Naturally tenebrescent (reversibly photochromic) materials have been known from geologists since the 1970's but were investigated seriously only recently. The adaptability of these materials, along with their high stability and good reversibility of the photochromism make them of great interest. The aluminosilicate sodalite (Na8Al6Si6O24Cl2) is one of them, with the assessed mechanism being a reversible photoinduced electron transfer from an impurity to a chloride vacancy, leading to the formation of a trapped electron in a crystal box. This trapped electron, called F-center, has quantified levels and absorbs in the visible light. The aim of this work is to develop a methodology based on quantum chemistry to confirm and get more insights on the mechanism at stake.
We first designed a simulation protocol to investigate the spectroscopic properties of point defects in sodalite minerals, using Time Dependent Density Functional Theory (TD-DFT). We highlighted the influence of the close environment and the vibronic coupling in these spectra, but also very interestingly the influence of the nature of the defect on the choice of some parameters in the methodology, such as the functional. The methodology was then successfully applied on other aluminosilicate materials, and other type of defects leading to a deeper understanding of these minerals.
Then, by adapting the methodology, the photoinduced charge was investigated to understand both the mechanism of the F-center formation and the process of bleaching. These last calculations, performed both at the TD-DFT and post-HF levels, provide paramount information for future development of, among others, specific wavelength sensors.