A Low-wavelength host absorption edge of LiB-=SUB=-3-=/SUB=-O-=SUB=-5-=/SUB=- and Li-=SUB=-2-=/SUB=-B-=SUB=-4-=/SUB=-O-=SUB=-7-=/SUB=- crystals

Abstract

We have carried out an experimental study of the VUF- edge of the optical host absorption of lithium borate crystals LiB3O5 (LBO) and Li2B4O7 (LTB). The transmission (T=293 K) and absorption (T=80, 293 K) spectra were studied, the short-wavelength edge of the transparency band (cutoff wavelength) and the energy position of the edge fundamental absorption at which the absorption coefficient k=50 cm-1 were determined. The absorption edge temperature shift coefficient -(3-4.7)·10-4 eV/K was determined. The dependence of the absorption edge parameters of oriented LBO normal Y and LBO normal X crystals has been studied. Based on low-temperature reflection spectra (T=10 K, theta=17o, E=4-32 eV) the Kramers-Kronig method was used to calculate the spectra of optical constants: refractive index (n) and absorption index (k), real (ε1) and imaginary (ε2) parts of the complex permittivity, as well as the absorption coefficient μ. The lowest energy peak, due to electronic transitions from the top of the valence band to the states of the bottom of the conduction band, was studied in the ε_2(E) spectrum, the thresholds for interband transitions were determined (Eg at T=10 K): 8.5-8.6 eV(LBO normal Y), 8.6-8.7 eV (LBO normal X) and 8.8-8.9 eV (LTB). The origin of the fundamental absorption edge of lithium borates is discussed. Keywords: Lithium triborate LiB3O5, lithium tetraborate Li2B4O7, host absorption edge, optical properties.