Abstract
EPR spectra of , S=(3/2), substituting for are reported as a function of temperature T in all four phases of . In the three ferroelectric phases (FEP’s), 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 FEP’s. The existence of the first term shows that the remains centered in the octahedral cell. The existence of the latter, not observed for , points to large thermal fluctuations of the . These are ascribed to the absence of antibonding, repelling electrons directed towards the oxygen atoms which are present for . Saturation of the (T) term for low T is accounted for by a Debye model for with an energy of only 236.6 K, proving independently a flat ionic potential for . The picture of considerable amplitude fluctuations agrees with an effectively reduced - distance of 0.02 Å compared to the - distance obtained from the superposition-model analysis. The latter yields the correct sign and magnitude of the crystal-field terms in all FEP’s. It confirms that a maximum of the intrinsic superposition-model parameter ¯(R) for , derived earlier by Müller and Berlinger, occurs for R between 1.95 and 1.96 Å.
- Received 6 June 1985
DOI:https://doi.org/10.1103/PhysRevB.32.5837
©1985 American Physical Society