Tenebrescent minerals by in silico Modelling – TeneMod

The details of the methodology of the computations of the project can be found in the articles related to the project. Here is a brief description of the key steps of the procedure used to simulate excited state properties in aluminosilicate:
1- Geometries were optimized with the CRYSTAL17 code along with the global hybrid PBE0 functional. This functional has been chosen since it reproduces the crystal structures with a good accuracy. The basis sets used were double-zeta or triple-zeta. For the trapped electron, we optimized a triple-zeta basis set 111G(d) for each system. The defects were placed in a 2x2x2 supercell (around 350 atoms) and calculations were done on a single k-point (le point gamma).
2- A cluster is then cut around the defect including the first sodium polyhedron and the first beta-cage leading to a system containing around 200 atoms when we investigate the charge transfer between the S22- ion and the chlorine vacancy.
3- The cluster is embedded in a sphere of cations replaced by pseudo-potentials (replacing the Si4+ and Al3+ ions) and an array of point charges fitted to reproduce the electrostatic potential of the crystal (around 46 000 point charges). To do so, we used a code called «Ewald« adapted to our system.
4- The TD-DFT calculations were done on the embedded cluster using the same basis set as for the periodic calculations and with a hybrid functional (such as PBE0) or a range separated hybrid functional (such as cam-B3LYP).

This project has two outcomes. The first one is the development of a computation methodology to simulate the excited states of aluminosilicates. The second one corresponds to the application of this methodology to understand the origin of the photochromism in these minerals.
Methodology development: We showed that the use of a cluster embedded by point charges fitted to reproduce the Madellung potential can give simulated spectroscopic properties by TD-DFT close to experimental results. A good agreement between theory and experiment was obtained for the absorption spectrum of the trapped electron, the emission spectrum of the trapped S2- and the charge transfer transition involved in the photochromism mechanism.
Understanding of the phenomena: By combining our calculations to the experimental results of the group of Mika Lastusaari, we showed that the stabilization of the trapped electron responsible of the coloration comes from a motion of a sodium atom in the system. In other words, this is the crystal structure of these minerals that allows the photochromism phenomenon. This allowed us to understand the origin of the short life time of the color observed with the minerals from the scapolite family. Our methodology allowed us to understand the photochromism phenomenon in sodalites when exposed to other irradiation sources (X-ray, Gamma, Alpha). Finally, the combination between our calculations and the experiments of the M. Lastusaari group allowed to understand the photoluminescence phenomenon also observed in these minerals (with lifetimes of several minutes).

The methodology now well assessed, we plan to complete to investigate of these minerals again in collaboration with the group of Mika Lastusaari. For instance, this group has shown that new colorations were accessible by insertion of calcium atoms in the material.
From a methodology point of view, we would like to improve the embedding technics by going beyond the point charges. To do so we will use the ALMO method (for Absolutely Localized Molacular Orbitals) and apply it not only to photochromism of sodalite but also to all excited state simulations happening in solids.

