LaBeB3O7: a new phase-matchable nonlinear optical crystal exclusively containing the tetrahedral XO4 (X = B and Be) anionic groups

Abstract

A new noncentrosymmetric lanthanum beryllium borate compound, LaBeB₃O₇, has been designed and synthesized by spontaneous crystallization with molten flux. In its crystal structure, boron and beryllium atoms are mutual disorder occupied and coordinated with four oxygen atoms forming XO₄ tetrahedra (X = B, Be). The XO₄ tetrahedra pile up with each other in the alignment toward the c-axis and build up BeB₅O₁₈ six-membered rings along the ab plane with La cations located inside the ring channels. Its structure is isotypic to the SrB₄O₇. LaBeB₃O₇ melts congruently at 1100 °C and exhibits the short wavelength absorption edge at 220 nm. Second harmonic generation (SHG) on polycrystalline samples was measured using the Kurtz and Perry technique, which indicated that this compound is phase-matchable and the measured SHG coefficient was about 1–2 times that of KH₂PO₄ (KDP). Theoretical calculations were carried out to explain the increasing of birefringence in this crystal structure. Our preliminary results indicate that LaBeB₃O₇ is a promising NLO crystal.