A comparison total scattering data from various sources: the case of a nanometric spinel

Nanocrystalline spinel gahnite (ZnAl2O4, 2-3 nm) has been measured with three different instruments: (i) ID31 high resolution diffractometer at the ESRF (9 line detectors, all with crystal analyzers, Qmax=22 Å-1), (ii) ID15B high energy beamline at the ESRF with 2D image plate detector (Qmax=32 Å-1) , (iii) PANalytical diffractometer with silver Kα, with an X'Celerator detector (Qmax=20.5 Å-1). The Pair Distribution Functions (PDFs) were obtained with PDFgetX3 [1], by subtracting the relevant empty capillary scattering, and correcting and normalizing the data. The same procedure was applied to all data sets. The diffraction geometries and the wavelengths of the incident X-rays were chosen in order to make the comparison as comprehensive as possible, and the material under analysis was chosen for its nanometric (and possibly disordered) nature, to give rise to a challenge for all the diffractometers involved. None of the latter should have a clear advantage: for instance, even the large resolution, in the direct space, of ID15B is not enough to resolve the very small difference between Zn-O and Al-O bonds in the first PDF peak; besides the material is nanometric (about 2-3 nm), so the very high resolution of ID31 does not bring a clear benefit to its PDF, if we disregard its smaller Qdamp, (i.e. a gentler function envelop at high r).The PDF and F(Q) functions were visually compared, and then the three PDFs were refined by means of PDFgui [2], using the simplest possible model of a spinel (totally direct, with space group symmetry respected). All the refinements were made exactly in the same way, by refining the same parameters, for the sake of a fair comparison. Small differences could be highlighted in the results, and all the data sets were perfectly and easily refinable. Differences in the refined parameters could also be detected on PDFs obtained with a different procedure for background subtraction, so attention should be paid to the procedure of PDF attainment from total scattering data. [1] P. Juhas, T. Davis, C.L. Farrow and S.J.L. Billinge, J. Appl. Cryst. 2013, 46, 560 [2] C.L. Farrow, P. Juhás, J.W. Liu, D. Bryndin, E.S. Božin, J. Bloch, Th. Proffen and S.J.L. Billinge, J. Phys.: Condens.