X-ray photoelectron spectroscopy and full potential studies of the electronic density of state of ternary oxyborate Na3La9O3(BO3)8

Density of electron states (DOS) measured by X-ray photoelectron spectra (XPS) on single crystals of a ternary oxyborate Na3La9O3(BO3)8 (NLBO) are reported for the first time. The Na3La9O3(BO3)8 ternary oxyborate nonlinear single crystals were synthesized by spontaneous crystallization on a iridium cylinder put in melts consisting of binary oxides. The novelty of the present work is that it describes the complete experimental picture of the total density of states (TDOS) for the valence band obtained by means of X-ray photoelectron spectroscopy. We compare the DOS obtained by XPS with band structure calculation. The latter was performed within the density functional theory, using a full-potential linear augmented plane wave (FP-LAPW) method. The valence band maximum (VBM) and the conduction band minimum (CBM) are located at Γ point of the Brilluoin zone resulting in a direct energy band gap of about 4.44 eV comparing with our measured energy band gap (5.22 eV). The VBM originates from O-p states, while the CBM is dominated by localized La-f states. We notice a different degree of hybridization between La-s, Na-s/p and O-s/p states in the valence and conduction bands. B-s is strongly hybridized with O-s states and B-p with O-p states in the VB at around −5 eV. O-p states are weakly hybridized with La-s, Na-s/p at the VBM.