Greatly enhancing birefringence of fluorosilicates through the synergetic effect of functional units

Short-wave ultraviolet optical crystals are widely used in various fields. Exploring organic–inorganic hybrid compounds is an effective approach for discovering crystal materials that simultaneously possess large band gaps and birefringence. In this work, two novel organic–inorganic hybrid fluorosilicates, (C2H7N4O)2SiF6 and (C2H4N4)4ZnSiF6·H2O, were obtained. (C2H7N4O)2SiF6 has the largest band gap among organic–inorganic hybrid fluorosilicates and a unique [(C2H7N4O)2SiF6]∞ layered structure, while (C2H4N4)4ZnSiF6·H2O has the largest birefringence (0.282@0.546 μm) among reported organic–inorganic hybrid fluorosilicates. In addition, we systematically analyzed the structural and optical properties upon transformation from (C2H7N4O)2SiF6 to (C2H4N4)4ZnSiF6·H2O.