beta-BaAlF5
Although this problem was solved from conventional X-ray data (J. Solid State Chem. 89, 1990, 282-291), it was interesting to try to determine the structure from the neutron powder data (which were used for structure accuracy improvement). The neutron data were collected from the ILL D2B instrument in low resolution mode (wavelength 1.5945 A), so that the U, V ,W are respectively 0.071, -0.166, 0.190 corresponding to FWHMs :
2-theta FWHM 10 0.42 30 0.39 50 0.36 70 0.33 90 0.31 110 0.31 130 0.40 150 0.74
With such high profile widths, can we expect to index the data ? ITO and TREOR90 proved not to be useful in such a case. DICVOL91 produced the expected cell with very low figures of merit (baalf51.html) so that it was not very credible. Applying the Le Bail method was encouraging (baalf51.gif), however there are 1387 reflections up to 147° 2-theta ! What confidence can we have in such a structure factors extraction when the FWHMs are so large ? The next step will give a response.
Try by yourself, downloading the full neutron data (baalf5.zip).
