Structure direction in zinc oxide and related materials by cation substitution: an analogy with zeolites

We explore the chemical and structural analogy between nanoporous aluminosilicates (zeolites) and co-substituted binary MX materials by theoretical means. Using global optimisation methods in combination with an empirical potential and density functional theory calculations we find and accurately characterise numerous low-energy co-substituted binary MX structures. Focussing on co-substituted ZnO, we demonstrate that, in line with experimental synthetic evidence, co-substitution of Zn2+ in ZnO by a combination of alkali metal cations (e.g. Li+/K+ to form vertical bar K vertical bar[LiZn3O4]) is clearly energetically favourable. We further show that, just as in the case of zeolites, co-substitution in ZnO leads to an energetic destabilisation of the dense packed structure-types found for the pure material, and, also in line with the few specific co-substituted MX syntheses reported in the literature, the stabilisation of open structure-types with large rings and cages. The global optimisation calculations further show that the specific open structure-types observed experimentally for the co-substituted materials are the likely global minima for the different substitution levels. Finally, we further consider the energetic stability against decomposition of co-substituted compounds in a combination of materials with lower and higher substitution levels and the effect of co-substitution on the optoelectronic properties of MX materials.