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.

The chromic phase transition from the high‐temperature/low‐pressure form of CuMo0.9W0.1O4 (γ‐phase, red‐brown) to the low‐temperature/high‐pressure form (α‐phase, green) is induced at 25 °C by simple immersion of the powder in an acidic aqueous solution.