Low energy models from scratch: Application to SiNF

A novel method is proposed to derive low energy models for simple materials through ab initio optimization in space group P1 of randomly nudged cell and atom coordinates of low multiplicity supercells of the primitive cell of a hypothetical cubic model of the material with non-competitive energy and a single formula unit per primitive cell. It is shown that the number of supercells to be tried is 1, 3, 6 and 17 respectively when searching for models with 1–4 formula units per unit cell. The approach is amazingly effective for small numbers of atoms per formula unit. Application to SiNF consistently leads to new layered tetrahedral structure types with tetrahedra triply connected at N apices while singly bonded F have van der Waals interactions with neighbouring layers. The total energy of those types is ∼0.6 eV lower per formula unit than currently accepted stishovite-based models. The hypothetical reaction Si3N4 + SiF4 ⇒ 4SiNF is then exothermic by at least 290 kcal, pointing to a plausible route for the synthesis of SiNF. The creation of corresponding quantum jobs has been automated in Materials Toolkit (http://www.tothcanada.com/toolkit).