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

Two novel noncentrosymmetric alkaline earth fluorooxoborates, M2B10O14F6 (M = Ca, Sr), were synthesized and characterized. Both of these two isostructural compounds had layered [B5O7F3](infinity) structures with large second harmonic generation (SHG) responses ranging from 2.3 to 2.5 x 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.