X-ray diffraction (XRD) can identify crystalline phases, quantify mixtures, measure internal strain, probe thin films, characterize nanoparticles, and even study materials that lack long-range order. The technique works by directing X-rays at a sample and recording how the atoms scatter them into a pattern of angles and intensities that acts like a structural fingerprint. What makes XRD so versatile is that researchers keep pushing it into new territory, from batteries charging in real time to paintings hanging on museum walls.
Identifying and Quantifying Crystalline Phases
The most familiar use of XRD is figuring out what crystalline phases a sample contains. Every crystalline material produces a unique diffraction pattern, so matching the peaks in a measured pattern against a database of known compounds tells you what is present. This is straightforward for simple samples, but real-world materials are often mixtures of many phases, and knowing that a phase is present is only half the story. You usually want to know how much of each phase is there.
Quantitative phase analysis got a major boost from a modified version of the Rietveld method, which fits a calculated diffraction pattern to the entire measured pattern rather than relying on individual peak heights. This approach does not require calibration standards or an internal reference, though you do need to know the approximate crystal structure of each phase in the mixture. When tested on synthetic mixtures, the method delivered errors generally below one percent absolute.1Journal of Applied Crystallography. Quantitative Phase Analysis Using the Rietveld Method The technique has since been validated on complex natural samples including granites, bauxites, and biogenic carbonates, making it a workhorse in geoscience and materials science.2American Mineralogist. Quantitative mineralogical analysis using the Rietveld full-pattern fitting method
If a sample contains an amorphous (non-crystalline) component, standard XRD won’t produce sharp peaks from that fraction. But you can get around this by adding a known amount of an internal standard. The amorphous content is then calculated from the discrepancy between the known standard amount and the amount the Rietveld fit reports.3Journal of Applied Crystallography. Quantitative Phase Analysis Using the Rietveld Method This trick is widely used in the cement industry, where the glassy fraction of clinker strongly affects how the final product behaves.
Identifying Clay Minerals in Geology
Clay minerals are a special case that illustrates how XRD analysis sometimes requires detective work beyond a simple pattern match. Clays are layered silicates whose basal spacings shift depending on what sits between the layers and how the sample is treated. Kaolinite shows a sharp reflection near 7.15 Å that stays put whether you expose it to ethylene glycol vapor or heat it to 550 °C. Illite has a prominent peak near 10 Å that is similarly stubborn. Smectites like montmorillonite, on the other hand, reflect near 12 to 15 Å when air-dried, expand to about 17 Å after glycolation, and collapse to about 10 Å on heating. Chlorite sits near 14.2 Å and holds steady under glycolation, but its secondary peak at 7.1 Å survives heating, which distinguishes it from kaolinite.4IntechOpen. Clay Mineral Identification Using X-Ray Diffraction
Geologists routinely run these sequential treatments on the same sample to confirm which clay minerals are present. The approach is essential for oil and gas exploration, soil science, and environmental remediation, where the type of clay controls everything from how water moves through rock to how contaminants are retained.
Measuring Crystallite Size and Lattice Strain
XRD peaks are not infinitely sharp. They broaden when crystallites are small or when the crystal lattice is strained. Separating these two contributions gives you useful microstructural information. The simplest way to estimate crystallite size is through the Scherrer equation, which relates peak breadth to size, but it ignores strain entirely. To tease apart size and strain effects, researchers commonly use the Williamson-Hall method, which plots peak broadening against diffraction angle. The size-dependent broadening scales with one angular function and the strain-dependent broadening scales with another, so plotting them together lets you separate the two.5Results in Materials. Easy and green synthesis of TiO2 (Anatase and Rutile): Estimation of crystallite size using Scherrer equation, Williamson-Hall plot, Monshi-Scherrer Model, size-strain plot, Halder- Wagner Model
This kind of analysis has become routine in nanomaterial research. For instance, ZnO nanoparticles have been characterized by both the Williamson-Hall method and the size-strain plot method to understand how synthesis conditions affect the final product’s microstructure.6Solid State Sciences. X-ray analysis of ZnO nanoparticles by Williamson–Hall and size–strain plot methods The results feed directly into decisions about catalytic activity, optical properties, and mechanical performance, since all of these depend on grain size and internal stress.
Probing Thin Films
Thin films present a challenge for conventional XRD because the film may be only nanometers to a few micrometers thick, sitting on a much bulkier substrate. If you aim X-rays at a normal angle, most of the signal comes from the substrate and very little from the film. Grazing-incidence XRD (GIXD) solves this by sending the X-ray beam in at a very shallow angle, so it skims along the film surface and interacts with the film material much more strongly.
Below the critical angle for total reflection, only the top tens of angstroms of material are sampled. By stepping the incidence angle in increments as small as 0.005°, researchers can profile the structure at different depths within the film.7Journal of Materials Research. Grazing incidence synchrotron x-ray diffraction method for analyzing thin films This capability is essential in semiconductor manufacturing, where a few atomic layers of the wrong phase can ruin device performance.
