Evidence of nickel ions dimerization in NiWO[formula omitted] and NiWO[formula omitted]-ZnWO[formula omitted] solid solutions probed by EXAFS spectroscopy and reverse Monte Carlo simulations

Abstract

The existence of exchange-coupled Ni2+ ions – the so-called magnetic dimers – in wolframite-type NiWO4 and ZncNi1−cWO4 solid solutions with high nickel content was discovered by X-ray absorption spectroscopy combined with reverse Monte Carlo (RMC) simulations. Temperature- (10–300 K) and composition-dependent X-ray absorption spectra were measured at the Ni K-edge, Zn K-edge, and W L3-edge of microcrystalline NiWO4, ZncNi1−cWO4 and ZnWO4. Structural models were obtained from simultaneous analysis of the extended X-ray absorption fine structure (EXAFS) spectra at three metal absorption edges using RMC simulations. The obtained radial distribution functions for different atomic pairs made it possible to trace in detail the changes in the local environment of metal ions and the effect of thermal disorder. Dimerization of Ni2+ ions within quasi-one-dimensional zigzag chains of [NiO6] octahedra was evidenced in NiWO4 in the whole studied temperature range. It manifests itself as the splitting of the Ni–Ni radial distribution function into two separate peaks. The effect is further preserved in solid solutions ZncNi1−cWO4 for c≤0.6, which is related to the probability to find two Ni2+ ions in neighbouring positions.