Page Comparison

It is important to characterise the samples for a PDF experiment as comprehensively as possible before your beamtime. The three key properties that are required are;

1. Sample composition - Quantitative analysis should be performed to identify the most accurate composition possible for the samples. This is the full composition for the sample, including any water, binder or support materials that may be present, not just the composition of the material of interest.

2. Density - The density of the sample should be known, preferably in g cm^-3

3. Packing fraction - This is a value between 0-1 and is often estimated to be ca. 0.6.

The sample composition, density and packing fraction are essential parameters that should be known as accurately as possible in order to perform the data reduction step and generate the PDFs.

A range of suitable analysis methods should be performed such as XRF, TGA/DSC-MS etc. to characterise the sample in advance of the beamtime.

Additional information such as amorphous content, concentration of impurity phases etc. will also be useful during analysis.

Additional Factors

There are additional factors that should be considered regarding the samples, such as:

• Are there any peaks at low Q? This may require the beam stop to be moved further from the sample. If you have a peak at Q ≤ 1 Å^-1 then your local contact should be made aware before the experiment. Q is defined as:

\begin{align}
Q&=4\pi\sin\theta/\lambda 
& = 2\pi/d
\end{align}

• Have you simulated the pdf and the scattering data for comparison?

• Do you have .cif files that you can bring with you?

• Which of your samples are likely to be the most strongly scattering? This can be effected by the concentration of highly scattering elements, the crystallinity of the sample and the presence of large crystallites in your powder.

• Is your sample a "good" powder? Are there likely to be larger crystallites that need further grinding to give a good powder average?

• Have you packed as many capillaries as you can at your home institution? We will post empty capillaries to you where necessary. Please refer to the "Know Your Setup" section and discuss with your local contact.

All samples should be labelled with the visit number, date, name of the investigator/group and a suitable description of the material (i.e. the full composition including any support materials/binders)