Study of the thermochromic phase transition in CuMo$_{1-x}$W$_x$O$_4$ solid solutions at the W L$_3$-edge by resonant X-ray emission spectroscopy

TL;DR

This study investigates the thermochromic phase transition in CuMo$_{1-x}$W$_x$O$_4$ solid solutions using resonant X-ray emission spectroscopy, revealing temperature-dependent structural changes and tungsten coordination variations correlated with color change.

Contribution

It provides new insights into the local atomic and electronic structures of tungsten in CuMo$_{1-x}$W$_x$O$_4$ during phase transitions, combining RXES measurements with ab initio calculations.

Findings

W ions remain octahedral across 90-420 K

Tungsten coordination shifts from tetrahedral to octahedral upon cooling for x <= 0.15

Hysteretic behavior observed in phase transition between α and γ phases

Abstract

Polycrystalline CuMo$_{1-x}$W$_x$O$_4$ solid solutions were studied by resonant X-ray emission spectroscopy (RXES) at the W L$_3$-edge to follow a variation of the tungsten local atomic and electronic structures across thermochromic phase transition as a function of sample composition and temperature. The experimental results were interpreted using ab initio calculations. The crystal-field splitting parameter for the 5d(W)-states was obtained from the analysis of the RXES plane and was used to evaluate the coordination of tungsten atoms. Temperature-dependent RXES measurements were successfully employed to determine the hysteretic behaviour of the structural phase transition between the $\alpha$ and $\gamma$ phases in CuMo$_{1-x}$W$_x$O$_4$ solid solutions on cooling and heating, even at low ($x$<0.10) tungsten content. It was found that tungsten ions have octahedral coordination for in the whole studied temperature range (90-420 K), whereas their coordination changes from tetrahedral to octahedral upon cooling for smaller ($x$ <= 0.15) tungsten content. Nevertheless, some amount of tungsten ions was found to co-exists in the octahedral environment at room temperature for $x$<0.15. The obtained results correlate well with the color change in these solid solutions.