Optical properties

A Low-wavelength host absorption edge of LiB$_3$O$_5$ (LBO) and Li$_2$B$_4$O$_7$ crystals

I. N. Ogorodnikov

Ural Federal University named after the First President of Russia B. N. Yeltsin, Ekaterinburg

Abstract: We have carried out an experimental study of the VUF-edge of the optical host absorption of lithium borate crystals LiB$_3$O$_5$ (LBO) and Li$_2$B$_4$O$_7$ (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) $\cdot$ 10$^{-4}$ eV/K was determined. The dependence of the absorption edge parameters of oriented LBO $\perp Y$ and LBO $\perp X$ crystals has been studied. Based on low-temperature reflection spectra ($T$ = 10 K, $\theta$ = 17$^\circ$, $E$ = 4–32 eV) the Kramers–Kronig method was used to calculate the spectra of optical constants: refractive index $(n)$ and absorption index $(k)$, real $(\varepsilon_1)$ and imaginary $(\varepsilon_2)$ parts of the complex permittivity, as well as the absorption coefficient $\mu$. 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 $\varepsilon_2(E)$ spectrum, the thresholds for interband transitions were determined ($E_g$ at $T$ = 10 K): 8.5–8.6 eV(LBO $\perp Y$), 8.6–8.7 eV (LBO $\perp X$) and 8.8–8.9 eV (LTB). The origin of the fundamental absorption edge of lithium borates is discussed.

Keywords: lithium triborate LiB$_3$O$_5$, lithium tetraborate Li$_2$B$_4$O$_7$, host absorption edge, optical properties.

Received: 14.03.2022
Revised: 14.03.2022
Accepted: 15.03.2022

DOI: 10.21883/FTT.2022.07.52568.314

Bibliographic databases:

•