Analysis of Raman Spectra of LiB3O5 Single Crystals

Abstract

Polarized Raman spectra of LBO single crystal were recorded in the temperature range 300-870 K. At room temperature, 26A,, 27A,, 26B, and 26B2 modes expected from group theoretical analysis are observed by various scattering geometries. Among them, the internal bands are tentatively assigned to vibrations of the BO, triangle and BO, tetrahedron in terms of factor group analysis. Based on the temperature dependence of the wavenumbers, the Griineisen parameters of A,(TO), &(TO) and &(TO) modes are calculated and discussed. No soft modes are observed in the above temperature range, implying that no displacive-type phase transition takes place there. The crystal structure of LBO below 870 K is stable. 105 optical vibration modes, 26A1 + 27A, + 26B, Modes A,, B , and B, are also infrared active, their polarizations being parallel to the c, a and b axes of the crystal, respectively.

Temperature-dependent birefringence of lithium triborate, LBO in the THz regime

“…2 and the following figures are not accurate region due to the limitation of the THz-TDS dynamic range. The absorption coefficient curves α X and α Z at all temperatures increase monotonically with frequency and do not appear any absorption peaks due to the fact that the lowest transverse optical phonon vibration mode is located at a higher frequency 3.3 THz at 300 K 20 . The absorption coefficients α Z and α X decrease with dropping temperature.…”

“…The absorption coefficients α Z and α X decrease with dropping temperature. This may be related to the following behaviors: (1) the low-frequency optical phonon modes B 1T and B 2T increase monotonically with decreasing temperature 20 ; (2) the Raman bands become narrow and blue shift at low-frequencies 21 . The absorption coefficient in the X-axis is not over 5 cm −1 in the range of 0.2–2 THz which is far less than that in the Z-axis.…”

“…Based on the group theory, the space group of the LBO crystal is Pna2 1 (). At zero wave vector, the optical phonon modes have the following form 20, 22 : , where all of the optical modes are Raman active, and A 1 , B 1 and B 2 are also infrared active. The polarizations of A 1 , B 1 and B 2 are parallel to the crystal axis c, a, b, respectively, which correspond to the crystal principal dielectric axes Y, X, Z.…”

“…As we could not find enough Raman or IR spectra data of LBO at 297–77 K, we discuss the changes from the qualitative perspective. The Raman spectra data of LBO at room temperature 300 K and liquid nitrogen temperature 80 K are cited from refs 20 and 22, respectively. The influence of 3 K temperature difference between the data and our experiments can be ignored since it can hardly affect the optical properties of LBO.…”

“…These outstanding merits make LBO a potential terahertz source 19 . The properties of LBO in the infrared and far infrared regions above 100 cm −1 (3 THz) have been studied by Raman and infrared reflection spectroscopy 20–23 . However, the optical properties of LBO crystal in the terahertz range have not been fully characterized and systematically studied, especially for different temperatures 24–27 .…”