ACaBO3 (A = Cs, Rb): two new cubic borates with isolated BO3 groups

Abstract

Two alkali metal and alkaline-earth metal borates, ACaBO₃ (A = Cs, Rb), were synthesized by a high temperature solution method. The powder SHG response of CsCaBO₃ is approximately one time that of KH₂PO₄ (KDP), and its experimental band gap is 4.20 eV. The SHG density calculations indicate that the SHG effect originates from the synergistic effect of B–O and Ca–O groups. The two compounds crystallize in the cubic space group P2₁3 and their structures feature a 3D framework composed of the CaO₆ octahedra, the CsO₉ polyhedra and the BO₃ triangles. However, borates containing anisotropic planar BO₃ triangles exhibit difficulty in forming an isotropic cubic structure. Through our investigation on all the ordered cubic structures with the BO₃ or the B₂O₅ (composed of two BO₃) groups, the MO₆ octahedra play important role in their structure formations. This discovery is essential for the design of new cubic borates containing the BO₃ triangles.