Assessing the design rules of electrides

There are three heuristic criteria commonly used to identify electrides: an apparent valence of plus one, empty space in the crystal structure and the presence of a strongly electron-donating cation. We evoke and explore these criteria by mapping probable charges to a database of all known materials and isolating around around 4000 compounds that are likely to exhibit an oxidation state of +1. Of these, we identify peaks in off-atom electron density by density functional theory and discuss the validity with which the design rules can be applied to these likely electrides. In doing so, we recover 4 experimentally confirmed electrides among 51 candidates identified as potential new electrides that were not considered previously. All results for each candidate are provided but perhaps of special significance is the material Ba₃AlO₄ that has similar composition and chemistry to the stable electride catalyst Ca₁₂Al₁₄O₃₂. Overall, we find that the valence and void space rules are surprisingly useful but that there is a breadth of chemistry to electrides that can be overlooked by considering only alkili and alkali earth metal compounds.