Optical absorption and ESR studies of nickel ions in sodium zinc sulphate tetrahydrate

The optical absorption spectrum of Ni doped Na2Zn(SO4)2 · 4 H2O shows that nickel enters the lattice in its divalent state. ESR spectra of X-irradiated sample show two doublets corresponding to the two sites of the unit cell. The spectra are fitted to a spin-Hamiltonian of orthorhombic symmetry and the doublet structure is found to be due to a superhyperfine interaction with a proton. A correlation of the ESR results and observed changes in optical absorption spectra after irradiation indicates the formation of Ni3+. Possible mechanisms responsible for the observed proton hyperfine interaction are discussed.