Fluoroaluminates crystallization

The common structural element in the crystal lattice of fluoroaluminates is the hexafluoroaluminate octahedron, AIF. The differing stmctural features of the fluoroaluminates confer distinct physical properties to the species as compared to aluminum trifluoride. For example, in A1F. all corners are shared and the crystal becomes a giant molecule of very high melting point (13). In KAIF, all four equatorial atoms of each octahedron are shared and a layer lattice results. When the ratio of fluorine to aluminum is 6, as in cryoHte, Na AlF, the AIFp ions are separate and bound in position by the balancing metal ions. Fluorine atoms may be shared between octahedrons. When opposite corners of each octahedron are shared with a corner of each neighboring octahedron, an infinite chain is formed as, for example, in TI AIF [33897-68-6]. More complex relations exist in chioUte, wherein one-third of the hexafluoroaluminate octahedra share four corners each and two-thirds share only two corners (14). 

The validity of this approach can be demonstrated by the example of several complex fluoride compounds that exhibit ferroelectric properties, such as compounds that belong to the SrAlF5 family [402, 403]. The crystal structure of the compounds is made up of chains of fluoroaluminate octahedrons that are separated by another type of chains - ramified chains. Other examples are the compounds Sr3Fe2Fi2 and PbsWjOgFio. In this case, the chains of iron- or tungsten-containing octahedrons are separated from one another by isolated complexes with an octahedral configuration [423,424]. 

Chemical dealumination involves reaction of the zeolite framework with any one of a variety of reagents(2). In this work, zeolites were reacted with ammonium hexafluorosilicate in aqueous solution(9-12) to prepare dealuminated products. Aluminum was extracted from the zeolite framework and removed from the crystal as a soluble fluoroaluminate complex the resulting lattice vacancies are believed to be filled by silicon in solution. Composition profiles of chemically dealuminated zeolites (AFS) are homogeneous and indicate the entire crystal is accessible for dealumination(13). Sorption data indicate that AFS zeolites do not possess a secondary pore system although pore blockage may occur due to occlusion of fluoroaluminate species(13). 

Izumitani, X, X Yamashita, M. Tokida, K.. Miura and H. Tajima, 1987, Physical, chemical properties and crystallization tendency of the new fluoroaluminate glasses, in Halide Glasses for Infrared Fiberoptics, ed. R.M. Almeida (Martinus Nijhotf, Dordrecht) pp. 187-196. Jacoboni, C, B. Boulard and P. Baniel, 1987, Mater. Sci. Forum 19-20, 253.. 

Unlike ordinary Portland clinker, alite-fluoroaluminate clinker is brownish in color, rather than dark gray. It typically possesses a very low porosity, and the alite and behte crystals that are present are distinctly smaller than those in ordinary Portland clinker. Only some of the fluorine is in the CjjA2.Cap2 phase the rest is incorporated into alite (Odler and Abdul-Maula, 1980). 

A1 EFG tensors in the crystal frame and allowed a quantitative interpretation of the tensor element orientations and magnitudes in terms of electron densities and octahedron distortions. Electron density maps (Figure 5.13) suggested that the magnitude and orientation of the A1 EFG tensors in fluoroaluminates mainly result from the asymmetric distribution of the A1 3p orbital valence electrons. 

This chapter is devoted to the crystallization of fluoroaluminates. Emphasis is given to hybrid organic-inorganic salts that are synthesized in hydro(solvo)thermal conditions numerous polyanions, derived from the primary building anion (AlFg) , are now evidenced, together with extended one- or two-dimensional architectures. These species are compared with the (Al, F) units that are found in inorganic fluoroaluminates for which a classification of minerals was proposed by F. C. Hawthorne in 1984 . ... 

L. R. Morss, Crystal stmcture of dipotassium sodium fluoroaluminate (elpasolite), J. Inorg. Nucl. Chem., 36, 3876-3878 (1974). 

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