Getting reliable intensities from GIXD experiments requires careful correction for polarization, absorption, and geometric factors. Recent work has derived detailed analytical equations for these corrections when using static area detectors, achieving reliability factors as low as about two percent for three-dimensional powder films.8Journal of Applied Crystallography. Intensity corrections for grazing-incidence X-ray diffraction of thin films using static area detectors That level of accuracy means GIXD can now be used not just to identify phases in thin films but to refine their structures quantitatively.
Characterizing Nanostructures with Small-Angle Scattering
Standard wide-angle XRD tells you about atomic-scale periodicity, but many questions in nanoscience involve structures at a larger length scale: how big are the nanoparticles, what shape are they, and how are they organized? Small-angle X-ray scattering (SAXS) captures this information by measuring the scattering pattern at very low angles, where the signal is sensitive to features ranging from roughly one nanometer to several hundred nanometers.
SAXS can determine a particle’s size, size distribution, shape, and hierarchical organization, and it works on samples in realistic conditions rather than requiring special preparation.9PubMed. Small Angle X-ray Scattering for Nanoparticle Research In biology, the related technique of small-angle solution scattering has become a powerful tool for studying protein folding, intrinsic disorder, and the assembly of multi-component complexes in solution, complementing crystallography and NMR.10PubMed Central. Emerging applications of small angle solution scattering in structural biology Researchers studying drug delivery nanoparticles, colloidal catalysts, or self-assembling polymers all lean heavily on SAXS because it gives structural information without drying, freezing, or otherwise altering the sample.
Studying Amorphous and Disordered Materials
XRD is often described as a technique for crystalline materials, and that reputation is earned: the sharp Bragg peaks it produces come from long-range atomic order. But materials that lack that order, like glasses, gels, and many pharmaceutical compounds, are not invisible to X-rays. They just produce broad, diffuse scattering instead of sharp peaks. The challenge is extracting useful structural information from that featureless hump.
Pair distribution function (PDF) analysis meets that challenge. Instead of interpreting individual peaks, PDF analysis Fourier-transforms the entire scattering pattern to produce a real-space map of atom-to-atom distances. For amorphous silica, PDF analysis reveals short-range periodicity within the basic tetrahedral building block, with characteristic Si-O and O-O distances that differ measurably from those in crystalline quartz.11Journal of Non-Crystalline Solids. Study of short range structure of amorphous Silica from PDF using Ag radiation in laboratory XRD system, RAMAN and NEXAFS The technique has also been adapted for thin films by using high-energy X-rays at normal incidence, opening up local-structure studies of amorphous coatings and functional layers.12PubMed Central. Demonstration of thin film pair distribution function analysis (tfPDF) for the study of local structure in amorphous and crystalline thin films
The pharmaceutical industry finds this particularly valuable. Many drugs are formulated in amorphous forms to improve solubility, but different degrees of local ordering within nominally amorphous material can affect stability and bioavailability. Standard XRD and thermal analysis cannot distinguish between those subtle structural differences, whereas PDF analysis can identify local ordering that conventional methods miss entirely.13PubMed. Probing the Amorphous State of Pharmaceutical Compounds Within Mesoporous Material Using Pair Distribution Function Analysis
Controlling Drug Polymorphs
Pharmaceutical polymorphism is one of the higher-stakes applications of XRD. The same drug molecule can crystallize in different arrangements, and these polymorphs can differ substantially in solubility, stability, melting point, and how easily they compress into tablets. Manufacturing steps like granulation, compression, coating, and drying can trigger conversion from one polymorph to another, so the polymorphic content of a drug needs to be monitored throughout production and stability testing.14PubMed. Quantitative determination of two polymorphic forms of imatinib mesylate in a drug substance and tablet formulation by X-ray powder diffraction, differential scanning calorimetry and attenuated total reflectance Fourier transform infrared spectroscopy
When researchers compared powder XRD against differential scanning calorimetry and infrared spectroscopy for quantifying two polymorphic forms of the cancer drug imatinib mesylate, XRD came out as the most accurate technique and was selected as the preferred method for validation.15PubMed. Quantitative determination of two polymorphic forms of imatinib mesylate in a drug substance and tablet formulation by X-ray powder diffraction, differential scanning calorimetry and attenuated total reflectance Fourier transform infrared spectroscopy Regulatory agencies expect drug makers to demonstrate control over polymorphic form, and XRD is typically the first-line analytical tool for meeting that requirement.
Watching Reactions Happen in Real Time
One of the more exciting frontiers for XRD is in situ and operando measurement, where you collect diffraction data while the sample is actively undergoing a process. In battery research, this means tracking the structural changes in electrode materials as a cell charges and discharges. A recently developed platform couples operando XRD with simultaneous strain monitoring, so researchers can correlate the electrochemical performance of a battery with mechanical stress measurements in real time.16PubMed. Tracking the Electrochemical-Mechanical Evolution by Coupled Operando X-ray Diffraction and Strain Monitoring This kind of integrated measurement reveals failure mechanisms that post-mortem analysis would miss, because the damaged structure often relaxes or transforms after the cell stops cycling.
