Two Novel Acentric Borate Fluorides: M3B6O11F2 (M = Sr, Ba)

Two novel, noncentrosymmetric borate fluorides, Sr3B6O11F2 and Ba3B6O11F2, have been synthesized hydrothermally and their structures determined. The compounds are isostructural, crystallizing in space group P21, having lattice parameters of a = 6.4093 (13) Å, b = 8.2898 (17) Å, c = 9.3656 (19) Å, and β = 101.51 (3)° for Sr3B6O11F2 and a = 6.5572 (13) Å, b = 8.5107 (17) Å, c = 9.6726 (19) Å, and β = 101.21 (3)° for Ba3B6O11F2. The structure consists of a complex triple-ring borate framework having aligned triangular [BO3] groups that impart polarity. Fluorine atoms are bound only to the alkaline-earth metals and are not part of the borate framework, resulting in a vastly different structure from those of the hydrated borates Sr3B6O11(OH)2 and Ba3B6O11(OH)2 with similar formulas. The title compounds are transparent to nearly 200 nm, making them potentially useful for deep-ultraviolet nonlinear-optical applications.