Powder X-ray diffraction (PXRD) is a fundamental analytical technique used across scientific and industrial disciplines. It offers a non-destructive way to examine the atomic and molecular structure of materials. Through the interaction of X-rays with a sample, PXRD provides detailed insights into the arrangement of atoms, aiding in the understanding and development of new materials.
What Powder X-ray Diffraction Is
Powder X-ray diffraction is a non-destructive method used to characterize crystalline materials. It operates by exposing a finely ground sample, composed of numerous tiny, randomly oriented crystals, to a beam of X-rays. The “powder” aspect ensures a comprehensive diffraction pattern by presenting numerous crystal orientations to the X-ray beam.
The technique relies on the interaction between X-rays and the electron clouds of atoms within the crystal lattice. When X-rays encounter the regularly spaced planes of atoms in a crystalline solid, they scatter. This diffraction produces a unique pattern characteristic of the material’s internal atomic arrangement, serving as a unique “fingerprint” for identification.
How Powder X-ray Diffraction Works
PXRD directs a monochromatic X-ray beam onto a powdered sample. An X-ray diffractometer consists of an X-ray source, a sample holder, and a detector. The X-rays are generated in a tube, producing X-rays of a specific wavelength, which are then collimated to focus the beam onto the sample.
When the X-ray beam strikes the crystalline powder, the X-rays interact with the electron clouds of the atoms arranged in repeating patterns within the crystal lattice. At specific angles of incidence, the scattered X-rays constructively interfere, producing diffraction peaks. This constructive interference occurs when the path length difference of X-rays scattered from different atomic planes is an integer multiple of the X-ray wavelength, a relationship described by Bragg’s Law.
The sample is placed in a small container and rotated at an angle (θ) relative to the X-ray source, while a detector simultaneously rotates at twice that angle (2θ). This synchronized movement captures all possible diffraction angles from the randomly oriented crystallites. The detector measures the intensity of the diffracted X-rays at various 2θ angles, generating a diffraction pattern that plots intensity against the diffraction angle. This pattern, with its distinct peak positions and intensities, provides information about the atomic spacing within the crystal.
What Powder X-ray Diffraction Reveals
The diffraction pattern obtained from PXRD analysis provides information about the crystalline material. This pattern allows for the identification of different phases within a material. By comparing the experimental pattern to reference databases, researchers can identify unknown crystalline materials, such as minerals and inorganic compounds.
PXRD can determine the crystal structure of a material, including the arrangement of atoms and the size and shape of its unit cell. It also allows for the calculation of lattice parameters, the dimensions of the repeating unit in the crystal structure. The technique can assess the degree of crystallinity, distinguishing between ordered crystalline regions and disordered amorphous components.
PXRD can estimate the average crystallite size within the powder and detect strain within the material. Both affect the width and shape of the diffraction peaks.
Common Applications of Powder X-ray Diffraction
Powder X-ray diffraction is widely used across diverse fields. In materials science, it is used for developing new alloys, ceramics, and other advanced materials by characterizing their phase composition and crystal structure. It also analyzes physical properties like crystallite size and lattice strain, important for material performance.
The pharmaceutical industry relies on PXRD for quality control and research. It helps in identifying different crystalline forms (polymorphs) of drug substances, which can affect their solubility, stability, and bioavailability. PXRD also ensures the purity of raw materials and monitors the stability of drug formulations over time.
In geology and mineralogy, PXRD is an important tool for identifying minerals in rocks and soils. This aids in understanding geological processes and in mineral resource assessments. Forensic science employs PXRD for analyzing unknown powders found at crime scenes, providing information for investigations. Environmental science uses the technique to characterize pollutants and understand their crystalline forms, which can influence their environmental impact.