Asymmetry of quadrupole doublet of Mössbauer spectra in LiNbO3∶Fe

Polycrystalline samples of LiNbO₃ doped by the enriched⁵⁷Fe isotope have been studied at room and liquid nitrogen temperatures by nuclear γ-resonance (NGR). Iron impurity concentrations were 0.07, 0.30, 1.02, 2.06 at.%, the Li/Nb ratio being 0.935, 0.990, 1.000. An asymmetry of the quadrupole doublet of NGR experimental spectra has been found. Acceptable hypotheses are discussed to explain the nature of the asymmetry: the Goldanskii-Karyagin effect, the effect of sample texture, and the electron and nuclear spin-lattice relaxation effect. On the basis of computed NGR spectra and thorough analysis of the models we can conclude that there is tendency to form coupled pairs of Fe²⁺ cations in neighbour lithium and niobium sites. The probability of the formation of such pairs exceeds considerably the statistical value.