At the extreme end, XRD has been coupled with dynamic diamond anvil cells and laser heating to study phase transitions at pressures up to hundreds of gigapascals and high temperatures simultaneously. Compression rates up to 500 GPa per second have been achieved, with millisecond time resolution sufficient to capture the kinetics of structural transitions under these extraordinary conditions.17Physical Review Research. Phase transition kinetics revealed by in situ x-ray diffraction in laser-heated dynamic diamond anvil cells This pushes XRD into planetary-science territory, simulating the pressures found deep inside Earth or other planets to understand what phases exist there.
Measuring Residual Stress
When a metal part is machined, heat-treated, or shot-peened, it retains internal stresses even after the external load is removed. These residual stresses affect fatigue life, corrosion resistance, and dimensional stability. XRD measures residual stress by detecting tiny shifts in the lattice spacing caused by internal strain. The principle is simple: if atoms are pushed closer together or pulled apart by stress, the diffraction peaks shift to slightly different angles than they would in a stress-free reference.
This measurement is a standard inspection method for aerospace turbofan bearings, where residual stress levels are critical to safe operation.18Residual Stress Applications. Residual Stress Measurement of Aerospace Turbofan Bearings via X-Ray Diffraction It is also used widely in automotive, nuclear, and biomedical implant industries. Synchrotron sources with micron-level spatial resolution can map strain variations across small regions of a sample, which has been demonstrated on martensitic steel samples and thin-film solar cells.19Journal of Synchrotron Radiation. X-ray diffraction with micrometre spatial resolution for highly absorbing samples High-energy photons above 35 keV allow this even in highly absorbing materials that would stop conventional laboratory X-rays.
Examining Art and Cultural Heritage Without Damage
Museums and conservation labs increasingly use portable XRD instruments to identify pigments, corrosion products, and other materials in artworks without taking samples. A portable diffractometer was brought to the Rijksmuseum and successfully analyzed two seventeenth-century oil paintings, one by Johannes Vermeer and one a copy after Jan Davidsz de Heem. The instrument identified ultramarine blue, lead-tin yellow type I, and Naples yellow directly from their diffraction patterns.20Microchemical Journal. Development of a new portable X-ray powder diffractometer and its demonstration to on-site analysis of two selected old master paintings from the Rijksmuseum
Another portable system combining XRD and X-ray fluorescence has been used on more than 70 works of art in situ, including ceramics, paintings, bronzes, and manuscripts.21Microscopy and Microanalysis. Portable Apparatus for In Situ X-Ray Diffraction and Fluorescence Analyses of Artworks Knowing which pigments an artist used helps establish authenticity, date works, and guide conservation treatments. XRF alone can tell you which chemical elements are present, but multiple pigments can share the same elements. XRD adds the crystallographic fingerprint that distinguishes them.
Why Sample Preparation Matters More Than You Think
For all its power, XRD is sensitive to how a sample is prepared, and poor preparation can introduce systematic errors that skew results. Two common culprits are preferred orientation and microabsorption. Preferred orientation happens when plate-like or needle-shaped crystals align during sample packing, overrepresenting certain crystal directions and distorting the relative peak intensities. Microabsorption occurs in mixtures of phases with very different X-ray absorption coefficients, causing the heavier phases to appear underrepresented.
Research on chromate waste treatment showed that micromilling the sample with a light, nonpolar, highly evaporative liquid like cyclohexane for just five minutes mitigated both effects. The milling produced finer and more uniform particle size distributions than hand pulverizing, while also being the fastest preparation method tested.22PubMed. Influence of x-ray diffraction sample preparation on quantitative mineralogy: implications for chromate waste treatment Anyone doing quantitative work with XRD needs to take preparation seriously, because the analytical method can only be as good as the sample it is looking at.
Pairing XRD with Other Techniques
XRD excels at identifying crystalline phases, but it has blind spots. It struggles with amorphous materials (unless you use PDF analysis), it cannot easily detect phases present below roughly one to two weight percent, and it tells you about crystal structure but not about elemental composition or chemical bonding in the way that spectroscopic methods do. This is why many analytical workflows combine XRD with complementary techniques.
A project within the EU’s SOLSA initiative developed an automated expert system that couples Raman spectroscopy, X-ray fluorescence, and XRD for mineral analysis. The researchers demonstrated cases where characterization by any single technique failed, but the combination of all three succeeded even on very complex samples.23Journal of Raman Spectroscopy. Mineralogical investigations using XRD, XRF, and Raman spectroscopy in a combined approach Raman spectroscopy is sensitive to molecular vibrations and can distinguish polymorphs and amorphous phases that XRD handles less gracefully, while XRF provides fast elemental analysis that narrows down which phases to look for in the diffraction data. The three together cover structural, vibrational, and compositional information, leaving far fewer ambiguities than any one alone.
In pharmaceutical settings, a similar philosophy applies. Infrared spectroscopy, thermal analysis, and solid-state NMR all complement XRD for characterizing drug substances. In biological research, SAXS is routinely combined with crystallography and solution NMR to build more complete structural models of proteins and their assemblies. The recurring theme is that XRD provides one essential perspective on a material’s structure, and pairing it with the right partner technique fills in what it cannot see on its own.