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

First ever intriguing reversible mechanochromism espied in and chromaticity study on polycrystalline CaCu3Ti4O12 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 dx2-y2 <-> dxy, dx2-y2 <-> dz2, and dx2-y2 <-> dzx(,) 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 CuO4 and ligand-metal charge transfer coupled with Cu2+ 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.