Synthesis, X-ray diffraction, and MAS n.m.r. characteristics of tetrahydroxoborate sodalite, Na8[AlSiO4]6[B(OH)4]2

Abstract Hydrothermal crystallization in the system Na2OSiO2Al2O3B2O3H2O reveals the dominant formation of tetrahydroxoborate sodalite, Na8[AlSiO4]6[B(OH)4]2, in the temperature range of 473–773 K at pressures of 100–150 MPa and concentrations of 17 Na2O:2 SiO2:Al2O3:xB2O3:yH2O (x = 0.1–10; y = 260–290). The high-temperature form (T ≥ 310 K) of tetrahydroxoborate sodalite shows cubic symmetry [space group P 4 3n, a 0 = 9.010(1) A ], whereas the superstructure diffraction pattern of the low-temperature form (T ≦ 270 K) has been indexed successfully by means of transformation of the cubic ∼ 9 A subcell to orthorhombic symmetry with a0 = 25.510(1), b0 = 12.750(1), and c 0 = 9.020(1) A . 29Si MAS n.m.r. shows complete ordering of the Si,Al atoms in the tetrahedral framework of tetrahydroxoborate sodalite. The observed single peak of the 11B MAS n.m.r. spectra correlates with the imbibition of [B(OH)4]− at the center of the sodalite cages. Substitution of B for Al in the tetrahedral sites of the sodalite framework and threefold coordinated boron within the sodalite cages or in the framework can be excluded.

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