Powder X-Ray Diffraction (PXRD): Analysis of Crystalline Materials
What Is Powder X-Ray Diffraction?
Powder X-Ray Diffraction (PXRD, also written pXRD or simply XRD) is a non-destructive analytical technique for characterizing crystalline materials by measuring the diffraction pattern produced when X-rays interact with a finely ground powder or polycrystalline solid. Each crystalline material produces a unique diffraction fingerprint — determined by its unit cell dimensions, crystal symmetry, and atomic positions — enabling unambiguous phase identification, crystal structure determination, quantitative phase analysis, and crystallite size measurement.
PXRD is one of the most versatile and widely used analytical methods in materials science, chemistry, and industry — applied across the metals, ceramics, pharmaceuticals, semiconductors, minerals, and polymer materials industries for quality control, research, and failure analysis.
How PXRD Works
X-rays are generated by a sealed tube (Cu Kα radiation at 1.5406 Å is most common) and directed at the powder specimen. Crystalline materials scatter X-rays at specific angles defined by Bragg’s Law:
nλ = 2d·sin(θ)
Where n is an integer, λ is the X-ray wavelength, d is the interplanar spacing between crystal planes, and θ is the angle of incidence/diffraction. Each family of crystallographic planes (hkl) with a specific d-spacing produces a diffraction peak at a defined 2θ angle. The resulting PXRD pattern — a plot of diffracted X-ray intensity versus 2θ — is the material’s crystallographic fingerprint.
Key Applications of PXRD
Phase Identification
Comparison of the measured PXRD pattern against the ICDD (International Centre for Diffraction Data) PDF-4+ database — containing reference patterns for over 500,000 crystalline phases — enables unambiguous identification of all crystalline phases present in a sample. This is the most common PXRD application across:
- Alloy phase verification (martensite, retained austenite, carbides)
- Mineral identification in geological and mining samples
- Cement and construction material phase analysis
- Active pharmaceutical ingredient (API) polymorph identification
Quantitative Phase Analysis (QPA) — Rietveld Refinement
The Rietveld method refines a full-pattern crystal structure model against the measured PXRD data, extracting quantitative weight fractions of all crystalline phases simultaneously. Applications include retained austenite quantification in hardened steel (ASTM E975), API polymorph ratios in multi-phase drug products, and phase ratio monitoring in ceramic sintering processes.
Crystallite Size and Microstrain (Scherrer Analysis)
Peak broadening in PXRD patterns provides information about crystallite size (Scherrer equation: L = Kλ / β·cosθ, where β is the peak full width at half maximum) and lattice microstrain. Nanoparticle size, grain growth monitoring, and cold-work-induced strain quantification all exploit this peak broadening analysis.
Residual Stress Measurement — sin²ψ Method
The d-spacing of specific crystallographic planes shifts measurably with applied or residual stress. The sin²ψ PXRD method measures d-spacings at multiple sample tilts (ψ) and extracts the biaxial surface residual stress non-destructively. This is widely used in aerospace, automotive, and tool manufacturing for quality verification of shot-peened, ground, or carburized components.
Crystallinity Index for Polymers and Cellulose
The ratio of crystalline diffraction peak area to total scattered intensity quantifies the degree of crystallinity in semi-crystalline polymers (PE, PP, PET, nylon) and natural materials (cellulose, starch). Crystallinity affects mechanical properties, barrier performance, and processability.
PXRD Standards
Application | Standard |
Retained austenite in steel | ASTM E975 |
Pharmaceutical polymorph | USP <941>, ICH Q6A |
Cement phase analysis | ASTM C1365 |
Residual stress | ASTM E2860, SAE J784a |
General phase ID | ICDD PDF database |
Conclusion
PXRD provides a uniquely direct window into the crystal structure of materials — delivering phase identification, quantitative composition, crystallite size, and residual stress data from a single non-destructive measurement. Its combination of information depth, minimal sample-preparation requirements, and applicability to virtually any crystalline material makes PXRD an indispensable tool in materials qualification, quality control, and failure analysis programs.
Why Choose Infinita Lab for PXRD and Crystallographic Analysis?
At the core of this breadth is our network of 2,000+ accredited labs in the USA, offering access to over 10,000 test types — including PXRD phase identification, Rietveld quantitative analysis, crystallite size measurement, and residual stress determination. We give clients unmatched flexibility, specialization, and scale — connecting you to the right analytical expertise every time.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you. Request a Quote
Frequently Asked Questions (FAQs)
What is Bragg's Law and why is it fundamental to XRD? Bragg's Law (nλ = 2d·sinθ) relates the X-ray wavelength (λ), interplanar crystal spacing (d), and diffraction angle (θ). It predicts exactly which angles produce constructive interference for each crystal plane family — making the diffraction pattern a direct measurement of the material's internal crystal geometry and enabling phase identification from peak positions.
Can PXRD distinguish between polymorphic forms of the same compound? Yes. Polymorphs are crystallographically distinct — different packing arrangements of identical molecules — and produce different PXRD patterns despite identical chemical composition. PXRD is the definitive method for polymorph identification and quantification in the pharmaceutical industry, where different polymorphs have different solubility and bioavailability.
What sample preparation is required for PXRD analysis? Samples are typically ground to <45 µm particle size to ensure random crystallite orientation (reducing preferred orientation artifacts). Bulk solids, powders, films, and thin coatings can all be analyzed. Minimal sample quantity — typically 100–500 mg — is required. Reactive or air-sensitive samples are measured in sealed sample holders under inert atmosphere.
How does PXRD differ from single-crystal X-ray diffraction? PXRD uses randomly oriented polycrystalline powder — all diffraction peaks appear simultaneously in a 1D pattern, providing phase and d-spacing information. Single-crystal XRD uses one oriented crystal — rotating it to collect full 3D diffraction data, enabling complete crystal structure determination including all atomic positions, bond lengths, and angles. PXRD is faster and more accessible; single-crystal XRD provides more complete structural information.
Can PXRD quantify retained austenite in hardened steel? Yes. ASTM E975 defines the direct comparison method for retained austenite quantification by PXRD — comparing integrated peak intensities of ferrite (martensite) and austenite phases using calibrated reference intensity ratios (RIR). Retained austenite content between approximately 0.5 and 30% can be accurately quantified, relevant to gear, bearing, and tool steel heat treatment quality control.
