Investigation of the spin Hamiltonian parameters and the local structure of two Ni3+ centers in KTaO3

The spin Hamiltonian anisotropic g factors g(parallel to) and g(perpendicular to) and the local structures of the Ni3+ centers I and II in K TaO3 are theoretically investigated by using the perturbation formulas of the spin Hamiltonian parameters for 3d(7) ions in tetragonally distorted octahedrons and dodecahedrons. By analyzing the electron paramagnetic resonance data of the studied systems, the centers I and II can be attributed to Ni3+ ions occupying octahedral Ta5+ (associated with a nearest-neighbour oxygen vacancy V-O along the C-4 axis) and the dodecahedral K+ (associated with a nearest-neighbour interstitial oxygen O-I along the C-4 axis) sites, respectively. Based on these studies, it is found that at the center I the impurity Ni3+ is displaced away from V-O by DeltaZ(I) approximate to -0.31(2) Angstrom along the C-4 axis. At the center II a large off-center displacement, DeltaZ(II) approximate to 1. 12(2) Angstrom, towards the O-I along the C-4 axis is obtained, due to Ni3+ -O-I covalent bonding.