Fig. 1. (a) The structure of KBBF crystal; (b) [Be₂BO₃F₂] layer; (c) KBBF crystal^[31]

Fig. 2. (a) The whole structure of RBBF; (b) [Be₂BO₃F₂] layer extended in the ab plane; (c) single crystal of RBBF^[35]

Fig. 3. (a)(b) Electronic densities of ABBF and KBBF; (c) the structure of ABBF; (d) a polished ABBF crystal wafer^［38-39］

Fig. 4. (a) Fundamental building block [B₄O₈F] of ABF structure; (b) [B₄O₆F] layer extended in the bc plane; (c) crystal structure of ABF; (d) block crystals of ABF^[40]

Fig. 5. (a) Fundamental building block of CBF; (b) [B₄O₆F] layer extended in the bc plane; (c) crystal structure of CBF; (d) CBF crystal grown from its stoichiometric melt, inset is the crystal plate of CBF^[41]

Fig. 6. (a) Crystal structure of GFB; (b) single crystal via solution method^[44]

Fig. 7. (a) Crystal structure of CBPO; (b) CBPO crystal grown by melt method^[45]

Fig. 8. (a) The alignments of the [CO₃] groups in type I configuration of KSrCO₃F, RbSrCO₃F and KCaCO₃F; (b) the alignments of the [CO₃] groups in type II configuration in RbCaCO₃F and CsCaCO₃F; (c) KSrCO₃F crystal grown from flux^[49,51]