Sodium aluminogermanate hydroxosodalite hydrate Na6+x[Al6Ge6O24](OH)x · nH2O (x ≈ 1.6, n ≈ 3.0): Synthesis,phase transitions and dynamical disorder of the hydrogen dihydroxide anion,H3O2−, in the Cubic high-temperature form

Crystalline sodium aluminogermanate hydroxosodalite hydrate Na_6+xAl₆Ge₆O₂₄](OH)_x · nH₂O with x ≈ 1.6 and n ≈ 3.0 has been synthesized by reacting Al₂O₃, GeO₂ and NaOH solution under hydrothermal conditions, and characterized by means of simultaneous thermal analysis, differential scanning calorimetry, X-ray and neutron diffraction as well as ¹H and ²³Na MAS NMR and IR spectroscopy. The material undergoes a reversible structural phase transition at T_c = 166 K (heating mode), which is actually a complex two-step transformation as detected in DSC measurements. Structure refinements of the cubic high-temperature form (cell constant a = 9.034(2) Å, room temperature) with single-crystal X-ray and powder neutron diffraction data have not yielded overall satisfactory results, probably due to the solid-solution character of the hydrosodalite. The refinements nevertheless demonstrate that (i) the sodalite host framework is a strictly alternating array of corner-linked AlO₄ and GeO₄ tetrahedra, and (ii) most polyhedral 4⁶6⁸] cavities are occupied by four sodium cations and one orientationally disordered hydrogen dihydroxide anion, H₃O₂^?, which possesses a strong central hydrogen bond. Variable-temperature ¹H MAS NMR spectra unambiguously confirm the presence of H₃O₂^? ions and, in addition, reveal a dynamical intraionic exchange between the central and terminal protons and a rotational diffusion of those anions to occur in the high-temperature form. The nature of the guest complexes filling the remaining cages could not be unambiguously determined. Results are compared with those obtained in recent studies on the related sodium aluminosilicate hydrosodalite system of the general formula Na_6+xAl₆Si₆O₂₄] (OH)_x · nH₂O.