2.2.1- High quality pattern

Here is the end of routine work. In order to obtain an accuracy of ± 0.02° 2-theta, a high quality powder pattern should be recorded. The diffractometer should be adjusted to its highest resolution leading to fittable patterns. On a Siemens D500 diffractometer equipped with a graphite monochromator in the reflected beam, the 0.15° receiving slit leads to minimal Full Widths at Half Maximum (FWHM) of the order of 0.14 to 0.20° 2-theta, this may be sufficient. At the cost of a notable decrease in intensity (1/3 to 1/10), minimal FWHMs can be lowered to 0.12 or 0.08° 2-theta with a receiving slit of 0.05° or 0.018°. A monochromator in the incident beam allows the Kalpha-2 suppression (some people think that there is no other way to do a good job, not me) care to fluorescency problems with this configuration. With such FWHMs, the counting step should be as low as 0.02° 2-theta, or even smaller. All this is possible on a conventional X-ray powder diffractometer. Now, don't forget all the problems described previously related to the sample preparation on its holder (preferred orientation...). The most recent diffractometers equipped with variable aperture slits allow to obtain FWHMs as low as 0.04° 2-theta at low diffracting angles (<40° 2-theta), whereas their resolution characteristics remain the same as those of the diffractometers with fixed slit aperture at larger angle (50-160° 2-theta). Such an excellent resolution at low angle is very decisive for succeeding in indexing a powder pattern, you need to decrease the counting step to 0.01° 2-theta or lower.

We were concerned in conventional in-laboratory diffractometers up to now. The 'Rolls' of the diffractometers consume monochromatic synchrotron radiation. This is another world. The recent performances claimed at the ESRF are FWHMs as low as 0.008° 2-theta. Such a value can be attained only if the sample is really free of structure imperfections and if the grain sizes are larger than 1 micron (should be lower than 60 or better, lower than 20 or 5 microns for expecting good statistics in grain orientation). Imagine that 4 points are necessary for the description of this quite narrow peak for the upper part above the FWHM, then the full number of points for the description of a pattern becomes hardly manageable by most of the old Rietveld packages (77500 points for a pattern ranging from 5 to 160° 2-theta, but in fact, low wavelengths are selected and the maximum diffracting angle can be chosen as low as 70 or 80° 2-theta, because an unpacked sample fall down for larger values in most of the synchrotron facilities). Small protein structure determination from powder diffraction data by means of these fantastic instruments is not utopia. These high (potential) performances are due to the quality of the synchrotron radiation allowing parallel beam geometry, few dispersion in the monochromatization process, high brilliance... there are only advantages to synchrotron radiation with one exception : it is generally not in your laboratory so that you cannot have immediate access, in principle.

The neutron powder diffractometers cannot offer comparable performances. Their minimal FWHMs are near of 0.12 or 0.20 or even 0.30° 2-theta depending on the instrument (see NIST or instruments D2B ou D1A at the ILL). An interesting point is that these performances are attained at large angles (90-130° 2-theta), in a range where the conventional X-ray diffractometers cannot do better (they may eventually perform worst in this range). Instruments measuring Time of Flight (ToF) neutrons from spallation sources (see ISIS) give results comparable to those measuring neutrons with fixed wavelength.

We come back now to conventional X-ray diffractometers and to indexing problems. Samples which would have diffracted synchrotron radiation and/or neutrons without having before diffracted conventional X-rays will not be numerous. The access to these prestigious facilities is selective. You have to submit a proposal in which you demonstrate that synchrotron or neutron radiations are absolutely required for the research project. This supposes that you have failed in solving your problem by conventional means or that you need an improvement in accuracy on atomic positions of light atoms (neutrons) or want to make use of the possibility of fine tuning the wavelength near of an absorption edge (synchrotron), etc.

Two techniques are useful in the objective of collecting perfect data for indexing purposes :

According to the technique selected, the high quality pattern which will produce data for indexation can be realized (on a standard in-laboratory instrument) : The time needed for recording such a high quality pattern may be reduced to 1 or 2 hours or less if your apparatus is equipped with a linear detector. Care that the resolution offered by such an instrument remains sufficiently good. Sometimes the whole data have to be corrected by applying a previously established calibration curve (for instance with the remarkable INEL detector covering 120° 2-theta).

Finally, we have yet discussed about the fact that excellent data (even better than those provided by some Bragg-Brentano diffractometers) can be obtained from a Guinier camera. This possibility should not be neglected.


Copyright © 1997- Armel Le Bail