First observation of reversible mechanochromism and chromaticity study on calcium–copper–titanate

Abstract First ever intriguing reversible mechanochromism espied in and chromaticity study on polycrystalline CaCu 3 Ti 4 O 12 composition milled up to 16 hours are reported. The composition changes its color from blackish‐brown (un‐milled sample)‐reddish‐brown (1 hours milled sample)‐ heina‐green (6 hours milled sample)‐spinach‐green (16 hours milled sample) mainly due to the phase separation and corresponding change in band‐gap energy on milling. The UV‐Vis spectra show three observable absorption edges correspond to , , and , d yz transitions. A marked mechanochromic effect with crystallite size reduction is explained in the light of the difference in the site group symmetry of CuO 4 and ligand‐metal charge transfer coupled with Cu 2+ d‐d transitions. The observed mechanochromism is reflected and well supported by photoluminescence spectral analysis. CIE chromaticity coordinates are located in the light blue region for an un‐milled sample and violet color region for 6 hours milled sample. The profound correlation between the structural and optical properties has been revealed.