Uniaxial thermal expansion-induced successively reversible thermochromism in zircon-type CaCrO4

Abstract

Thermochromic materials based on solid-state oxides exhibit many potential photo-thermal regulation applications. Herein, we report a new thermochromic pure-phase CaCrO₄ pigment with merits of high chromatic aberration, fully reversible thermochromism, and high stability. CaCrO₄ crystalized into a tetragonal zircon-type structure, which showed a successively reversible color change from greenish yellow to reddish orange with the highest chromatic aberration of 59.60. In situ variable temperature X-ray diffraction analysis results revealed that the expansion of the lattice was uniaxial along the c-axis with a dilation rate of 2.06 × 10⁻⁵, while that along the a-axis remained nearly unchanged in the temperature range of 25–575 °C. In situ variable temperature UV–vis absorption spectra of CaCrO₄ revealed a successive bandgap reduction with an average temperature dependency rate (dE_g/dT)_average of 0.27 meV K⁻¹. Continuous color change or bandgap reduction originated from the successive elongation of Cr–O bonds in the CrO₄ tetrahedral chromophore in the lattice. The most sensitive temperature range was between 175 °C and 400 °C, which rendered a naked-eye-detection accuracy rate of over 85% for a standard temperature deviation (ΔT) of 25 °C, demonstrating the potential of the pigment as a pan-bottom temperature indicator for healthy cooking.