Investigation of the Chromic Phase Transition of CuMo_0.9W_0.1O₄ Induced by Surface Protonation

Abstract

CuMo0 .9 W 0.1 O 4 oxide is known to exhibit two allotropic forms (R and γ forms) linked by a firstorder phase transition that can be induced by pressure and/or temperature. In this work, we show that this R f γ phase transition (with a chromic effect from green to brown color) can also take place at 25°C by a simple immersion of the powder in an aqueous solution with acidic pH. This chromic phase transition from the high-temperature/low pressure form into the low-temperature/highpressure form is shown to be induced by surface protonation, whereas the copper-molybdate powder is dispersed in acidic solutions. Wettability measurements on both phases were performed in order to confirm the affinity of the R-form for protons in comparison with the γ-form. Then pH metric titration (addition step by step of an acid solution on the powder, controlling at each step the pH evolution) has allowed describing the kinetic aspects of the surface protonation (kinetic laws were approached and discussed). Finally, an interpretative model of the "halochromic" phenomenon based on the modification of the cations environment at the liquid-solid interface was proposed. For the first time, a superficial interaction between a solid oxide and a liquid medium, for which no partial dissolution-recrystallization sequence is occurring, is shown to be sufficient for producing a phase transition of the whole oxide bulk.

Effect of W doping on phase transition behavior and dielectric relaxation of CuMoO4 obtained by a modified sol-gel method

“…Figure 3(a) shows the temperature dependence of dielectric loss tan δ of CMW (x=0∼0.12) at the same frequency. It is an intriguing and valuable discovery that there is an upward dielectric-anomaly peak in the curve for all the samples, and the position of the peak moved towards high temperature with increasing the doping content of W. Many studies have demonstrated that CuMoO 4 undergoes a first-order phase transition (γ → α) from low temperature to high temperature [9,[18][19][20], and the phase-transition temperature ranges from 200 to 250 K under the environmental pressure [9,10]. Therefore, it is reasonable to believe that CuMoO 4 has a phasetransition peak of relaxation type with dielectric properties.…”

“…Therefore, the material can also be used as pressure indicators [2]. The phase-transition temperature of CuMoO 4 is generally within the range of 200 to 250 K [9,10]. Whether as a pressure sensor or a temperature sensor, precise control of its phase-transition temperature is the key to the practical application.…”

X-ray absorption near edge spectroscopy of thermochromic phase transition in CuMoO4

“…It attracts much attention because of its potential applications for temperature sensing in the ranges where majority of organic compounds and liquid crystals are unstable. Possible chromic-related applications extend from the user-friendly temperature and pressure indicators [14][15][16][17] to "smart" inorganic pigments [18]. Besides, copper molybdate has also promising catalytic properties [19][20][21][22][23].…”

The influence of Zn$^{2+}$ ions on the local structure and thermochromic properties of Cu$_{1-x}$Zn$_x$MoO$_4$ solid solutions

“…Both types of thermochromic behaviour are found in copper molybdate (CuMoO 4 ) [4,5,6,7]. Moreover, it has been demonstrated that replacing molybdenum with tungsten or copper with other divalent metals makes it possible to change the thermochromic response [5,8,9,10,11,12,13]. Thermochromic properties have recently also been found in other molybdates such as NiMoO 4 [14] and CoMoO 4 [15].…”