High-temperature, high-pressure hydrothermal synthesis, crystal structure, and solid state NMR spectroscopy of a lead borosilicate with boron–silicon mixing: Pb6B2Si8O25

Abstract

A new lead(II) borosilicate, Pb₆B₂Si₈O₂₅ (1), has been synthesized by a high-temperature, high-pressure hydrothermal reaction at 480 °C and 990 bar. Its structure was determined by single-crystal X-ray diffraction. The reaction product was phase-pure as indicated by powder X-ray diffraction and whole pattern fitting using the Pawley method. Compound 1 has a 2D layer structure with the lead ions being located at interlayer regions. Each layer is formed of corner-sharing BO₄ or SiO₄ tetrahedra and contains an eight-ring window. The layer consists of a new fundamental building block (FBB) with the formula T₈O₂₃ (T: B or Si) formed by two (B(1)_0.8Si(1)_0.2)O₄ tetrahedra and six (Si(2)_0.933B(2)_0.067)O₄ tetrahedra. The FBB can be described as double open-branched triple tetrahedra. Another interesting structural feature of 1 is boron–silicon mixing which is uncommon in borosilicates. There are three unique tetrahedra in the structure: B(1)O₄ tetrahedra with 20% substitution of Si for B, Si(2)O₄ tetrahedra with 6.67% substitution of B for Si, and Si(3)O₄ tetrahedra without substitution. We have applied ¹¹B and ²⁹Si MAS NMR spectroscopy to study the substitutional disorder. The NMR study results not only confirm the mixing of B and Si atoms in the structure, but also verify quantitatively the results from single-crystal X-ray diffraction. The reasons for boron–silicon mixing in the structure are discussed. Crystal data for 1: trigonal, Rc (no. 167), a = 9.5090(2) Å, c = 42.3552(8) Å, V = 3316.7(2) ų, Z = 6, R₁ = 0.0147, and wR₂ = 0.0309.