Investigations on the g factors and superhyperfine parameters for various tetragonal Ni+ centers in RbCaF3

The anisotropic g factors g∥ and g⊥ and the planar F− superhyperfine parameters A∥ and A⊥ in the tetragonal impurity Ni+ I, II and III centers (as well as the axial F− superhyperfine parameters A∥′ and A⊥′ in I center) on the substitutional Ca2+ site in RbCaF3 are theoretically investigated in a uniform way using the perturbation formulas of these parameters for a 3d9 ion in tetragonally elongated octahedra. In the calculations, the ligand unpaired spin densities are determined quantitatively from the related molecular orbital coefficients based on the cluster approach. These defects are attributed to the substitutional Ni+ associated with none, one and two axial nearest neighbour F− vacancies for I, II and III centers, respectively. Ni+ I center is found to suffer the relative elongation (≈5%) along the [0 0 1] (or C4) axis due to the Jahn-Teller effect, while the tetragonal elongation distortions are mainly ascribed to the axial vacancies in II and III centers. The local structures of the various impurity centers are discussed.