New polymorph γ-CuWO4 inspired by γ-CuMoO4: Experimental identification and theoretical verification

In the context of our efforts to develop hydrogen gas sensors, two samples of ternary CuWO₄ with the same crystalline structure have been prepared by two different non-equilibrium techniques. We show that the materials' structure is significantly different from the known stable one, and we identify that it is very similar to that of previously reported γ-CuMoO₄. We use ab-initio calculations to confirm that the newly identified phase, γ-CuWO₄, represents a local energy minimum. We present very similar calculated and measured lattice constants and X-ray diffractograms. We make a case that the low-temperature formation of the metastable γ-CuWO₄ phase is facilitated by easier kinetics and/or by Cu-rich composition of our samples, and we show that it converts to the stable phase after annealing to 600 °C.