Band gap structure- Lewis acid/base feature correlation for potassium/sodium polyniobates and polytitanates

Polyniobates and Polytitanates are extensively studied because of their characteristic structure, electronic and acid/alkali feature as perovskite oxide based materials. Based on the experimental data from references and theoretical results calculated by density functional theory (DFT), respectively, the qualitative correlation between band gap structure and Lewis acid/base feature is investigated in the present paper. When it comes to band gap structure, the bond angle O and bond length are considered as the quite influential factors. As the M-O-M bond angle deviates from 180 degrees and the bond length elongates, the band gap energy increases. The conduction band potential and valence band potential are associated with Sanderson charge fraction 5i. It can be drawn from this study that the conduction band potential is positively correlated with Lewis acid (Nb5 +/- or Ti5 +/-) strength and valence band potential is negatively correlated with Lewis base (O5-) strength for potassium polyniobates and polytitanates with different phase structure. The conduction band potential and valence band potential are positively correlated with Lewis acid (Nb5 +/- or Ti5 +/-) strength and Lewis base (O5-) strength for sodium poly-niobates and polytitanates with same phase structure. Sanderson electronegativity and the charge for the O 1s confirm the correlation between band gap structure and Lewis acid and base. In addition, the phase structures of polyniobates and polytitanates affect the Lewis acid feature of the conduction band and Lewis base feature of valence band respectively, resulting in changes in their band gap structure.