Electronic structure and photoluminescence properties of Eu2+-activated Ca2BN2F

The electronic structure of undoped and luminescence properties of Eu2+-doped Ca2BN2F have been investigated. First-principles calculations for Ca2BN2F show that the valence band is mainly composed of F and N 2p, B 2s and 2p orbitals, while the Ca 4s and 3d are almost empty, indicating that Ca2BN2F is a very ionic compound. The valence band close to the Fermi level is dominated by the N 2p states, and the bottom of the conduction band is determined by the Ca 3d and N/B 3s orbitals. The direct energy gap is calculated to be about 3.1 eV, in fair agreement with the experimental data of ∼3.6 eV derived from the diffuse reflection spectrum. Due to the high degree of ionic bonding of the coordinations of Eu with (N, F) on the Ca sites, Ca2BN2F:Eu2+ shows strong blue emission with a maximum at about 420 nm upon UV excitation in the absorption range of 330-400 nm. - Graphical abstract: Diffuse reflection spectrum, excitation and emission spectra of Ca2BN2F:Eu2+, showing that only the ionic Eu2+ center offers deep blue emission at about 422 nm under excitation at 339 nm.