Study of the thermochromic phase transition in CuMo1−xWxO4 solid solutions at the W L3-edge by resonant X-ray emission spectroscopy

Polycrystalline CuMo_1−xW_xO₄ solid solutions were studied by resonant X-ray emission spectroscopy (RXES) at the W L₃-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 Δ 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 α and γ phases in CuMo_1−xW_xO₄ solid solutions on cooling and heating, even at low (x<0.10) tungsten content. It was found that tungsten ions have octahedral coordination for x>0.15 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.