Electronic structures and linear optics of A(2)B(2)O(5) (A = Mg, Ca, Sr) pyroborates

The electronic structure and linear optical properties of the pyroborate A( 2)B(2)O(5) (A = Mg, Ca, Sr) compounds are reported here. These compounds, w hich crystallize with four formula units in the monoclinic space group P2(1 )/c, are modeled in terms of the cluster units (A(2)B(2)O(5))(2). The calcu lated 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 a re obtained in the framework of the INDO/SCI approximation together with th e sum-over-states method. It is found that the refractive index increases w ith an increase of alkaline-earth cationic radius for pyroborates, and the charge-transfer for O2- anion orbitals to A(2+) cation orbitals appears to provide a significant contribution to the linear polarizability of these co mpounds.