CRYSTAL FIELD THEORY AND OPTICAL ABSORPTION OF COBALT AND NICKEL BORACITES

In the boracite structure the local symmetry of paramagnetic ions is characterized by a strong tetragonal distortion of the cubic component of the crystal field caused by spatial and charge non‐equivalency of nearest neighbours of 3d‐ions and also by the lack of an inversion centre. The crystal field operator for ferroelectric and paraelectric phases has been calculated using the point charge model. The expressions connecting crystal field parameters and ion displacements have been derived. The optical absorption of boracites Me3B7O13X where Me = Co, Ni and X = Cl, Br, I has been studied in the spectral range 0.25 to 25 μm from 77 to 700 °K. The absorption spectra are much more complicated than the known spectra of Co2+ and Ni2+ in octahedral cubic crystal field. It was found that the intensity of many transitions in boracites drops when the temperature is raised. The most rapid decrease was observed in the vicinity of the ferroelectric Curie temperature Tc. Such unusual behaviour was assigned to the change of the crystal field odd component connected with unequivalent displacements of halogen ions relatively to the 3d‐paramagnetic ion. The temperature dependence of the tetragonal crystal field parameters Ds and Dt was studied in cobalt boracites. An increase of Ds and decrease of the Dt was observed at Tc in accordance with the theoretical predictions. Copyright © 1969 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim