Effect of cation substitution on structural transition: synthesis, characterization and theoretical studies of NaCa4B3O9, NaCaBO3, NaSrBO3 and Li4CaB2O6

Abstract

Single crystals of NaCa₄B₃O₉, NaCaBO₃, NaSrBO₃ and Li₄CaB₂O₆ have been successfully synthesized through conventional high-temperature solid-state reactions. They are structurally characterized by single crystal X-ray diffraction and exhibit three-dimensional crystal structures consisting of isolated planar BO₃ as fundamental building blocks. Interestingly, for the centrosymmetric crystal structure of NaCaBO₃ (Na₃Ca₃B₃O₉), as 2/3 of the Na⁺ ions are substituted by Ca²⁺ ions, NaCa₄B₃O₉ is obtained and crystallizes in the noncentrosymmetric space group Ama2 (crystal class mm2). A second harmonic generation (SHG) test of the title compound by the Kurtz–Perry method shows that NaCa₄B₃O₉ can be phase matchable with an effective SHG coefficient approximately one-half that of KH₂PO₄ (KDP). Studies of their optical properties as well as band structure calculations based on density functional theory methods have been also performed. NaCa₄B₃O₉ possesses a moderate birefringence of about 0.05 at 1064 nm. To explain the difference in optical nonlinearity we compared the electronic structures of NaCa₄B₃O₉, KCa₄B₃O₉ and KSr₄B₃O₉ crystals, in particular at the bottom of the conduction band (CB) and the top of the valence band (VB), since they are known to play a primary role in SHG. These electronic structures are responsible for the optical-nonlinearity of NaCa₄B₃O₉, KCa₄B₃O₉ and KSr₄B₃O₉ crystals.