Phase identification with XRD
The identification of major and minor single or multiple phases in an unknown sample is the main application of classical X-ray powder diffraction. A phase is a crystalline solid with a regular 3-dimensional arrangement of the atoms. The measured diffraction peak positions and intensities are like a fingerprint of a particular crystalline phase. Identification is accomplished by comparison of the measured pattern with the entries in reference databases using a search-match algorithm. This is also known as qualitative phase analysis.
Optimized measurement geometries
Phase identification is the most important application of X-ray powder diffraction (XRD or XRPD). XRPD is not only applied on powder samples but also on polycrystalline solids, suspensions and thin films. Inorganic powder samples are most often measured in the classical Bragg-Brentano reflection geometry. On the other hand, a transmission geometry is normally preferred for organic materials (e.g. pharmaceuticals and polymers), liquid crystalline materials and for suspensions, as it generally gives a better low-angle (below 5 degrees 2theta) performance. In case of thin films a grazing incidence setup is most appropriate.
Typical examples where phase identification is being applied are:
- Identification of minerals in geological samples: phase identification aids understanding of the formation mechanisms of samples, which can provide valuable information regarding the presence of ores or fuels.
- Grade control of ores and rocks: for exploration of mineral deposits.
- Detection of polymorphs to distinguish substances with the same chemical composition in different phases of a certain material, an important task for the pharmaceutical industry.
- Quality control: determination of the presence of impurities in a pure phase. With modern X-ray optics and detectors, impurities down to 0.1 weight-% can be detected
- Detection of phase transitions under non-ambient conditions, such as variable temperature or humidity
- Forensics: phase identification can be a deciding factor in determining the origin of traces found at crime scenes.
- Corrosion in boilers and power plants: the phases found give valuable information about the conditions and reactions leading to problems. Indirectly they give hints on how to prevent or minimize corrosion processes.
- Nanomaterials: for example, the rutile phase of nano-titania is required for UV blocking applications (e.g. in sunscreens) whereas photocatalytic activity requires the anatase phase.
- Polymers and plastics: identification of crystalline phases and polymorphs, as well as of filler materials by WAXS (wide-angle X-ray scattering).
- Liquid crystals: identification of thermotropic and lyotropic liquid crystalline phases (mesophases). In surfactant systems by low-angle diffraction measurements, for example.
XRD solutions for phase ID
PANalytical's Empyrean and X'Pert³ Powder X-ray diffraction systems with their vertical goniometer platforms are well suited for phase identification in powders, thin films, solids and suspensions. These multi-purpose instruments are predominantly used in research environments. High-throughput polymorph screening on a well plate is also supported.
X'Pert³ MRD (XL) systems are ideal for phase identification in polycrystalline and epitaxial thin films.
The CubiX³ instruments are designed for specific industrial applications with sample changer and automation options to accommodate high sample volumes.
HighScore (Plus) is a powerful software package that allows for peak search and easy phase identification, even in complex phase mixtures. HighScore (Plus) allows simultaneous searching across multiple reference databases and offers many options for automation and reporting. Cluster analysis for grouping samples with a similar phase composition is also supported.