What Is Powder Diffraction and How Is It Used?
Analyzing the interaction of X-rays with a crystalline solid's atomic structure provides a unique pattern used for definitive material identification.
Powder diffraction is a non-destructive analytical technique used to characterize crystalline materials. Every crystalline substance has a distinct diffraction pattern that acts as a unique fingerprint, similar to a product’s barcode. This method is widely applied because it can analyze polycrystalline materials directly, often without extensive sample preparation.
Principles of Diffraction in Crystalline Powders
Crystalline solids have a highly ordered internal structure where atoms, ions, or molecules are arranged in a repeating, three-dimensional lattice. This lattice contains parallel atomic planes separated by specific distances. When a beam of X-rays is directed at a crystalline material, these planes scatter the X-rays in a predictable manner known as diffraction.
The specific conditions required for diffraction are described by Bragg’s Law. For a strong reflection to be observed, X-rays scattered from adjacent atomic planes must interfere constructively. This occurs only when X-rays strike the planes at a precise angle, the Bragg angle. At incorrect angles, the scattered waves interfere destructively, and no signal is detected.
A powdered sample is used because it consists of millions of randomly oriented microscopic crystals, or crystallites. This random arrangement ensures that for any set of atomic planes, enough crystallites will be positioned at the correct Bragg angle for diffraction. This allows all possible diffraction signals to be measured simultaneously, providing a complete picture of the crystalline structure.
The Powder Diffractometer
A powder diffractometer is the instrument used for this analysis. It has three main components: an X-ray source, a sample holder, and a detector. The source generates an X-ray beam of a known wavelength, which is directed onto the powdered sample on the holder.
When the X-ray beam strikes the sample, crystallites diffract the X-rays in various directions. A sensitive detector captures these diffracted beams and measures their intensity. The X-ray source remains stationary while the sample stage and detector rotate in a coordinated manner. The sample rotates by an angle (θ), and the detector moves to an angle of two-theta (2θ) relative to the initial beam path.
This synchronized movement allows the instrument to scan a range of angles and measure the intensity of diffracted X-rays at each step. The output is a graph plotting X-ray intensity against the detection angle (2θ). This graph is the powder diffraction pattern and serves as the data for analysis.
Analyzing a Diffraction Pattern
After a diffraction pattern is collected, it is analyzed to identify the material. Each crystalline substance produces a unique pattern with a series of peaks at distinct 2θ angles. The most common analytical approach is phase identification, which involves matching the experimental pattern to a known reference.
This matching is done by comparing the peak positions and intensities in the measured diffractogram to entries in a database. The Powder Diffraction File™ (PDF®), from the International Centre for Diffraction Data (ICDD), contains patterns for hundreds of thousands of known materials. If the experimental pattern matches a database entry, the substance’s identity is confirmed.
Different features of the diffraction pattern provide specific structural details. The position of each peak on the 2θ axis is related to the spacing between the atomic planes in the crystal lattice, as dictated by Bragg’s Law. The intensity of each peak is determined by the arrangement and type of atoms within those planes. Analyzing these characteristics allows scientists to identify a compound and gain insights into its atomic structure.
Real-World Uses of Powder Diffraction
In the pharmaceutical industry, the technique ensures a drug is produced in its correct crystalline form, or polymorph. Different polymorphs of the same compound can have varying properties, like solubility and stability, which impact a drug’s effectiveness and shelf life. Powder diffraction is also used to check for crystalline impurities in the final product.
In geology and environmental science, powder diffraction identifies minerals in rocks, ores, and soil samples. This information supports mining operations, geological surveying, and the assessment of soil composition for agriculture or environmental remediation. Geologists analyze a rock’s mineral content to understand its formation history and economic value.
Materials science and engineering use powder diffraction for quality control and research. It verifies the composition of products like cement, ceramics, and metal alloys to ensure they meet performance specifications. Researchers also use it to develop new materials, such as those for high-capacity batteries, solar cells, and semiconductors.
The technique also has applications in forensic science. When an unknown crystalline substance is found at a crime scene, forensic scientists use powder diffraction for identification. This could involve illicit drugs or minerals in a soil sample that could link a suspect to a location, providing objective evidence for legal investigations.