Adaptable thermochromism in the CuMo1-xWxO4 series (0 ≤ x < 0.1):: A behavior related to a first-order phase transition with a transition temperature depending on x

The members of the CuMo1-xWxO4 series (0 <= x < 0.1) undergo a first-order phase transition at normal pressure, which can be induced by temperature. The two allotropic forms exhibit two distinguishable colors, green for the high-temperature form (alpha) and brownish-red for the low temperature one (gamma), which opens up a new market for user-friendly temperature indicators. From X-ray diffraction and microprobe analyses as from optical properties, the tungsten substitution rate for molybdenum is limited to 12%. Beyond, a third, parasitic wolframite-type phase, CuMo0.6W0.4O6, systematically crystallizes besides the alpha/gamma CuMo0.9W0.1O4 compounds. Within the CuMo1-xWxO4 solid-solution domain, the dependence of the transition temperatures was followed by calorimetry, optical reflectivity, and magnetism. On the basis of these measurements, the transition is characterized for all of the chemical compositions by a hysteresis loop of about 90 K in width with a temperature transition strongly dependent on the tungsten content. Namely, the gamma -> alpha transition can occur between 260 and 360 K, and the alpha -> gamma transition between 175 and 275 K as a function of x. The control of the alpha/gamma transition temperatures with x is related to the larger propensity of tungsten compared to molybdenum, to adopt a tetrahedral environment.