The calculated absorption coefficient (20×20×20 k-point grid) and band structure of Cs 2 NaEuF 6 are Figure , showing a 6.9 eV band gap at the Γ point. Interestingly, the Cs 2 NaEuF 6 polycrystalline product was gray instead of white. When the sealed tube contained starting powders in a boron nitride crucible and excess CsF in a separate crucible, the excess CsF also had a darker hue after synthesis. EDS revealed oxygen contamination in the perovskite product (Supporting Information). The gray hue, therefore, likely comes from the oxygen contamination or Eu 2+ impurities, but no unidentified peaks were present in our XRD patterns.