X-ray Diffraction (XRD) is a powerful, non-destructive analytical technique used to reveal the atomic and molecular structure of materials. This method provides a unique “fingerprint” for crystalline substances. By analyzing how X-rays interact with a sample, researchers gain insights into its internal arrangement. XRD is widely employed across various scientific and industrial disciplines for material characterization.
The Anatomy of an XRD Pattern
The standard output from an X-ray diffraction experiment is presented as a plot. The x-axis represents the diffraction angle, commonly denoted as two-theta (2θ) in degrees, while the y-axis shows the intensity of the diffracted X-ray signal. This plot visualizes the interaction between the incident X-rays and the material’s atomic planes.
Sharp, distinct peaks are key features observed within this pattern. These peaks arise from constructive interference, occurring when X-rays diffract coherently off the orderly, repeating planes of atoms in a crystalline structure. Each peak corresponds to a specific set of atomic planes within the crystal lattice.
An underlying curve, referred to as background noise, is also present beneath these peaks. This background signal originates from various sources, including incoherent scattering of X-rays, diffuse scattering from amorphous components, and contributions from the sample holder. Minimizing this background enhances peak clarity.
Bragg’s Law, expressed as nλ = 2d sinθ, is the fundamental equation explaining these peaks at specific angles. In this equation, ‘n’ represents an integer indicating the order of diffraction, ‘λ’ is the wavelength of the incident X-rays, ‘d’ signifies the spacing between atomic planes (d-spacing), and ‘θ’ is the Bragg angle (half of the 2θ angle). This law demonstrates a direct relationship between the observed diffraction angle and the interplanar spacing within the crystal lattice.
Key Information Extracted from the Data
The precise two-theta (2θ) positions of peaks within a diffraction pattern provide unique identification for a specific crystalline material. This characteristic set of angles acts like a distinctive fingerprint, allowing for the identification of unknown crystalline phases. Experimental patterns are compared against extensive reference databases, such as those maintained by the International Centre for Diffraction Data (ICDD), to confirm substance identity.
The relative intensities of these diffraction peaks offer information for quantitative analysis of multi-phase mixtures. In a sample containing multiple crystalline compounds, the peak intensities can estimate the relative proportion of each phase present. This is useful in materials science for determining the percentages of different mineral components in a rock sample, such as quartz versus calcite.
The shape of the diffraction peaks, particularly their width, reveals the material’s microstructure. Broad peaks indicate very small crystallites or microstrain within the crystal lattice. Microstrain refers to small, localized variations in the interplanar spacing caused by defects or impurities.
The Scherrer equation estimates crystallite size from the observed broadening of diffraction peaks. This equation relates the peak width at half maximum to the average size of coherent diffracting domains. It is a tool for understanding the nanoscale structural features of crystalline materials.
Differentiating Crystalline and Amorphous Materials
Crystalline materials are characterized by a highly ordered, repeating arrangement of atoms extending over long distances. When subjected to X-ray diffraction, these materials produce sharp diffraction peaks. These peaks result from long-range atomic periodicity, which facilitates coherent scattering of X-rays at specific angles.
Conversely, amorphous materials, such as glass or certain polymers, lack long-range atomic order. Instead of sharp peaks, their diffraction patterns display one or two broad, diffuse humps, called an “amorphous halo.” This broad scattering indicates short-range order but an absence of the periodic atomic planes necessary for sharp diffraction.
Some materials exhibit both crystalline and amorphous regions, known as semi-crystalline materials. Many polymers fall into this category, possessing ordered crystalline domains embedded within a disordered amorphous matrix. Their XRD patterns show a combination of features, with sharp peaks superimposed upon a broad, diffuse hump.
Common Applications in Science and Industry
In the pharmaceutical industry, X-ray diffraction identifies and quantifies different crystalline forms, known as polymorphs, of a drug substance. Different polymorphs of the same chemical compound can exhibit varying solubilities, dissolution rates, and stabilities, directly impacting drug effectiveness and shelf-life. This analysis ensures consistent quality and performance of pharmaceutical products.
Geology and mineralogy utilize XRD for mineral identification in rock and soil samples. This technique allows geologists to determine the mineralogical composition of geological formations, aiding in exploration for resources and understanding Earth’s crust. It provides a non-destructive way to characterize complex mineral assemblages.
Materials science and engineering use XRD to characterize new materials, analyze crystal structures of alloys, and for quality control of thin films in microelectronics. Researchers confirm the synthesis of novel compounds, investigate phase transformations in metals, and assess epitaxial growth and strain in semiconductor layers. This ensures materials meet specific performance criteria for various applications.
In forensic science, XRD analyzes unknown crystalline substances found at a crime scene. By comparing the diffraction pattern of an unknown sample to databases of known materials, forensic scientists can identify illicit drugs, explosives, or other crystalline residues. This provides objective evidence in criminal investigations.
Manufacturing industries, such as cement production, use XRD for quality control to ensure the final product has the correct phase composition. XRD analysis helps manufacturers maintain consistent product quality and performance.