Photochromic Sodalites: From Natural Minerals to Advanced Applied Materials

ConspectusWhile photochromic natural sodalites, an aluminosilicate mineral, were originally considered as curiosities, articles published in the past ten years have radically changed this perspective. It has been proven that their artificial synthesis was easy and allowed compositional tuning. Combined with simulations, it has been shown that a wide range of photochromic properties were achievable for synthetic sodalites (color, activation energy, reversibility, etc.), making them interesting as alternatives to organic photochromes but with the stability offered only by inorganic materials.The photochromism in this mineral originates from a photoinduced electron transfer from a sulfur based impurity toward a chlorine vacancy generating a trapped electron called an F-center. This F-center gives the color of the mineral. To investigate further the mechanism of the coloration and design artificial forms of these minerals, we built a multidisciplinary international consortium (Finland, Norway, United Kingdom, Belgium, and France). By combining experimental and computational characterizations, specifically designed for these minerals, we discovered that the stability of the F-center is due to the motion of a single sodium atom, able to move by more than 1 Å inside the structure to stabilize the trapped electron.Our international consortium, combining expertise in geology, material science, spectroscopy, and computational chemistry, has leveraged this understanding to design artificial sodalites for targeted applications. By adjusting their chemical composition, we can now fine-tune their photochromic properties (activation energy, color, thermal stability, etc.). As a result, photochromic sodalites have emerged as a highly promising platform for inorganic photonics, inspiring exciting new research directions. Notably, we have already demonstrated proof-of-concept applications as detectors and sensors for UV, X-ray, and α-, β-, or γ-irradiation. The first tests to use them as photosensitive films toward visible light and X-rays have also been performed to open the way of new imaging technologies.Beyond the development of technological applications for synthetic sodalites, their investigation was a driving force in the design of new experimental and computational techniques to study photochromism in the solid state. For instance, the "thermotenebrescence" technique was formulated to extract the stabilization energy of the F-center, and a methodology to compute excited states by quantum chemistry based on a multilayer electrostatic embedding was conceived for these systems. The latter has already been applied to understand other optical properties in other minerals, such as the photochromism in scapolite and tugtupite, the polychromism of cordierite and alexandrite, or the persistent luminescence in sodalites.This Account highlights the strength of a multidisciplinary approach to tackle the investigation of complex phenomena.