1.2- The routine powder diffraction pattern

The scope of a fast recorded powder pattern is mainly for identifying your sample in order to avoid redetermining a known structure.

1.2.1- Preparing the sample

It should be kept in mind that problems could arise from small grain sizes, preferred orientation effects , sensitivity to hydration...

About grain size : by definition, a powder adequate for X-rays should present a large number of very small crystals oriented in all directions. The optimal grain size is near of one micron, this is very difficult to realize. As a minimum, one should mill the sample by hands, pass it through a 63µ (or less) sieve. Not all samples let you to do that easily, it is extremely difficult to do it manually through a smaller sieve. Be careful, milling can reduce the crystallinity degree of your sample (unusual if the milling is by hand, almost general if a crushing machine is used).

Once the powder prepared, you have to deposit it on the holder :

- It can be pressed on the holder with a glass slide. Doing this will probably lead to favouring a preferred orientation of the individual grains. This can prevent the later use of reflection intensities. Preferred orientation effects can be reduced by making a suspension of the sample in chloroforme that will be evaporated on the sample holder. The mixture could be spread out when pasty with a glass slide just before the full liquid evaporation.

- A very efficient way for avoiding preferred orientation is to dust the sample through a sieve on the holder. The disadvantage is that a flat surface will be diifficult to prepare, such a flat surface being absolutely necessary for not to degrade the resolution (excepted when working with parallel beam).

- If you have enough sample (500 mg or 1 g), use a special sample holder which can be vertically loaded. However don't shake it down too much. Samples very sensitive to preferred orientation can show a non-negligible effect even with this special holder.

In short, the ideal sample is quite hard to prepare.

1.2.2- Measurement conditions

Warning : the sample must be in the diffracting plane otherwise systematic errors will occur on the reflection positions. Verify the diffractometer settings, the zero, the theta-2-theta coupling...

For a routine pattern with a 0.15° receiving slit, the following recording conditions may lead to a successful identification :

a- From 5 to 77°2-theta, with a counting step of 0.08°2-theta, a total of 900 points measured in 30 minutes if counting 2 seconds per point.

b- From 5 to 41°2-theta could be sufficient with 4 seconds per point, all other conditions similar.

The figures 6 and 7 present patterns measured in the a) conditions for one of the samples in the scenario, testing two preparation ways (figure 6 : sample pressed on a glass slide and figure 7 : inserted in the vertical loading holder) : a preferred orientation is obvious as can be concluded from notable differences on intensities.

1.2.3- Data treatment

According to the scenario (Bragg Brentano diffractometer with a diffracted beam monochromator), the alpha 1-2 doublet and the background have to be subtracted first. This operation is usually done very fast by a PC software generally proposed by the diffractometer vendor (for instance EVA as part of the DIFFRACT-AT suite from the Socabim society is proposed with the Siemens diffractometers). This point will be examined at the stage of recording a high quality pattern (chapter 2.2.1). Once the pattern cleaned, it can be compared with those inside your databank, expecting for identification.


Copyright © 1997- Armel Le Bail