Modelling amorphous materials via a joint solid-state NMR and X-ray absorption spectroscopy and DFT approach: application to alumina

Understanding electronic structure is a crucial component in the development of many functional materials including semiconductors, transparent-conducting oxides, and batteries, and is necessarily directly dependent on their underlying atomistic structure. The elucidation of atomistic structure is impeded, both experimentally and computationally, by structural disorder, presenting a huge challenge for designing functional amorphous materials. Amorphous materials may be characterised through their local atomic arrangements using, for example, solid-state NMR and X-Ray Absorption Spectroscopy (XAS). By using these two spectroscopy methods to inform the sampling of configurations from ab initio molecular dynamics we devise and validate an amorphous model, choosing amorphous alumina to illustrate the approach due to its wide range of technological uses. Our model predicts two distinct geometric arrangements of AlO5 coordination polyhedra and determines the origin of the pre-edge features in the Al K-edge XAS. We finally construct an average electronic density of states for amorphous alumina, and identify localized states at the conduction band minimum (CBM). We show that the CBM is comprised of Al 3s states and connect this localization and the presence of the pre-edge in the XAS. Deconvoluting this XAS by coordination geometry reveals contributions from both AlO4 and AlO5 geometries at the CBM give rise to the pre-edge, which provides insight into the role of AlO5 in the electronic structure of alumina. This work represents an important advance within the field of solid-state amorphous modelling, providing a method for developing amorphous models through comparison of experimental and computationally derived spectra, which may then be used to determine the electronic structure of amorphous materials.