t-AlF₃

Discovering new allotropic form of any element is always remarkable. It is now rare too to discover new forms of binary compounds. Although 100 or so zeolites are known, a new way to build a 3D four-connected net (in tridimensional AB₂ frameworks of AB₄ tetrahedra connected exclusively by corners, the A atoms, being connected to 4 other A atoms through the B atoms, lie at the points at a 3D 4-connected net) is of interest a fortiori if the net was not in the list of 1000 or so guessed possibilities built from combinations of previously existing nets. This is why t-AlF₃ is distinguished here (t stands for tetragonal in order to avoid confusion with the previously known alpha-, beta- and gamma-AlF₃ varieties). The 3D six-connected net family is poor and t-AlF₃ represents the ninth recently discovered AB₃ framework of AB₆ octahedra connected exclusively by corners. Its structure was never predicted. No isotypic coumpound has been prepared to date. It is probably not yet mentioned in any inorganic chemistry book at the AB₃ chapter.

The serendipitous discovery of t-AlF₃ came from the neutron time-resolved study of the thermolysis of an amorphous AlF₃.xH₂O (J.L. Fourquet and A. Le Bail, ILL Report, Experiment 5.22.313, 1987, p. 150). The low resolution of the neutron powder pattern did not allow to select a cell among tetragonal and cubic possibilities. Reproducing the experiment by using a limited quantity under an X-ray beam proved to be difficult, the beta-AlF₃ form growing first followed by more or less t-AlF₃ quantity appearing only after the water complete loss. So the problem stayed unsolved in spite of renewed efforts during several years.

Then t-AlF₃ was observed as the thermal decomposition product of [(CH₃)₄N]AlF₄.H₂O and a tetragonal cell was evidenced (U. Bentrup, Eur. J. Solid State Inorg. Chem. 29, 1992, p. 51). The compound was almost pure and relatively well crystallized but in powder form only. We made use of the recent development in structure determination from powder diffraction data to solve it ab initio from its X-ray powder pattern. The space group proved to be P4/nmm, with Z=16, a = 10.1843(3) and b = 7.1738(2) Angstroms as refined by the Rietveld method. The direct methods applied to 426 reflections extracted from the powder pattern allowed to locate all the 11 independent atomic sites, as practically required because Al³⁺ and F^- are isoelectronic (A. Le Bail, J.L. Fourquet and U. Bentrup, J. Solid State Chem. 100, 1992, p. 151).

The t-AlF₃ structure did not presented new rings of AB₆ octahedra. The whole 6-connected 3D net family is built from rings of 3, 4, 5 and 6 octahedra. However, four new building units were inside t-AlF₃. The 6 previously known are tetrahedra of octahedra 3⁴ (a geomerical object bounded by 4 triangular faces), prisms with triangular basis 3²4³ (2 triangular faces and 3 tetragonal faces), tetragonal basis 4⁶ (six tetragonal faces : a cube), pentagonal basis 4⁵5², hexagonal basis 4⁶6² and finally the Friauf polyhedra 3⁴6⁴ characterizing the pyrochlore structure. Only the A atoms at the net points are shown below for the four new building units existing in t-AlF₃ :

The t-AlF₃ discovery allowed to recognize Na₄Ca₄Al₇F₃₃ (A. Hemon and G. Courbion, J. Solid State Chem. 84, 1990, p. 153) as a stuffed polytype which should be added to the 6-connected 3D net family, the AB₃ octahedral framework being [Ca₄Al₇F₃₃]. Compact clusters of 10 octahedra and even 15 are part of the t-AlF₃ and Na₄Ca₄Al₇F₃₃ structures, respectively, they are built exclusively from the 4³ units :

As a conclusion, the reality recalls us the inefficiency of our current efforts in order to predict crystalline structures, even relatively simple. We cannot be so sure that rings of 7 or 8 octahedra will never be encountered in the small 6-connected 3D nets family. To obtain new structure-types is easy for quaternary or even ternary coumpounds, but how many will again surprise us with binary formula or how many allotropic forms remain to be discovered for elements ? Even if we could successfully predict them, could we predict how to synthesize them ? A challenge would be to obtain t-ReO₃, t-WO₃ or t-FeF₃, for instance, there is no crystallochemical obstacle to their existence, but how to make them ?!