t-AlF3
This compound is obtained from an amorphous AlF3.xH2O or from [(CH3)4N]AlF4.H2O by dehydration (J. Solid State Chem. 100, 1992, 151-159). It transformed into the cubic AlF3 variety at high temperature without anymore weight loss so that it was unbelievably a new AlF3 variety (see the negative result from a PDF-2 search by EVA-2) ! Such disclosing is more and more rare so that the structural study was quickly undertaken. Some proof that t-AlF3 is really interesting may be found in the fact that it was recently redetermined, also from powder data (Chem. Mater. 7, 1995, 75-93) and evenmore, it was renamed theta-AlF3. We found no possibility for obtaining single crystal from this soft chemistry synthesis. Impurities were always present in large quantity when the sample was made from the amorphous starting material. Fortunately, the second preparation was purest with only small residue. The powder pattern was recorded from 10 to 150° 2-theta on a Siemens D500 diffractometer, using the side-loaded sample holder. The guessed harmonics method provided a zeropoint of 0.02° 2-theta. The TREOR90/ITO/DICVOL91 results were finally consistent with a tetragonal cell although many orthorhombic or even cubic cells were proposed. By carefully examining the data, then the space groups P4/nmm or P4/n were obvious. The cell confirmation was concluded by the Le Bail method, excluding a few angular zones where the impurities were contributing (talf32.gif).
Try by yourself, downloading the full data (talf3.zip).
