Cation regulation achieves symmetry breaking: from centrosymmetric M2Cs2B10O17 (M = Na, K) to noncentrosymmetric Na3CsB10O17

Abstract

A new noncentrosymmetric crystal, Na3CsB10O17 was obtained through a symmetry-breaking strategy of cation regulation. Meanwhile, two isostructural crystals, K2Cs2B10O17 and Na2Cs2B10O17 were also synthesized and belong to the C2/c space group. All of them exhibit the 2[B10O17]∞ double-layered framework built by the fundamental building blocks, [B5O11], however, alterations in the cationic Na/Cs ratio and coordination number disrupt the inversion symmetry of the 2[B10O17]∞ double-layer in Na3CsB10O17. The UV-Vis-NIR diffuse reflectance spectra indicate that the three compounds show a short cutoff edge, remarkably, Na3CsB10O17 achieves a deep-UV cutoff edge (<190 nm). In addition, theoretical calculations were carried out to confirm the related results and reveal the origin of their optical properties. This work establishes a paradigm for designing NCS borates and advances development of NLO crystals.