LiRb2LaB2O6: a new rare-earth borate with a MOF-5-like topological structure and a short UV cut-off edge

A new rare-earth borate LiRb₂LaB₂O₆ (LB-1) has been synthesized using a high-temperature solution method. Interestingly, it shares its topological structure with the famous porous material MOF-5, which is formed from Zn₄O(BDC)₃ (BDC = 1,4-benzenedicarboxylate). The nodes [OZn₄] and organic linkers [BDC] of MOF-5 are carefully substituted with La³⁺ rare-earth cations and the inorganic linkers [BO₃] and [LiO₄], respectively, in order to construct a pure inorganic open framework. LB-1 exhibits good thermal and water stability. Moreover, it possesses a short cut-off edge (<190 nm) and moderate birefringence (exp. 0.040@589.5 nm, cal. 0.038@589.5 nm). Further insights into the cut-off edge were given by first-principles calculations. The factors that influence the birefringence of LB-1 were discussed in view of the spatial arrangement of the [BO₃] anionic groups.