Fig. 2

Crystal structural features of Pna2₁-Ba₃Mg₃(BO₃)₃F₃. a, b The structural evolution from Sr₂Be₂B₂O₇ to Pna2₁-Ba₃Mg₃(BO₃)₃F₃. c, d The orientation of [BO₃]³⁻ groups in Pna2₁- and P\(\bar 6\)2m-Ba₃Mg₃(BO₃)₃F₃, respectively. e The layered structure of derivative P\(\bar 6\)2m-Ba₃Mg₃(BO₃)₃F₃ derived by phase transition from Pna2₁-Ba₃Mg₃(BO₃)₃F₃