Investigation of impurity centers in lithium metaniobate doped with iron by the method of gamma-resonance spectroscopy

The mehtod of gamma-resonance spectroscopy has been used to study iron-doped crystals of lithium metaniobate of a variable composition. From the NGR spectra the types of the impurity centres have been established and their relative concentrations have been determined. The distribution of the impurity ions of iron in cation positions in the structure of lithium metaniobate has been studied. It has been found that the ions Fe2+ substitute for the Li ions, whereas the Fe3+ ions form three types of centres, substituting for Li, Nb, and occupying an empty octahedron. The values for the isomeric shift and the quadrupole separation Δ Esub(i) for iron ions occupying different positions in the crystal structure have been calculated. The dependence of Δ Esub(i) on the crystal composition has been studied. It has been shown that, as the crystal composition approaches the stoichiometric one, the fraction of Fesub(Li)sup(2+) decreases, whereas the relative concentration of Fe3+ ions in the position of Nb and in the empty octahedron increases. The relative content of Fesub(Li)sup(3+) ions varies insignificantly