ORIENTATIONAL DISORDER OF THE HYDROGEN DIHYDROXIDE ANION, O2H3-, IN SODIUM HYDROXOSODALITE DIHYDRATE, NA8[AL6SI6O24](OH)2.2H2O - SINGLE-CRYSTAL X-RAY AND POWDER NEUTRON-DIFFRACTION AND MAS NMR AND FT IR SPECTROSCOPY

The crystal structure of sodium hydroxosodalite dihydrate, Na8[Al6Si6O24](OH)2.2H2O, is cubic at 173 K with space group P43nBAR (Z = 1) and the cell constants a = 8.875 (2) angstrom (X-ray), a = 8.87 (5) angstrom (neutron, nondeuterated form), and a = 8.86 (5) angstrom (neutron, deuterated form). The 1:1 aluminosilicate framework is a strictly alternating arrangement of corner-sharing AlO4 and SiO4 tetrahedra. Each [4(6)6(8)] polyhedral cage is occupied by four Na cations, located close to oxygen atoms of six-membered rings of the framework, and, near the center, by a 6-fold or 12-fold orientationally disordered hydrogen dihydroxide anion, O2H3-, of point symmetry 2 (C2) with a very strong central hydrogen bond O...H...O with O...O distances at 2.36 (4) angstrom (nondeuterated powder sample) and 2.28 (4) angstrom (deuterated powder sample). The hydrogen atom in the central hydrogen bond is probably (dynamically) disordered between two positions near the 2-fold axis, i.e. the anion is probably of the type [HO...(H,H)...OH]-. No hydrogen bonding exists between the terminal OH groups of the anion and framework oxygen atoms. The structure model is in line with the H-1 MAS NMR spectrum with chemical shifts at 16.3 ppm (central hydrogen atom) and -0.1 ppm (terminal hydrogen atoms) and the mid IR spectra of the nondeuterated and deuterated forms. The structural formula [Na4(O2H3)]2[Al6Si6O24] is suggested for hydroxosodalite dihydrate. Reversible structural-phase transitions were verified by DSC measurements which showed two heat effects at 150 +/- 1 K and 154 +/- 1 K for nondeuterated, but only one heat effect at 150 +/- 1 K for deuterated samples. Structure chemical considerations suggest that a recently published orthorhombic crystal structure for the low-temperature phase may be incorrect.