X-ray diffraction is a technique for investigating the fine structure of matter. It was discovered by Von Laue, in 1912, who observed that the manner in which a crystal diffracts X-rays can reveal its structure. In 1916, Debye and Scherrer proposed to use this technique on powder for the identification and quantification of crystalline compounds, and later developed the Debye-Scherrer X-ray diffraction camera. In the 1940’s, the technique became commonly used for phase identification with the publication of the “Powder Diffraction Files” database. Today, Xray diffraction is the prime technique for studying crystalline structure and identifying crystallized compounds on the basis of their structure.

Soft X-rays interact with matter in several ways, one of which being a coherent elastic scattering by electron shells of atoms. When solid matter is structurally organized in a crystal, the coherently scattered radiation emitted from periodically arranged atoms leads to constructive interference in specific directions and destructive interference in all others. As shown in the figures below, a constructive interference is consistent with the reflection of the X-ray beam by a crystalline plane in a particular orientation condition determined by Bragg’s law, 2d.sinθ=nλ, with d the interplanar distance of the considered plane, λ the wavelength of the radiation, and θ the incident and reflected angles relative to the atomic plane.

Bragg reflection

Performing an XRD analysis consists of measuring the angles and intensities at which crystalline matter reflects X-rays. Each crystal structure has a set of possible reflections that occur when the crystal is appropriately oriented in an X-ray beam. Measuring all possible reflections of a sample in an angular range allows determination of its crystalline structure or identification of its nature on the basis of its structure.  It is critical to the understanding of XRD to realize that diffraction occurs only under very specific conditions. Indeed, a given crystal placed in a monochromatic beam at any given orientation would most likely lead to no diffraction at all. A diffraction beam can be generated from this crystal only if its orientation in the beam is such that the Bragg condition is met for a type of plane of the lattice.

 Powder XRD

The most commonly used crystallographic approach of XRD is powder diffraction (pXRD). The sample in pXRD is a powdered (polycrystalline) material, which is composed of many small crystallites that randomly assume all possible orientations with respect to the incident beam. As illustrated in  below, in a pXRD experiment a relatively small proportion of the grains contribute to a given diffracted beam. Higher numbers of randomly oriented grains exposed to the X-ray lead to better statistical representation for any given diffraction direction. This is referred to as “particle statistics”.

Classic pXRD instruments require a sample to be analyzed be finely ground in order to provide sufficient number of random oriented particles. Recent advances brought as a result of the inXitu XRD program has resulted in similiar diffraction patterns from particles which are much larger in size, and therefore easier to prepare. This is brought about through the process of sample convection. Relying on the principles of fluid dynamics, the patented inXitu sample vibration chamber, allows for a sample whose particle size is between 5-150um to "flow" in the sample cell causing a spinning effect on the crystal particles. This spinning effect (best seen through this video) provides for a similar random orientation as one would obtain through more complicated sample preparation. The impact of this methodology can bee seen in the actual CCD images of a sample which has no convection on the left, and a sample XRD image obtained using the vibration chamber.