where 𝜈 * is the spectrometer frequency in MHz, and 𝜂 $ is the asymmetry parameter of the electric field gradient. Both 𝐶 $ and 𝜂 $ were obtained from DFT calculations of the magnetic shielding parameters. A spectrometer frequency of 182.4 MHz for 27Al, corresponding to a 1H Larmor frequency of 700 MHz, was used for 𝜈 * . First principles XAS calculations were carried out by specifying a single atom at which to calculate the absorption edges. One Al atom from each of the 45 configurations was selected at random, in order to obtain a set of atoms on which to calculate the spectra, such that the total 45 Al atoms were in coordination environments which were representative of the 50%, 38%, and 12% of Al(IV,V,VI) environments present in experiment. For each configuration an XAS spectra was calculated using the core-hole pseudopotential method within CASTEP v19.11 . To calculate the XAS for the Al K-edge, a pseudopotential with a 1s core-hole was placed on the randomly selected atom within the configuration and charged balanced by placing a total positive charge of +1 on the cell. The absorption spectra was calculated at a plane-wave energy cutoff of 1000 eV, using the HARD pseudopotential library in CASTEP v19.11 . The absorption spectrum was produced by OptaDOS v2.1 and broadened using the adaptive broadening scheme . This spectrum was referenced using the method of Mizoguchi et al. such that the transition energy is referenced to the difference in energy between the ground state configuration of a-Al2O3 and the configuration which includes the core-hole pseudopotential.