Peculiarities of the local structure in new medium- and high-entropy, low-symmetry tungstates

TL;DR

This study investigates the local atomic structure of new medium- and high-entropy tungstates with low symmetry, revealing how different metal ions organize locally and how CuO6 octahedra become more regular with increased dilution.

Contribution

It provides detailed local structural analysis of novel medium- and high-entropy tungstates, highlighting ion-specific distortions and the impact of dilution on CuO6 octahedra shape.

Findings

Ni²⁺ ions tend to form slightly distorted [NiO₆] octahedra.

Mn²⁺, Co²⁺, and Zn²⁺ ions have strongly distorted octahedral coordination.

CuO₆ octahedra become more regular with increasing dilution, contrasting pure CuWO₄.

Abstract

New monoclinic ($P2$/$c$) tungstates - a medium-entropy tungstate, (Mn,Ni,Cu,Zn)WO$_4$, and a high-entropy tungstate, (Mn,Co,Ni,Cu,Zn)WO$_4$ - 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 spectra at multiple metal absorption edges - five for (Mn,Ni,Cu,Zn)WO$_4$ and six for (Mn,Co,Ni,Cu,Zn)WO$_4$ - by means of reverse Monte Carlo simulations. In both compounds, Ni$^{2+}$ ions have the strongest tendency to organize their local environment and form 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 (Mn,Ni,Cu,Zn)WO$_4$ and (Mn,Co,Ni,Cu,Zn)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 degree of dilution.