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The calculation formulas of g-factors g(parallel to) and g(perpendicular to) for 3d(7) ion in tetragonal octahedral crystals are established from a cluster approach. In these formulas, the parameters related to covalency effect, configuration interaction and low-symmetry crystal field can be determined from the optical spectra and the structural data of the studied system. Based on these formulas, the structural parameters of ligand octahedra of Co2+ in Rb2MgF4 crystal are obtained by fitting the calculated g(parallel to) and g(perpendicular to) to the observed values. The result suggests that the CoF6 (and hence MgF6) octahedra in Rb2MgF4:Co2+ are tetragonal compressed. The relationship between the sign of Delta g( = g(perpendicular to) - g(parallel to)) and the sign of distortion (elongated or compressed) of ligand octahedron and the causes of the mistakes of octahedron distortion for Rb2MgF4:Co2+ in the previous papers are discussed. (C) 2000 Elsevier Science B.V. All rights reserved.