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 <i>P</i>2₁, having lattice parameters of <i>a</i> = 6.4093 (13) Å, <i>b</i> = 8.2898 (17) Å, <i>c</i> = 9.3656 (19) Å, and β = 101.51 (3)° for Sr₃B₆O₁₁F₂ and <i>a</i> = 6.5572 (13) Å, <i>b</i> = 8.5107 (17) Å, <i>c</i> = 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