In addition to screening proposed candidates with DFT, we searched for existing entries in the Materials Project. First, using the database's API, we queried every binary, ternary, and quaternary entry that contained the ions we used to propose structures. Then we filtered out entries with a Eu 3+ minimum separation less than 3.95 Å and entries with non-polar Eu 3+ site symmetries. After filtering, seven compounds remained, each experimentally realized. They are listed in Table . Their database IDs are in the Supporting Information, as are compounds with a smaller Eu 3+ separation. Six of the seven are phosphates, which stems from our chemistry restrictions. Phosphorous, unlike the other cations, consistently forms polyatomic ions with oxygen. These polyatomic ions exist in various geometries (see extensive overviews ) that allow for large Eu 3+ cation separation and provide space for the mononuclidic alkali metal cations Na + and Cs + .