Ba₄Bi₂(Si_8- B₄₊ O₂₉) (x = 0.09): a new acentric metal borosilicate as a promising nonlinear optical material

A new acentric metal borosilicate, namely Ba₄Bi₂(Si_8- B₄₊ O₂₉) (x = 0.09), has been synthesized by a standard solid-state reaction. The title compound crystallizes in noncentrosymmetric (NCS) space group I4[combining macron]2m with lattice parameters a = 11.0254(4) Å and c = 10.3961(9) Å. Structure refinements indicate that mixing of B atoms and Si atoms exists for a few atomic sites. In the "ideal" Ba₄Bi₂(Si₈B₄O₂₉), BO₄ or SiO₄ tetrahedra are inter-connected by corner-sharing to cyclic B₄O₁₂ or Si₄O₁₂ units. These B₄O₁₂ and Si₄O₁₂ units are further interconnected via corner-sharing to an "ideal" [Si₈B₄O₂₉]^14- 3D network. The Ba²⁺ and Bi³⁺ act as the counter cations and are located at the cavities of the structure. Ba₄Bi₂(Si_8- B₄₊ O₂₉) (x = 0.09) melts incongruently at a high temperature of 929 °C. Powder second-harmonic generation (SHG) measurements reveal that Ba₄Bi₂(Si_8- B₄₊ O₂₉) (x = 0.09) is a type I phase-matching compound with a good SHG response of about 5.1 times that of KDP (KH₂PO₄), which is the highest among the borosilicates reported so far. The SHG source has been studied by DFT theoretical calculations. Our preliminary results indicate that Ba₄Bi₂(Si_8- B₄₊ O₂₉) (x = 0.09) is a new second-order nonlinear-optical crystalline material candidate.