PREPARATION AND STRUCTURAL CHARACTERIZATION OF 2 NEW PHASES OF ALUMINUM TRIFLUORIDE

When certain salts, R(+)AlF(4)(-) are heated in flowing nitrogen, the equivalent of R(+)F(-) is lost at temperatures <500 degrees C, leaving microcrystalline AlF3. This transformation proceeds via an intermediate material of formula HalF(4) 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 corner-shared octahedra of [AlF6], are delineated using powder diffraction (X-ray and neutron) techniques. When HAlF4 is an intermediate, the ultimate eta-AlF3 phase has a structure identical to pyrochlore materials of formulae FeF3 and AlFx(OH)(3-x). When there is no discrete intermediate, the ultimate theta-AlF3 phase has a structure which has now also been reported by Bentrup et al. During attempts to crystallize HAlF4 from formamide, a new beta-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 beta-phase of RbAlF4. The sheet structure loses NH4F when pyrolyzed, producing yet another new phase, kappa-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 KxWO3 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 alpha-AlF3 farm at temperatures between similar to 450 and similar to 650 degrees C.