Electronic Structures and Linear Optics of A2B2O5 (A = Mg, Ca, Sr) Pyroborates

Abstract

The electronic structure and linear optical properties of the pyroborate A2B2O5 (A = Mg, Ca, Sr) compounds are reported here. These compounds, which crystallize with four formula units in the monoclinic space group P21/c, are modeled in terms of the cluster units (A2B2O5)2. The calculated electronic structures show that the top of the valence band consists of mostly the O-2p orbitals and the bottom of the conduction band consists of cationic orbitals. The dynamic refractive indices of these pyroborates are obtained in the framework of the INDO/SCI approximation together with the sum-over-states method. It is found that the refractive index increases with an increase of alkaline-earth cationic radius for pyroborates, and the charge-transfer for O2- anion orbitals to A2+ cation orbitals appears to provide a significant contribution to the linear polarizability of these compounds.