Electron spin resonance of axially symmetric Cr³⁺centres in KMgF₃and KZnF₃

The ESR spectra of five Cr 3+ centres in KMgF 3 and KZnF 3 are reported. All are axially symmetric and can be described by the spin-Hamiltonian H=g/sub /// beta B z S z +g perpendicular to beta (B x S x +B y S y )+D(S z 2 - 1 / 3 S(S+1)) with S= 3 / 2 . In KMgF 3 one finds spectra with D=-0.1528 cm -1 (trigonal), D=-0.2167 cm -1 (tetragonal) and D=-2.4 cm -1 (tetragonal). In KZnF 3 , centres are found with D=-0.1613 cm -1 (trigonal) and D=-0.0522 cm -1 (tetragonal). The trigonal spectra are ascribed to Cr 3+ ions associated with K + vacancies. The tetragonal spectra are attributed to Cr 3+ ions with divalent cation vacancies with O 2 ions or with other impurities on the tetragonal axis.