M₂B₁₀O₁₄F₆ (M = Ca, Sr): Two Noncentrosymmetric Alkaline Earth Fluorooxoborates as Promising Next-Generation Deep-Ultraviolet Nonlinear Optical Materials

Abstract

Two novel noncentrosymmetric alkaline earth fluorooxoborates, M₂B₁₀O₁₄F₆ (M = Ca, Sr), were synthesized and characterized. Both of these two isostructural compounds had layered [B₅O₇F₃]_∞ structures with large second harmonic generation (SHG) responses ranging from 2.3 to 2.5 × KH₂PO₄ (KDP) and short UV absorption edges (<200 nm). The first-principles calculation demonstrated that their nonlinear optical (NLO) properties were superior to those of KBe₂BO₃F₂ (KBBF). In contrast to the alkali fluorooxoborates, these two fluorooxoborates showed not only remarkable stability against air and moisture but also high thermal stability. Therefore, M₂B₁₀O₁₄F₆ (M = Ca, Sr) should be promising deep-ultraviolet (DUV) NLO materials.