Investigations on the Spin Hamiltonian Parameters and the Local Structures for Various Rh2+ Centers in NaCl

The spin Hamiltonian parameters (the g factors, the hyperfine structure constants, and the superhyperfine parameters) and the local structures for various Rh2+ centers OI, OII, and RTAX in NaCl are theoretically investigated from the perturbation formulas of these parameters for a 4d7 ion in tetragonally and orthorhombically elongated octahedra. The related molecular orbital coefficients and the ligand unpaired spin densities are determined quantitatively from the cluster approach in a uniform way. The centers OI, OII (orthorhombic) or RTAX (tetragonal) are attributed to the substitutional Rh2+ on Na+ site, associated with two, one or none next nearest neighbour cation vacancies VNa along [100] (or [010]) axis, respectively. The ligand octahedra in the orthorhombic centers OI and OII are found to suffer the relative elongations ΔZ ≈0.071 and 0.068 °A along the [001] axis due to the Jahn-Teller effect, and the intervening ligand(s) in the VNa and the Rh2+ may undergo the inward displacements ΔX ≈ 0.001 and 0.011 A° towards Rh2+, respectively. As for the tetragonal center RTAX, the uncompensated [RhCl6]4− cluster is found to experience the relative elongation ΔZ ≈ 0.067 °A along the [001] axis of the Jahn-Teller nature. The calculated spin Hamiltonian parameters based on the above local structures show good agreement with the observed values for all the centers.

Investigations of the Defect Structure for Cu2+ in SrLaAlO4

The defect structure for Cu2+ in SrLaAlO4 is theoretically investigated by the perturbation formulas of the spin Hamiltonian parameters (g factors g‖, g⊥ and the hyperfine structure constants A‖ and A⊥) for a 3d9 ion in tetragonally elongated octahedra. Based on these studies, the tetragonal center may be attributed to Cu2+ occupying the host Al3+ site, associated with one hole delocalized at the four oxygen ligands in planar coordination. Furthermore, the four Cu,O bonds (perpendicular to the four-fold axis) are found to suffer an outward stretch of about 0.06 Å due to (i) the local tenseness in this plane, arising from the size mismatching substitution of the smaller Al3+ by the larger Cu2+ and (ii) the weaker electrostatic attraction of the less charged Cu2+ acting upon the four oxygen ions containing the delocalized hole. The calculated spin Hamiltonian parameters agree well with the experimental data. The defect structure of this center is discussed.

Studies on the Local Structure of the Tetragonal Er3+ Center in CaO

The local structure of the tetragonal Er3+ center in CaO is theoretically studied by using the perturbation formulas of the g factors for a 4f11 ion in tetragonal symmetry. In these formulas, the contributions to the g factors from the second-order perturbation terms and the admixtures of various states are taken into account. Based on the investigations, this center is suggested to be the impurity Er3+ substituting the host Ca2+ site, associated with a Ca2+ vacancy VCa in the [100] (C4) axis due to charge compensation. By studying the g factors of the tetragonal center, impurity Er3+ is expected to undergo an off-center displacement ΔZ(≈ 0.2 Å ) towards the VCa along the C4 axis because of the electrostatic attraction. The calculated g factors based on the displacement ΔZ show reasonable agreement with the observed values.