An investigation on the defect structures and spin Hamiltonian parameters for the two orthorhombic Ti3+ centers in ZnWO4

Abstract By employing the perturbation formulae of the spin Hamiltonian parameters (SHPs) ( g factors g xx , g yy , g zz , hyperfine structure constants A xx , A yy , A zz and superhyperfine parameters A xx ׳, A yy ׳, A zz ׳) for a 3d 1 ion in orthorhombically elongated octahedra and tetrahedra, the defect structures and the experimental EPR spectra are theoretically and systematically investigated for the two orthorhombic Ti 3+ centers C 1 and C 2 in ZnWO 4 . Center C 1 is ascribed to the impurity Ti 3+ at host W 6+ site associated with two nearest neighbor oxygen vacancies due to charge compensation. The resultant tetrahedral [TiO 4 ] 5– cluster is determined to undergo the local orthorhombic elongation distortion, characterized by the axial distortion angle Δ θ (= θ – θ 0 ≈–6.84°) of the local impurity-ligand bond angle θ related to θ 0 (≈54.74°) and the perpendicular distortion angle Δ e (= e – e 0 ≈2.5°) related to e 0 (≈45°) of an ideal tetrahedron because of the Jahn–Teller effect. Center C 2 is attributed to Ti 3+ on Zn 2+ site, and this octahedral [TiO 6 ] 9– cluster may experience the local axial elongation Δ Z (≈0.001 Ǻ) and the planar bond angle variation Δ φ (≈9.1°) due to the Jahn–Teller effect, resulting in a more regular oxygen octahedron. All the calculated SHPs (i.e., g factors for both centers, the hyperfine structure constants for center C 2 and superhyperfine parameters of next nearest neighbor ligand W for center C 1 ) show good agreement with the observed values. However, the theoretical results based on the previous assignment of center C 1 as Ti 3+ on W 6+ site with only one nearest planar oxygen vacancy (i.e., five-fold coordinated octahedral [TiO 5 ] 7– cluster) show much worse agreement with the experimental data. The defect structures and the SHPs (especially the g anisotropies) are discussed for both centers. The present studies on the superhyperfine parameters of ligand W 6+ for center C 1 would be helpful to further investigations on the superhyperfine interactions of cation ligands which were rather scarcely treated before.