We then successfully synthesized the DFTpredicted compound Cs 2 NaEuF 6 , an air-stable and insulating double perovskite halide with a large Eu 3+ separation that leads to paramagnetic behavior. Magnetic susceptibility, optical spectra, and EDS indicate that the compound contains several defects. Though our attempts to synthesize the other predicted compounds were unsuccessful, we believe more extensive tests are merited for Cs 3 Eu(PO 4 ) 2 and CsEuP 2 O 7 because of the compounds' favorable Eu 3+ separations and site symmetries as well as the wide range of tunable synthesis parameters involved.