture of considerable Cr3+ amplitude fluctuations agrees with an effectively reduced Cr3+-O2- distance of 0.02 A compared to the Fe3+-O2- distance obtained from the superposition-model analysis. The latter yields the correct sign and magnitude of the crystal-field b20 terms in all FEPs. It confirms that a maximum of the intrinsic superposition-model parameter b2 (R) for Cr3+, derived earlier by Müller and Berlinger, occurs for R between 1.95 and 1.96 A. © 1985 The American Physical Society.