The structural origin of the efficient photochromism in natural minerals

Abstract

Significance Natural photochromic minerals have been reported by geologists for decades. However, the understanding of the photochromism mechanism has a key question still unanswered: What in their structure gives rise to the photochromism’s reversibility? By combining experimental and computational methods specifically developed to investigate this photochromism, this work provides the answer to this fundamental question. The specific crystal structure of these minerals allows an unusual motion of the sodium atoms stabilizing the electronic states associated to the colored forms. With a complete understanding of the photochromism mechanism in hand, it is now possible to design new families of stable and tunable photochromic inorganic materials-based devices.

Reusable radiochromic hackmanite with gamma exposure memory

“…To understand the origin of the additional reflectance side bands we recall two properties of the hackmanites structure reported before: (i) we have recently shown that sodium atoms can move from one b-cage to another (see the ESI † for more information about the structural details) during the UV tenebrescence process proving that these ions are mobile. 31 (ii) we also reported that under X-ray exposure, NaCl can exit from the hackmanite 5 generating Na and Cl vacancies. In other words, Na atoms can move inside the hackmanite and even exit the structure.…”

Engineering aluminosilicate's photochromism by quantum chemistry

“…Here, we can introduce the fact that our model a priori overestimates the bleaching energies (i.e., underestimates the metastability of the colored form) but reproduces qualitatively good experimental observations. 7 The calculations also show the possibility to slightly tune the bleaching energy (E b ) essentially by modifying the halogen atom. Indeed, whatever A (A = Al, Ga) and B (B = Si, Ge) in ABX's composition, (X = Cl, Br, I) we have…”

“…The same methodology as the one reported here was actually used to simulate F-center's absorption in marialite and was able to quantitatively reproduce this red-shift. 7 In Fig. 7, we also report the absorption spectrum for a system with an additional sodium atom placed in the vicinity of the F-center (structure referred to as Na 5 ).…”

“…Recently, we showed that sodium atoms surrounding the different impurities move during the charge transfer (up to more than 1 Å), stabilizing the different forms of the material (i.e., white or colored). 7 More precisely, during the activation step, the volume of the tetrahedron surrounding the sulfur impurity will increase, while it will decrease for the one surrounding the trapped electron. The reverse process is observed during bleaching.…”

“…For a deeper description of the electronic structure of this system, the interested readers can refer to the following ref. 6 and 7.…”