Electronic structures and nature of host excitation in borates

We report the electronic structures of three large band gap borates β-BaB₂O₄, YBO₃ and LuBO₃, and corresponding binary oxides BaO, Y₂O₃, Lu₂O₃ and B₂O₃. The main objective of this investigation is to explore how the electronic structures of complex oxides relates to their binary oxides, and how tightly bound anionic groups lead to large gap oxides. The calculated electronic structures lead to good agreement between observed optical gap and structure of the valence band for these oxides. While the borates lead to large gap materials, the excitations across the band gap do not always involve bonding and antibonding states of borate groups that are common to all the ternary oxides considered in this work. Although the top of the valence band is characterized by hybridized O 2p- and B 2p-like states, the bottom of the conduction band can be due to metallic states. The presence of such states near the bottom of the conduction band was speculated earlier from the excitation properties of YBO₃. We have also explored the possibility of designing a multi-photon phosphor through core valence luminescence from a ternary oxide of Ba.