Rational Design of a Series of Non-centrosymmetric Antiperovskite and double Antiperovskite Borate Fluorides

Abstract

Non-centrosymmetric (NCS) compounds can exhibit many symmetry-dependent functional properties yet their rational structure design remains a great challenge. Herein, a strategy to introduce the F-centered octahedra to construct the peroveskite-type framework filled by the π-conjugated [B2O5]4− dimers is proposed to obtain NCS compounds. The first examples of antiperovskite or double antiperovskite borate fluorides, [(M/Ba)2Ca]F[B2O5] (M = K, Rb) and [CsBaCa]F[B2O5] have been successfully designed and synthesized. All three compounds exhibit a novel three-dimensinoal framework constructed by [F(M/Ba)4Ca2] (M=K, Rb), [FCs4Ca2] and [FBa4Ca2] octahedra, which are further filled by [B2O5]4− dimers to form antiperovskite-type structures. They all crystallize in the NCS space group of P-421m, and can exhibit moderate second harmonic generation (SHG) responses (~0.5 × KDP@1064 nm), short UV cut-off edges (~190 nm) as well as suitable birefringence (Δn = 0.0405 - 0.0548@532nm). These suggest their potentials as UV nonlinear optical crystals.