Modelling the S-Doped Sodalites Using DFT, TD-DFT and SAC-CI Methods

A. Curutchet and T. Le Bahers, Inorg. Chem. 2017, 56, 414-423

It corresponds to my first published work in the field of tenebrescent materials. It is a purely computational work aiming to demonstrate that quantum chemistry can be used to model the photochromism in sodalites.

Among the most important results, we show that to model the absorption spectrum of the F-center by a cluster approach, the full β-cage must be considered in the quantum model along with the vibronic-coupling. The electronic transition of the trapped electron strongly couples with the breathing mode of the Na4 tetrahedron.

 Figure 1: (a) TD-DFT absorption wavelength of the [Na4VCl]3+ system surrounded by a cluster of point charge (Mulliken or Bader charges) of increasing size. In inset, the green triangle is the quantum part and the points are the charge positions. (b) TD-DFT computed absorption wavelength of a cluster including β-cage, without point charges. (c) Simulated vibronic coupling of the absorption, including only the vibration of the Na4 tetrahedron.

We prove that the charge transfer from the S22- impurity to the chlorine vacancy is responsible of the F-center formation with transition energies in agreement with the experiment.

 


 Figure 2: (a) Structure of the small and large clusters. (b) and (c) SAC-CI transition energies computed for the two clusters for two geometries.

 

Solar UV Index and UV Dose Determination with Photochromic Hackmanites: From the Assessment of Fundamental Properties to the Device

I. Norrbo, A. Curutchet, A. Kuusisto, J. Mäkelä, P. Laukannen, P. Paturi, T. Laihinen, J. Sinkkonen, E. Wetterskog, F. Mamedov, T. Le Bahers, M. Lastusaari, Mater. Horiz. 2018, 5, 569-576

This article is the first of the collaboration with the group of Mika Lastusaari. In this work, we proved both experimentally and computationally that a partial replacement of Na by other larger alkaline atoms (K and Rb) leads to a lowering of the transition energy of the electron transfer from S22- to the chlorine vacancy.

Figure 3: (a) Tenebrescence excitation spectra of (Na,M)8Al6Si6O24(Cl,S)2 (M = none, K and Rb) with 6% of S doping. (b) DFT computed evolution of the energy gap (in eV) between the last occupied orbital π*(S22−) and the first unoccupied orbital a1(VCl) as a function of the number of Na substituted around the Cl vacancy.

 Furthermore, in this work the group of Mika Lastusaari presents for the first time the thermotenebrescence experiment allowing to measure the activation energy for the bleaching of the material. Interestingly, the activation energy measured (around 0.4 eV) is in very good agreement with the one proposed by DFT calculation in our previous article.

Finally, we propose to use artificial tenebrescent hackmanites as UV indicator because the sensitivity of the material toward UV light can be tuned and adapted for specific UV light (such as UV from the sunlight).

 Figure 4: Color intensity of (Na,K)8Al6Si6O24(Cl,S)2 at different UV index values.