Electronic structures and optical properties of LiB3O5, Li2B4O7 and KBe2BO3F2

Abstract

The structures of the electronic energy bands of LiB₃O₅, Li₂B₄O₇ and KBe₂BO₃F₂ solid compounds, containing both ionic and covalent bonds, have been calculated using the INDO/S method first, and the dynamic refractive indices and frequency-dependent second susceptibilities have been obtained in terms of INDO/SCI following combination with the sum-over-states method. It is shown that the covalent interactions contribute to the lowest valence band and the highest conduction band, and the ionic interactions contribute to the highest valence band and the lowest conduction band, except that the lowest edge (LUMO) of the conduction band consists of antibonding interaction orbitals between boron and oxygen atoms. The wavelengths of the absorption edge and the refractive indices are predicted to increase in the order: KBe₂BO₃F₂ < Li₂B₄O₇ < LiB₃O₅. The calculated absorption edges of the bulk compounds LiB₃O₅(169 nm) and KBe₂BO₃F₂(147 nm) are in agreement with the observed bulk values, LiB₃O₅(160 nm) and KBe₂BO₃F₂(155 nm), and the calculated average linear refractive indices, n, of bulk KBe₂BO₃F₂(1.311), Li₂B₄O₇(1.357) and LiB₃O₅(1.563) at an input wavelength of 1064 nm are comparable to the observed refractive indices, KBe₂BO₃F₂(1.450), Li₂B₄O₇(1.590) and LiB₃O₅(1.594) respectively. The total second susceptibilities, X_t, are estimated to be in the order: LiB₃O₅(7.73 × 10^–9 esu) > KBe₂BO₃F₂(4.75 × 10^–9 esu) > Li₂B₄O₇(3.38 × 10^–9 esu) at an incident wavelength of 1064 nm. The excited charge-transfer states from the 2p orbitals of the anion to the boron atomic valence orbitals make the major contributions to the susceptibility.