Publication(s):
• S. Vuoria, P. Colinet, J.-P. Lehtiöb, A. Lemiere, I. Norrbo, M. Granström, J. Konu, G. Ågren, P. Laukkanen, L. Petit, A. J. Airaksinen, L. van Goethemi, T. Le Bahers*, M. Lastusaari*, Reusable radiochromic hackmanite with gamma exposure memory, Mater. Horizons accepted (IF : 15.7)
• P. Colinet, H. Byron, S. Vuori, J.-P. Lehtiö, P. Laukkanen, L. Van Goethem, M. Lastusaari*, T. Le Bahers*, The Structural Origin of the Efficient Photochromsim in Natual Minerals, Proc. Natl. Acad. Sci. USA PNAS, 2022, 119, e2202487119. (IF = 12.8)
• S. Vuori, P. Colinet, I. Norrbo, R. Steininger, T. Saarinen, H. Palonen, P. Paturi, L.C.V. Rodrigues, J. Göttlicher, T. Le Bahers, M. Lastusaari*, Detection of X-Ray Doses with Color-Changing Hackmanites: Mechanism and Application Adv. Opt. Mater., 2021, 9, 2100762. (IF : 10.0)
• P. Colinet, A. Gheeraert, A. Curutchet, T. Le Bahers*, On the Spectroscopic Modelling of Localized Defects in Sodalites by TD-DFT, J. Phys. Chem. C 2020, 124, 8949-8957. (IF : 4.1)
• C. Agamah, S. Vuori, P. Colinet, I. Norrbo, J. Miranda de Cavalho, L. K. O. Nakamura, J. Lindblom, L. van Goethem, A. Emmermann, T. Saarinen, T. Laihinen, E. Laakkonen, J. Lindén, K. Konu, H. Vrielinck, D. Van der Heegen, P. F. Smet, T. Le Bahers*, M. Lastusaari*, Hackmanite - The Natural Glow-In-The-Dark Mineral, Chem. Mater. 2020, 32, 20, 8895-8905. (IF : 10.5)
• Norrbo, A. Curutchet, A. Kuusisto, J. Mäkelä, P. Laukannen, P. Paturi, T. Laihinen, J. Sinkkonen, E. Wetterskog, F. Mamedov, T. Le Bahers*, M. Lastusaari*, Solar UV Index and UV Dose Determination with Photochromic Hackmanites: From the Assessment of Fundamental Properties to the Device, Mater. Horizons 2018, 5, 569-576. (IF : 15.7)

Photochromic compounds are the base of numerous potential high-tech devices including adaptive glasses, photo-switches, optical memories etc. Up to now, this research field has been dominated by organic materials because of the large versatility offered by organic synthesis to design and tune molecules. The lack of adaptability of inorganic photochromic materials is the main brake on their development although they possess some interesting properties such as good working condition stability. The lack of adaptability of inorganic materials could be solved by natural minerals of the sodalite family. The natural photochromism of these minerals has been known from geologists (who call this phenomenon tenebrescence) for around 50 years but has been started to be investigated seriously only recently. Up to now, the only assessed mechanism of the photochromism was a photo-induced electron transfer from a sulfur-based impurity toward a chlorine vacancy creating a F-center, giving a colour to the system. But no atomistic model of the system was developed to clearly confirm this mechanism. The TeneMod project stands there. We want to develop a methodology based on quantum chemistry to uncover the elementary steps of photochromism mechanism in sodalite leading to an accurate simulation of the phenomenon and of the final colour of the system. With this methodology we will perform a large benchmark of colour simulation and activation energy of all photochromic (tenebrescent) natural minerals of the sodalite family known in geology and on potential artificial sodalites to determine the adaptability of these minerals for further investigations.
The TeneMod project is organized in five tasks. The Task 1 is dedicated to the development of the methodology on the most simple photochromic sodalite. This approach involves geometries determined by DFT in periodic boundary conditions and sophisticated excited state simulations (TD-DFT, SAC-CI…) computed on clusters extracted from the PBC geometries surrounded by an environment simulating the Madelung potential of the crystal. The methodology is then tested on natural tugtupite and scapolite tenebrescent mineral in Task 2 with the objectives first to improve the methodology if necessary then to understand the influence of the crystal structure on the photochromism. The Task 3 will focus on artificial sodalite with the simulation of their photochromic properties, by changing the chemical nature of the constituting atoms and the dopants, to demonstrate the adaptability of these minerals for the practical applications. In the Task 4, we will move toward a realistic colour simulation including the light scattering of the powder nature of the material. The final Task has the objective to prove that the methodology developed by TeneMod can be used for other spectroscopic modelling, like the difficult simulation of alexandrite and cordierite polychromism.
From a practical point of view, a funding of ~160k€ is asked for this project. A large part of this funding will be used for a PhD fellowship. Beyond the PhD student and myself, TeneMod will involve one of the world expert in the synthesis of artificial sodalite, Pr. Mika Lastusaari, and some colleagues of my host-laboratory. The TeneMod project corresponds to a new field of research in my host laboratory. This project will be the first large quantum chemical investigation of this family of material, bringing my team as the leader on the modelling of these minerals with the objective to offer to the community both an atomistic point of view that is missing to inorganic photochromism research field and the proof that an adaptable, stable and cheap mineral can be developed for photochromic devices.