Electronic structure calculations of SrB4O7 and SrB4O7 :Eu crystals

The electronic structures of SrB4O7 and SrB4O7:Eu have been calculated by density functional method with the local density approximation. The respective contributions of cations and anionic groups to the band structures are evaluated. An indirect band gap of 9.71 eV and a direct gap of 11.18 eV at U are obtained for SrB4O7. The calculated total and partial densities of states indicate that the top valence band of SrB4O7 is mainly constructed from the O-2p state, and the low conduction band mostly originates from B-2p and Sr-4d states. From the calculation, we find that the host absorption of SrB4O7 in VUV region is from both the BO4 group and Sr–O charge transfer transition. With the doping of Eu3+, the energy band of SrB4O7 moves downwards and new energy levels appear in the band gap, which is consistent with the experimental band models.