Optical and Microwave Studies of Divalent Vanadium in Octahedral Fluoride Coordination

Abstract

We have measured the optical absorption spectrum of V²⁺ introduced as a substitutional impurity into the octahedral site of MgF₂ and KMgF₃, and its spin resonance spectrum in MgF₂ and in uniaxially strained KMgF₃. The cubic field splittings are best interpreted in terms of the weak‐field formalism of Ferguson and Wood: The parameters of the electron–electron interaction obtained from this formalism show a much more rational trend with increasing covalency than do those obtained from a strong‐field analysis. The crystal fields induced by uniaxial strain in KMgF₃ are close to those expected from the point charge model. The noncubic splittings of the optical and microwave spectra of V²⁺ in MgF₂ are in reasonable agreement with the ligand field calculations of Ferguson et al., with parameters quite close to those obtained for Ni²⁺. However, it is found that the ground state splitting in MgF₂ is much larger than expected on the basis of the data on strained KMgF₃, indicating a large contribution from ions outside the nearest neighbor shell. Finally, there is some indication of a Jahn–Teller interaction (involving the $B_{\text{3g}}$ mode of the octahedron) in the ${}^4{T}_2$ excited state of V²⁺ in MgF₂, analogous to the well‐established case (previously reported) in KMgF₃.