A Temperature-Responsive Copper Molybdate Polymorph Mixture near to Water Boiling Point by a Simple Cryogenic Quenching Route

Smart temperature-responsive inorganic materials in accessible temperature ranges open up new positions in the technology. Herein, we present for the first time a CuMoO₄ polymorph mixture prepared by a simple cryogenic quenching approach, which offers a fast temperature response close to water boiling temperature for use as a permanent temperature recorder. The new cryogenic quenching technique initiates the formation of a unique polymorph mixture of a deep brown color with a nonuniform combination of γ- and α-CuMoO₄, with the γ phase being confined to the outer region of α-CuMoO₄, which has been prepared by conventional solid-state synthesis. In situ structural analysis and refinement results confirm the presence of CuMoO₄ α and γ polymorphs in which the amount of γ polymorph decreases and that of the α phase increases with temperature, accounting for the irreversible thermochromic behavior. The thermal analysis reveals that the polymorph mixture exhibits a fast response with the color changing from deep brown to bright green with intermediate colors of light brown, yellowish green, and light green depending on the exposure temperature as observed from reflectance measurements.