Two novel centrosymmetric barium strontium borates with a deep-UV cut-off edge: Ba2Sr3B4O11 and Ba3Sr3B4O12

Abstract

Two novel strontium barium borates, Ba2Sr3B4O11 and Ba3Sr3B4O12 have been synthesized and their crystal structures were characterized by a single-crystal X-ray diffraction. Ba2Sr3B4O11 is monoclinic, sp.gr. C2/c and based on isolated BO3 triangles and B2O5 pyroborate groups in relation of 1:2. The crystal structure of Ba2Sr3B4O11 has no structural analogues and belongs to a new structural type. Ba3Sr3B4O12 is tetragonal, sp.gr. I4/mcm and based on isolated BO3 triangles oriented in a non-parallel manner. Ba3Sr3B4O12 is a member of the “anti-zeolite” borate family, and structurally close to the Ba3B2O6 borate. Sr and Ba cations are ordered in Ba2Sr3B4O11 and partially disordered in Ba3Sr3B4O12. The structure of Ba3Sr3B4O12 contains BO3 triangles orientationally disordered over the 4 and 8 orientations, while the structure of Ba2Sr3B4O11 contains B2O5 pyroborate groups orientationally disordered over the 2 orientations. Ba2Sr3B4O11 and Ba3Sr3B4O12 exhibit a short cut-off edge lower than 210 and 206 nm, respectively. Thermal behaviour of these borates including Ba5B4O11 was studied by high-temperature X-ray powder diffraction in the 20–800°С temperature range. Thermal expansion is strongly anisotropic for monoclinic Ba2Sr3B4O11 (α11 = 2.8(3), α22 = 5.6(1), α33 = 38.0(5) × 10−6 °С−1 at 200°С), less anisotropic for orthorhombic Ba5B4O11 (α11 = 9.9(2), α22 = 11.7(3), α33 = 20.1(2) × 10−6 °С−1 at 200°С) and close to the isotropic for tetragonal Ba3Sr3B4O12 (α11 = α22 = 12.8(2), α33 = 20.0(1) × 10−6 °С−1 at 200°С). The new compounds were also characterized by thermal analysis, vibrational and UV–Vis–NIR spectroscopy and Ba2Sr3B4O11 was characterized by DFT calculations.