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

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 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 − 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 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.