Rational design of a series of non-centrosymmetric antiperovskite and double antiperovskite borate fluorides

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 F-centered octahedra to construct a perovskite-type framework filled by 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-dimensional framework constructed from [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 P4̄2₁ m, and can exhibit moderate second harmonic generation (SHG) responses (∼0.5 × KDP@1064 nm) and short UV cut-off edges (∼190 nm), as well as suitable birefringence (Δn = 0.0405-0.0548@532 nm). This suggests their potential as UV nonlinear optical crystals.