Vdovin O. Vibrational spectrum and optical phenomena in Li2B4O7 pyroelectric crystals

The object of research is the physical properties of Li2B4O7 (LB4) crystal. The aim of the work is to ascertain the dynamic nature of optical and nonlinear optical phenomena and the reasons of temperature anomalies of physical properties in LB4 crystals. The research techniques are Raman and IR-spectroscopy, second optical harmonic generation and hyper-Rayleigh light scattering. The complete information about optical vibrational spectrum of LB4 crystals in the temperature range of 100-500 K is obtained. The presence of phase transition at 235 K is established. The isostructural character of phase transition is substantiated and the dominating role of lithium ions in the mechanism of transition is established. The presence of growth striae, which manifest itself in nonlinear diffraction experiments, is established. The temperature anomalies of hyper-Rayleigh scattering intensity, characteristics of nonlinear diffraction and mixed vibrations involving lithium ions due to thermoactivational disordering of Li+ ions are detected at temperatures greater than 375 K. The regions of temperature stability of physical characteristics of LB4 crystals are revealed, that is important in development of devices of functional electronics and nonlinear optics on its basis. The results of work can be useful for the analysis of physical properties of affined borate compounds.