Superhyperfine interactions and their origins for n d1 ions in rutile-structure oxides

Abstract

Paramagnetic resonance spectra of 4d1 (Nb4+ and Mo5+) and 5d1 (W5+) ions at substitutional cation sites in single crystals of tetragonal GeO2 have been studied at 92 K. Spin-Hamiltonian parameters obtained from hyperfine and superhyperfine (shf) structures are reported and compared with previously reported results for these ions and for V4+ (3d1), in rutile-structure GeO2, TiO2, and SnO2. Analysis of superhyperfine interactions with these nd1 impurity cations in GeO2 indicates that the unpaired spin is highly delocalized into nearest-neighbor ligand cations. On the basis of an LCAO-MO treatment, delocalization probabilities are inferred from measured shf interactions for these impurities; for each, the isotropic shf interaction is attributed to admixture of the unoccupied ligand-cation 4s orbital into the outer d orbital of the central impurity ion. The anisotropic contribution to the shf interaction is attributed to admixture of 4p ligand-cation orbitals. The results suggest an increase in the probability of delocalization of the unpaired spin onto the nearest-neighbor cations as n increases, however, for the Nb impurity, the degree of delocalization is found to be anomalously high. Extension of this type of analysis to treatment of these impurities in the isomorphs TiO2 and SnO2 is discussed.