The very first normal-pressure tin borate Sn3B4O9, and the intermediate Sn2[B7O12]F

Abstract

Aside from amorphous phases, only a single crystalline tin borate had been synthesised so far under high pressure. In this work, we present the very first crystalline ternary tin borate Sn₃B₄O₉ synthesised under ambient pressure by decomposition of Sn₃[B₃O₇]F above 500 °C. The crystal structure of Sn₃B₄O₉ (P2₁/c, Z = 4, a = 768.07(3) pm, b = 1206.78(4) pm, c = 924.96(3) pm, β = 101.847(1)°, 10 550 data, 147 parameters, R₁ = 0.028) determined by single-crystal X-ray diffraction comprises open layered borate polyanions with Sn(II) ions in-between showing the presence of a stereochemically active lone pair. Sn₃B₄O₉ was further characterized by DFT calculations and vibrational spectroscopy. Its optical band gap was calculated to approx. 3.5(1) eV. Tin borate was gained by thermal decomposition of Sn[B₂O₃F₂] via a further new tin borate fluoride Sn₂[B₇O₁₂]F (C2/c, Z = 8, a = 1037.99(2) pm, b = 859.78(2) pm, c = 2370.71(8) pm, β = 93.5650(10)°, 2674 data, 199 parameters, R₁ = 0.045).