Preparation and Structural Characterization of Two New Phases of Aluminum Trifluoride

When certain salts, R⁺AlF₄^- are heated in flowing nitrogen, the equivalent of R⁺F^- is lost at temperatures <500 °C, leaving microcrystalline A1F3. This transformation proceeds via an intermediate material of formula HAlF₄ when R⁺ = pyridineH⁺ or occurs in a single step when, e.g., R⁺ = an organic cation such as N(CH₃)₄⁺. In each case a different, metastable phase of AlF₃ is produced and their structures, both based on comer-shared octahedra of [AlF₆], are delineated using powder diffraction (X-ray and neutron) techniques. When HAlF₄ is an intermediate, the ultimate η-AlF₃ phase has a structure identical to pyrochlore materials of formulae FeF₃ and AlF_X(OH)_3-X. When there is no discrete intermediate, the ultimate 0-AlF3 phase has a structure which has now also been reported by Bentrup et al. During attempts to crystallize HA1F4 from formamide, a new β-phase of NH₄AlF₄ was isolated and characterized by powder diffraction as a layered material having NH₄⁺ ions between fluoroaluminate sheets and with connectivity within the sheets identical to the β-phase of RbAlF₄. The sheet structure loses NH₄F when pyrolyzed, producing yet another new phase, k-AlF₃, which retains the connectivity of the layers from the precursor and simply fuses these layers together in a pseudotopotactic transformation. The resultant structure is closely related to that of the hypothetical end member of the tetragonal tungsten bronzes such as K_xWO₃ when x = 0. All three new phases contain [Al-F-Al] rings which dictate a limited nanoporosity and all convert irreversibly to the thermodynamically stable α-AlF₃ form at temperatures between ~450 and ~650 °C.