DFT-based ab initio study of dielectric and optical properties of bulk Li2B3O4F3 and Li2B6O9F2 Original Research Article

We present results of ab initio DFT-based and experimental studies on electrical, piezoelectric and optical properties of the newly synthesized noncentrosymmetric lithium fluorooxoborates, Li2B3O4F3 and Li2B6O9F2. Polar Li2B6O9F2, space group Cc, was found to possess a relatively large spontaneous polarization and some large components of the elasto-electric tensor, compared to non-polar but noncentrosymmetric Li2B3O4F3, space group P212121. Linear and nonlinear optical properties of the crystals are studied theoretically. Second harmonic generation is possible for both crystals, which is demonstrated by calculations of the phase matching conditions and the second order electronic susceptibility. Experimental measurements have confirmed the expected pyroelectric (but not ferroelectric) and piezoelectric properties of Li2B6O9F2. The intensity of second harmonic generation of Li2B6O9F2 has been revealed experimentally to be of the same order of magnitude as that observed in α-SiO2.