Effect of anion dimensionality on optical properties: the ∞[B7O10(OH)2] layer in CsB7O10(OH)2vs. the ∞[B7O12] framework in CsBaB7O12

Abstract

Two new caesium borates, CsB₇O₁₀(OH)₂ and CsBaB₇O₁₂, were synthesized in a high-temperature vacuum system and high-temperature open system, respectively. They all crystallize in the monoclinic space groups, C2/c (no. 15) and P2₁/c (no. 14), respectively. Their structures show different dimensional anionic architectures. CsB₇O₁₀(OH)₂ has two-dimensional (2D) _∞[B₇O₁₀(OH)₂] anionic layers with the [B₇O₁₂(OH)₂] fundamental building block (FBB) which displays a novel connection mode. CsBaB₇O₁₂ possesses a three-dimensional (3D) _∞[B₇O₁₂] anionic open-framework with the [B₇O₁₅] FBB. The characterization of their properties combined with experimental and theoretical methods reveals that the two hepta-borates show differences in their optical properties due to their different optical anisotropies which are influenced by their anionic structures. The former compound CsB₇O₁₀(OH)₂ presents a wide band gap of 6.6 eV and the latter compound CsBaB₇O₁₂ presents a band gap of 5.6 eV, which is smaller than that of CsB₇O₁₀(OH)₂. The first-principles calculation results suggest that the birefringences are 0.08 and 0.05 at 1064 nm for CsB₇O₁₀(OH)₂ and CsBaB₇O₁₂, respectively. Furthermore, the detailed summaries and structural comparisons have been provided for all available disorder-free mixed alkali- and alkaline-earth metal caesium borates.