Li2Ca5Tb(BO3)5: An orthoborate with large spherical hollow cages

A new rare-earth orthoborate Li₂Ca₅Tb(BO₃)₅ with an unusual 3D framework structure has been successfully obtained from a high-temperature solution in a platinum crucible. It crystallizes in the orthorhombic space group Pnma (62) with lattice parameters of a = 19.264(4) Å, b = 10.408(2) Å, c = 6.5336(13) Å and Z = 4. Its structure consists of near spherical hollow cages with size of d_max = 7.897, d_mid = 6.532 and d_min = 5.643 Å built by (CaǀTb)O₈, CaO₈, CaO₆, LiO₅ and BO₃ polyhedra. Furthermore, the thermal stability was investigated using differential scanning calorimeter (DSC). It shows strong green luminescence due to the characteristic emission ⁵D₄ to ⁷F₅ transitions of Tb³⁺ ions.