ELECTRONIC-STRUCTURES AND OPTICAL-PROPERTIES OF LIB3O5, LI2B4O7 AND KBE2BO3F2

The structures of the electronic energy bands of LiB3O5, Li2B4O7 and K Be2BO3F2 solid compounds, containing both ionic and covalent bonds, ha ve been calculated using the INDO/S method first, and the dynamic refr active indices and frequency-dependent second susceptibilities have be en obtained in terms of INDO/SCI following combination with the sum-ov er-states method. It is shown that the covalent interactions contribut e to the lowest valence band and the highest conduction band, and the ionic interactions contribute to the highest valence band and the lowe st conduction band, except that the lowest edge (LUMO) of the conducti on band consists of antibonding interaction orbitals between boron and oxygen atoms. The wavelengths of the absorption edge and the refracti ve indices are predicted to increase in the order: KBe2BO3F2 < Li2B4O7 < LiB3O5. The calculated absorption edges of the bulk compounds LiB3O 5 (169 nm) and KBe2BO3F2 (147 nm) are in agreement with the observed b ulk values, LiB3O5 (160 nm) and KBe2BO3F2 (155 nm), and the calculated average linear refractive indices, n, of bulk KBe2BO3F2 (1.311), Li2B 4O7 (1.357) and LiB3O5 (1.563) at an input wavelength of 1064 nm are c omparable to the observed refractive indices, KBe2BO3F2 (1.450), Li2B4 O7 (1.590) and LiB3O5 (1.594) respectively. The total second susceptib ilities, chi(1), are estimated to be in the order: LiB3O5 (7.73 x 10(- 9) esu) > KBe2BO3F2 (4.75 x 10(-9) esu) > Li2B4O7 (3.38 x 10(-9) esu) at an incident wavelength of 1064 nm. The excited charge-transfer stat es from the 2p orbitals of the anion to the boron atomic valence orbit als make the major contributions to the susceptibility.