Peculiarities of the Local Structure in New Medium- and High-Entropy Low-Symmetry Tungstate Oxides

Abstract

New monoclinic ( P 2 /c ) tungstate oxides – a medium-entropy (MnNiCuZn)WO 4 and high-entropy (MnCoNiCuZn)WO 4 tungstate – were synthesized and characterized. Their phase purity and solid-solution nature were confirmed by powder X-ray diffraction and Raman spectroscopy. X-ray absorption spectroscopy was used to probe the local structure around metal cations. The atomic structures based on the ideal solid-solution model were optimized by a simultaneous analysis of the extended X-ray absorption fine structure (EXAFS) spectra at multiple metal absorption edges – five edges for (MnNiCuZn)WO 4 and six edges for (MnCoNiCuZn)WO 4 – using reverse Monte Carlo simulations. In both compounds, Ni 2+ ions have the strongest tendency to organize their local environment with the formation of slightly distorted [NiO 6 ] octahedra, whereas Mn 2+ , Co 2+ , and Zn 2+ ions have a strongly distorted octahedral coordination. The most intriguing result is that the shape of [CuO 6 ] octahedra in (MnNiCuZn)WO 4 and (MnCoNiCuZn)WO 4 differs from that found in pure CuWO 4 , where a strong Jahn-Teller distortion is present: [CuO 6 ] octahedra become more regular with increasing the degree of dilution.

Keywords: Tungstates, High-entropy oxides, EXAFS, Reverse Monte
Carlo, Solid solutions