Flat-plate samples for Bragg-Brentano diffractometers

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Preparing flat-plate samples for Bragg-Brentano diffractometers

Flat-plate samples are often considered the easiest ones to prepare and are therefore very popular for both academic and industrial use despite several disadvantages, namely the relatively large amount of sample required and the potential for samples to show preferred orientation effects. The effect of the latter may (and should) be easily tested for by the use of different sample geometries, namely with the use of flat-foil and capillary holders.

This page describes how to prepare a typical flat-plate sample holder for use on the Bruker D4 diffractometer at UCL (or for use on the much older Siemens D500 instrument).

A typical empty sample holder is shown below:

NEVER CLEAN THE HOLDER WITH ORGANIC SOLVENTS
These sample holders are manufactured from plastic that disolves in solvents such as acetone! Note that the holders have been manufactured for undergraduate use with a number engraved on the rear side. The number can be seen correctly when viewed from the sample side with the digits at the bottom (as seen above). The sample holder has grooves to discourage the sample from sliding sideways in the holder during loading. The overall diameter of the holder is about 50 mm and the sample diameter is about 25 mm. This can be compared to a typical beam width of around 8 to 12 mm depending on setup.

Using a spatula, add a small heap of powdered material to the middle of the holder and flatten with a glass microscope slide. Do not add too much: if you have added too much, the simplest procedure is to empty the holder completely and start again. Use the flat face of the glass slide to move material sideways away from the middle and towards the edge. Initially (assuming that only a small amount of material was added), the sample will not reach the edge. Once flat, add more to the centre of the holder and continue to flatten out towards the edges with the glass slide. It is crucial that a flat finish is obtained as shown in the picture below:

The top surface of the holder is the reference height for the instrument and the sample must be level with this. For this reason, one should never use a procedure for always filling the holder with, say, 1 g of material since the mass of material in the holder will vary from one sample type to the next.

Some common errors with flat-plate sample preparation

At an early stage during the sample loading, the sample may look like picture below in the holder, i.e. with a flat top surface level with the holder but the edge around the centre not filled with sample:

The sample should not be run like this, but more sample should be added to the middle of the holder and flattened out with the glass slide towards the edge of the inner circle. An attempt to use this sample on the diffractometer runs the risk of the sample falling out of the holder during the measurement and contaminating the instrument. You will not be popular if this happens!

It is not common to see sample holders underfilled though the picture below is one genuine example showing a case where a student intended to run a sample for which he or she did not have enough material to fill the holder. This sample as prepared will definitely fall out of the holder; fortunately, it was spotted before being loaded onto the diffractometer θ circle.

A good test of whether the sample has been prepared correctly is to gently tilt the holder sideways (but not upside down!). A well-prepared sample will normally stay in the holder when tilted through 80°.

The most common error (shown in an extreme case below) is for sample holders to be overfilled. It is not uncommon to break the glass slides in this case since pressing on opposite ends of the slide will result it in cracking in the middle where the sample is too high. Do not be tempted to remove sample and to re-flatten it: simple empty the holder and start again!

Overfilled sample holders results in the peak positions being shifted away from the true position and, in extreme cases, near total loss of peak intensities.

A variety of different types of flat-plate sample holder for PXRD use can be seen on the on-line powder diffraction course.

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