X-ray diffraction (XRD) is a powerful, non-destructive analytical technique primarily used to determine the atomic and molecular structure of crystalline materials.
1. How It Works
Crystals are regular, repeating 3D arrays of atoms. The spacing between these atomic planes typically ranges from 0.1 to 0.2 nanometers—which happens to be the same magnitude as the wavelength of X-rays.
When a material is bombarded with X-rays:
- The waves scatter off the electrons surrounding the atoms in the crystal lattice.
- If the scattered waves are out of phase, they undergo destructive interference and cancel each other out.
- However, at specific angles, the waves undergo constructive interference, reinforcing one another to produce a sharp intensity peak.
This relationship is governed by Bragg's Law:
Where:
- n = an integer (order of reflection)
- λ = wavelength of the incident X-rays
- d = distance between atomic planes (interplanar spacing)
- θ = Bragg angle (the angle between the incident beam and the scattering planes)
2. Key Applications
Because every crystalline substance produces a unique diffraction pattern—acting essentially as a structural "fingerprint"—XRD is widely utilized across various industries and scientific fields:
- Phase Identification: Rapidly identifying unknown crystalline substances, minerals, and compounds by matching their patterns against standardized reference databases (e.g., the ICDD database).
- Crystal Structure Determination: Mapping the precise positions of atoms, bond lengths, and angles within a molecular lattice.
- Material Characterization: Assessing physical properties such as crystallite size, percent crystallinity, lattice strain, and preferred orientation (texture) in nanomaterials, metals, and polymers.
- Pharmaceutical Industry: Detecting drug polymorphism (different crystal packing forms of the same active pharmaceutical ingredient), which impacts drug stability and bioavailability.
- Geology and Archaeology: Identifying mineral compositions in rocks, soils, clays, and ancient artifacts.
Read also Applications of X-Ray Diffraction
3. Advantages and Limitations
- Advantages: It is non-destructive, requires minimal sample preparation (especially for powder diffraction), provides definitive phase identification, and yields high-resolution data on atomic geometry.
- Limitations: It struggles significantly with entirely amorphous (non-crystalline) materials, and analyzing complex mixtures or overlapping peaks can sometimes require advanced computational refinement (such as Rietveld refinement).
XRD helps detect drug polymorphism, which affects stability and bioavailability of medicines.