Modeling the polychromism of oxide minerals: The case of alexandrite and cordierite

International audienceIn this work, we investigate the spectroscopic properties of photochromic alexandrite and cordierite by TD‐DFT. The objective is to assess the TD‐DFT for the simulation of pleochroism (change of color depending on the crystallographic direction of the observation) and the change of color as a function of the light source. For these simulations, we compared an embedding where dangling bonds are saturated by hydrogen atoms and an electrostatic embedding. The electrostatic embedding provided numerically more stable results and allowed a good reproduction of the pleochroism of cordierite, based on a Fe 2+ ‐Fe 3+ intervalence charge transfer transition. However, the pleochroism of alexandrite is not as well reproduced, suggesting that TD‐DFT has some difficulties to reproduce the anisotropy of the transition dipole moment, an aspect that is not deeply documented in the literature

Highly Tuneable Photochromic Sodalites for Dosimetry, Security Marking and Imaging

Photochromic sodalites are considered for a plethora of possible applications, such as UV indexing and X-ray imaging, but for many of these the materials are yet to be optimized. UV indexing can be improved through incremental adjustment of the activation energy of coloration from 300 to 410 nm through replacement of sulfur with selenium. By combining this and other methods of tuning presented in the literature, the excitation threshold and photochromism color can be tuned independently of one another. The range of possible absorption maxima is expanded to 420–680 nm, or almost the entire visible spectrum. Mixing low-cost and easy-to-synthesize sodalites further broadens the possible range of colors and facilitates development of a unique sodalite mix capable of quantifying the doses of two types of UV radiation simultaneously. Finally, the response to X-rays of these highly tuned sodalites is investigated, and it is found that they can be sensitized to produce clear, high-contrast X-ray images at significantly lower doses of radiation than those required by classic photochromic sodalite, Na8(AlSiO4)6(Cl,S)2