X-ray diffractometry (XRD)

Description

X-ray diffraction (XRD), or X-ray diffraction, is a physical examination method that can be used to study any type of crystalline matter.

Application

The main field of application of X-ray diffraction is in the geosciences and especially in mineralogy, where it is used for the identification of minerals, phase analysis and structural elucidation. But X-ray diffraction can also be useful for archaeological and archaeometric research. For example, pigments or dyes can be identified, and even mixtures of substances usually cause hardly any problems. In the case of authenticity analyses of metal objects or works of art, the method can also help to detect suspicious substances. For example, if the patina of a metal object contains a corrosion product that does not occur under natural conditions, the object can be exposed as a fake.

Basics

In XRD, the material to be examined is irradiated with X-rays – similar to X-ray fluorescence analysis. In contrast to XRF, however, it is monochromatic, i.e. it consists of X-rays of a specific element with a discrete energy spectrum. Of the different interactions of X-rays with matter, the method makes use of diffraction at the crystal lattice. The X-rays are diffracted at adjacent crystal planes. The subsequent superposition of the electromagnetic waves causes interference and, depending on the path difference of the waves, amplification or extinction. This results in material-characteristic diffraction patterns which, when compared with databases, allow the identification of the substance under investigation. As a rule, a qualitative determination is carried out, but calculation algorithms (e.g. Rietveld method) can also be used to quantitatively determine the quantity of the substances contained.

Limitations

Due to the physical principle, only crystalline substances can be analysed. Amorphous substances, such as glass or amorphous corrosion products, do not provide diffraction patterns and therefore cannot be analysed.

Sample properties

Solid samples that have to be pulverized for analysis by powder diffraction can be examined. Typical sample quantities here are 20-50 mg. Analyses of smaller samples are possible using a µ-diffractometer, which also allows in situ analysis. Therefore, even small areas of intact artifacts can be examined without damaging the object.