Modeling short-range substitution order and disorder in crystals: Application to the Ga/Si distribution in a natrolite zeolite

Abstract

Atomic substitutions are a central feature of the physicochemical properties of an increasing number of solidstate materials. The complexity that this chemical disorder locally generates in otherwise crystalline solids poses a major challenge to the understanding of the relationships between the structure and properties of materials at the atomic and molecular level. Strategies designed to efficiently explore the ensemble of local chemical environments present in disordered crystals and predict their signatures in local spectroscopies such as solidstate nuclear magnetic resonance (NMR) are therefore essential. Focusing on the Ga/Si disorder in the framework of rubidium-exchanged gallosilicate natrolite zeolite (Rb-PST-1) with a high Ga content (SiGa=I.28), we show how the structure-generation approach implemented in the new program supercell (Okhotnikov et al. [26]) provides an excellent basis for the understanding of complex experimental spectroscopic data. Furthermore, we describe how exhaustive explorations of atomic configurations can be performed to seek local structural ordering and/or disordering factors. In the case of Rb-PST-1, we more specifically explore the possibility to form and to detect the presence of thermodynamically unfavorable Ga-O-Ga connectivities. While particularly adapted to the description of dense materials, we demonstrate that this approach may successfully be used to reproduce and interpret the distributions of local structural distortions (i.e., the geometrical disorder) resulting from the chemical disorder in systems as complex as microporous zeolites.