Pressure-Induced Crystallization and Characterization of the Tin Borate β-SnB₄O₇

In the last years, several investigations were performed in the ternary system Sn−B−O as a simplified variant of the tin-based amorphous composite oxide (TCO), a material in use as negative electrode of lithium-ion rechargeable batteries. All compounds in this system are glasses, so the synthesis of crystalline approximands for a more detailed structural investigation would be favorable. The use of high-pressure/high-temperature conditions (7.5 GPa and 1100 °C; Walker-type multianvil apparatus) led to the synthesis of the first crystalline tin borate β-SnB₄O₇. The single-crystal structure determination of β-SnB₄O₇ showed Pmn2₁, a = 1086.4(2) pm, b = 444.80(9) pm, c = 423.96(8) pm, Z = 2, R₁ = 0.0155, and wR₂ = 0.0324 (all data). In contrast to the isotypic phases MB₄O₇ (M = Sr, Pb, Eu, β-Ca, and β-Hg), the position of the tin atom in β-SnB₄O₇ is influenced by the existence of a stereochemically active lone pair. Furthermore, we report on thermoanalytical aspects (DTA-TG and temperature-resolved in situ powder diffraction), DFT calculations, IR spectroscopy, Mössbauer spectroscopic results, and solid-state NMR investigations on β-SnB₄O₇. The latter method allows us to make a general differentiation of Sn²⁺ and Sn⁴⁺ in Sn−O systems on the basis of well-separated ^117/119Sn chemical shifts.