Evidence of nickel ions dimerization in NiWO4 and NiWO4-ZnWO4 solid solutions probed by EXAFS spectroscopy and reverse Monte Carlo simulations

The existence of exchange-coupled Ni²⁺ ions – the so-called magnetic dimers – in wolframite-type NiWO₄ and Zn_cNi_1−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, Zn_cNi_1−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 Zn_cNi_1−cWO₄ for c≤0.6, which is related to the probability to find two Ni²⁺ ions in neighbouring positions.