X-ray Diffraction (XRD) Academy

X-ray diffraction (XRD) is a foundational technology in solid-state science, providing unique insights into the composition and structural arrangement of crystallized samples. The relevant data are obtained by measuring the diffracted intensity of a primary X-ray beam against the diffraction angle, as per Bragg’s Law. In compliance with this law, different geometries in transmission or reflection mode can be arranged to analyze the samples.

X-Ray diffraction is a phenomenon in which the atoms of a crystal, by virtue of their uniform spacing, cause an interference pattern of the waves in an incident beam of X-rays. The crystal's atomic planes act on the X-rays in the same way a uniformly ruled grating acts on a beam of light (see polarization). The interference pattern is specific to each substance and gives information on the structure of the atoms or molecules in the crystal.

In an XRD measurement, a sample is placed within a chamber and is bombarded with a beam of primary X-rays at an angle defined by the relevant diffraction geometry. The Bragg-Brentano geometry is among the most common methods used in powder XRD, otherwise known as XRPD. It relies on multiple moving parts to detect the secondary scattered X-rays from a sample and obtain a full diffraction pattern.

Provided Bragg’s Law has been satisfied, when the primary X-ray beam interacts with the sample it will diffract and produce a secondary beam that can be acquired by a detector. This yields a diffraction pattern that is characteristic of the unique crystallographic structure of the sample.

In contrast to this Bragg-Brentano geometry that requires a goniometer with mobile parts, it is possible to obtain a real-time full pattern without this motor constraint by using a Thermo Scientific PSD (Position Sensitive Detector). Schematic of this geometry is shown here.

XRD patterns allow to perform a variety of structural investigations, such as fingerprinting of phase content, which requires a comparison of the relative intensities and positions of reflections against existing data in available databases. This is one of the most widely accepted methods for phase control (including the polymorphism considerations) and quantification in many research and industrial processes.