Na3AEZn2B3O9 (AE = Mg, Ca): two new short-wave ultraviolet beryllium-free Sr2Be2B2O7-type zincoborates designed by chemical cosubstitution

Abstract

Exploration of short-wave ultraviolet beryllium-free Sr₂Be₂B₂O₇-type borates is of great significance to the development of short-wave ultraviolet lasers. As a structural analogy to BeO₄, the ZnO₄ unit has been demonstrated as a facile building unit for the design of beryllium-free borates with balanced overall performance. Nevertheless, despite the discovery of numerous zincoborates, no Sr₂Be₂B₂O₇-type zincoborates with characteristic bilayer structures constituted by ZnO₄ and BO₃ units have been found, highlighting the great challenge in the design and syntheses of Sr₂Be₂B₂O₇-type zincoborates. Herein, two new beryllium-free Sr₂Be₂B₂O₇-type zincoborates, namely, Na₃AEZn₂B₃O₉ (AE = Mg and Ca), were rationally obtained by a chemical cosubstitution design strategy. With the cosubstitution of Sr²⁺, Be²⁺ and out-of-plane bridging O²⁻ of {Sr[Be₂(BO₃)₂O]}_∞²⁻ bilayers of Sr₂Be₂B₂O₇, Na₃AEZn₂B₃O₉ (AE = Mg and Ca) featuring a new Sr₂Be₂B₂O₇-type bilayer structure of {Na₃[Zn₂(BO₃)₂(BO₃)]}_∞²⁻ were successfully isolated, indicating the feasibility of chemical cosubstitution for the design of Sr₂Be₂B₂O₇-type zincoborates. In particular, Na₃CaZn₂B₃O₉ exhibits a wide band gap of 4.43 eV, a moderate birefringence of 0.045@1064 nm and a high melting point of 843 °C, being favorable to practical applications.