X-Ray Diffraction (XRD)
Introduction:
X-Ray Diffraction (XRD) is one of the most commonly used techniques for identifying compounds and phases present in materials. In X-ray diffraction analysis, X-rays with extremely short wavelengths interact with matter at atomic distances, allowing the study of atomic structures using this technique. By analyzing the X-ray diffraction spectrum, known as the X-ray reflection pattern, it is possible to determine crystal structures, grain size, atomic spacing, lattice parameters, and crystal defects.
Principle of Analysis:
In X-ray diffraction (XRD) analysis, the sample is exposed to monochromatic X-rays. As a result of the interaction between X-rays and the sample, emitted X-rays are collected and analyzed by a detector. Both the X-ray source and detector rotate, allowing the X-rays to hit the sample at different angles. The instrument produces a pattern that consists of a set of peaks, and the shape of these peaks is related to the atomic structure of the sample.
Most materials are composed of crystals with a regular atomic arrangement. Each atom consists of a nucleus and a surrounding electron cloud. The concept of X-ray diffraction is based on these atomic dimensions. X-rays are high-energy waves with a specific wavelength, and their wavelength is comparable to the distance between atoms in a crystal. Due to this proximity, interference and specific phenomena occur.
When X-rays interact with matter, their energy is absorbed by the atoms. If the absorbed energy is insufficient to eject an electron from the atom, it is re-emitted as new X-rays with energy similar to the incident rays. The interference resulting from the interaction of emitted X-rays depends on their synchronization; if the waves are synchronized, they reinforce each other, known as constructive interference. Otherwise, destructive interference occurs.
The diffraction phenomenon refers to areas where constructive interference occurs. Consequently, diffraction or X-ray reflection occurs at specific angles depending on the crystal structure of the material. The peaks in the graph generated by the instrument represent the locations of these angles. These patterns are like fingerprints, indicating the unique crystal structure of each material.
Main Components of the XRD Instrument:
1. X-ray Tube:
Generates a monochromatic X-ray beam using a vacuum tube. Electrons are accelerated towards a metal target (usually copper), producing characteristic X-rays and white X-rays. The resulting beam passes through a filter that allows only the desired wavelength to pass.
2. Sample Holder:
Holds powdered or polycrystalline samples and can rotate around its axis. Some holders are equipped with heaters to heat the sample before analysis.
3. Detector:
Consists of a semiconductor plate sensitive to X-rays. When X-rays strike the detector, a current is generated, which is measured and recorded as a function of time to determine the X-ray intensity at different angles.
Benefits of XRD Analysis:
Crystallinity Determination:
Taller and narrower peaks indicate a more crystalline material, while broader and shorter peaks indicate an amorphous material.
Phase Ratio Determination:
The ratio of the highest peak between two phases is calculated using a reference material for accurate estimates.
Crystal Size Measurement:
Performed using equations such as Debye-Scherrer and Williamson-Hall.
Peak Shift Examination:
Used to identify substitutions within the crystal system.
Metal and Mineral Identification:
XRD is used to determine the types of minerals present in specific samples, aiding in geological composition analysis.
Steel Structure Analysis:
Used to analyze phase types and grain sizes in steel, contributing to improved mechanical properties.
Non-Ferrous Alloy Studies:
Enables the analysis of the crystal structure of non-ferrous alloys and identifies different phases.
Measurement of Interplanar Spacing:
XRD is used to measure the distances between crystal planes in materials, helping to understand their crystal structure.
Thin Film Analysis:
Used to examine phases in thin films, such as those used in electronics and coatings.
Layered Material Analysis:
Determines interlayer distances in materials like graphene, graphene oxide, and clay, providing valuable insights into structure and properties.
Types of Measurable Samples:
1. Crystalline Materials:
Includes minerals, base metals, and alloys, where crystal structure and phase identification are analyzed.
2. Amorphous Materials:
Such as glass and non-crystalline substances, which can be examined to determine their crystallinity.
3. Non-Ferrous Alloys:
Includes aluminum, copper, and zinc, where crystal structures and phases can be identified.
4. Thin Films:
Used for studying thin films in electronic applications and coatings.
5. Powdered Samples:
Easily measurable, as particles are uniformly distributed to improve results.
6. Composite Materials:
Can be analyzed to determine their crystal structure and chemical composition.
XRD Measurement Conditions:
Sample Preparation:
The sample must be homogeneous and finely ground (for powdered materials) to ensure accurate results.
Sample Size:
The sample should be appropriately sized according to instrument requirements, usually with a thin layer in the case of thin films. If the sample is a powder, an adequate amount of approximately 0.2 g is required.
Note: Liquid samples cannot be measured in XRD unless they are deposited on a glass slide or the solvent is evaporated.
Data Analysis:
Graph Interpretation:
XRD data is typically represented as a graph, where the horizontal axis represents the angle (2θ) and the vertical axis represents X-ray intensity. Identifying peaks in the graph is the first step.
Peak Position Identification:
Determining peak positions helps in understanding the crystal structure. Each peak represents X-ray scattering at a specific angle, which corresponds to the spacing between crystal planes.
Bragg’s Law Application:
Bragg’s equation can be used to calculate interplanar distances (d) based on angles (θ) and the wavelengths used.
Phase Identification:
Comparing peak positions with crystal material databases (PDF – Powder Diffraction File) helps determine the phases present in the sample.
Peak Width Analysis:
Peak width provides information about crystal size. Equations like Williamson-Hall or Debye-Scherrer are used to calculate grain size.
Crystallinity Estimation:
The shape and height of peaks help estimate the crystallinity of a sample. Taller and narrower peaks indicate higher crystallinity.
Phase Percentage Determination:
The percentages of different phases in a system can be calculated by comparing the heights of main peaks.
Impurity Analysis:
Unexpected additional peaks may indicate the presence of impurities or unwanted phases in the sample.
XRD result analysis is a multi-step process that requires precision and a deep understanding of the data. Using these steps, researchers can gain valuable insights into the crystal structure and physical properties of studied materials.