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A topic from the subject of Analysis in Chemistry.

  • 1 year ago
  • 6 min read
X-Ray Diffraction (XRD) in Chemistry
Introduction

X-Ray Diffraction (XRD) is a non-destructive technique used to determine the structure of a crystal. It is based on the principle that X-rays are scattered by electrons in the crystal, and the resulting diffraction pattern can be used to determine the arrangement of atoms in the crystal.

Basic Concepts

When an X-ray beam is incident on a crystal, it is scattered by the electrons in the atoms of the crystal. The scattered X-rays interfere with each other, and the resulting diffraction pattern is a unique fingerprint of the crystal. This pattern is governed by Bragg's Law (nλ = 2d sin θ), relating the wavelength of X-rays (λ), the interplanar spacing (d), the angle of incidence (θ), and an integer (n). The diffraction pattern can be used to determine the crystal's unit cell, which is the smallest repeating unit of the crystal.

Equipment and Techniques

XRD is performed using an X-ray diffractometer. The diffractometer consists of an X-ray source (often a sealed tube or rotating anode), a sample holder, and a detector (e.g., a scintillation counter or a position-sensitive detector). The X-ray source produces a beam of X-rays, which is collimated (focused) and then directed at the sample. The scattered X-rays are detected by the detector, and the resulting diffraction pattern (typically a plot of intensity vs. 2θ) is recorded.

Types of Experiments

There are two main types of XRD experiments: powder diffraction and single-crystal diffraction. In powder diffraction, the sample is a powder, and the diffraction pattern is an average over all possible orientations of the crystallites. In single-crystal diffraction, the sample is a single crystal, and the diffraction pattern is recorded for a specific orientation of the crystal, providing more detailed structural information.

Data Analysis

The diffraction pattern is analyzed to determine the crystal's unit cell parameters (lattice constants, angles) and the arrangement of atoms within the unit cell (atomic positions). This involves indexing the diffraction peaks (assigning Miller indices (hkl) to each peak), and subsequently using software packages to refine the crystal structure model. The data analysis can be performed using a variety of software programs such as Rietveld refinement software.

Applications

XRD has a wide range of applications in chemistry, including:

  • Determining the structure of new materials
  • Characterizing the structure of known materials (phase identification, crystallinity, particle size)
  • Studying the phase transitions of materials
  • Measuring the residual stress in materials
  • Determining the texture (preferred orientation) of materials
  • Quantitative phase analysis
Conclusion

XRD is a powerful and versatile tool for studying the structure of materials. It is a non-destructive technique that can be used to determine the crystal structure of a wide range of materials. XRD has a wide range of applications in chemistry, materials science, and other fields, contributing significantly to the understanding and development of new materials and technologies.

X-Ray Diffraction (XRD)

Overview:

  • XRD is a non-destructive analytical technique used to determine the crystal structure and composition of materials.
  • It involves exposing a crystalline sample to X-rays and analyzing the resulting diffraction pattern. The interaction of X-rays with the sample's crystal lattice causes constructive interference at specific angles, producing a diffraction pattern.

Key Points:

  • Bragg's Law: nλ = 2d sin θ, where n is the order of diffraction (an integer), λ is the wavelength of the X-rays, d is the spacing between the crystal planes (d-spacing), and θ is the angle of incidence (and diffraction).
  • XRD Patterns: Consist of peaks (or reflections) at specific 2θ angles. The positions and intensities of these peaks are unique to the material's crystal structure and composition. The 2θ angle is twice the Bragg angle θ.
  • Identification: XRD patterns can be compared to databases (like the International Centre for Diffraction Data (ICDD) database) to identify unknown crystalline materials.
  • Structure Determination: Analysis of peak positions and intensities allows for the determination of the crystal structure (unit cell parameters, space group, atomic positions) and lattice parameters (unit cell dimensions and angles).
  • Applications: XRD is used extensively in various fields, including materials science (characterizing materials like metals, polymers, and ceramics), chemistry (analyzing crystalline compounds), mineralogy (identifying minerals), and pharmaceutical sciences (analyzing drug polymorphs and crystallinity). It can also be used to determine crystallite size, strain, and preferred orientation.
  • Instrumentation: XRD instruments typically consist of an X-ray source, a sample holder, a detector, and a goniometer to precisely control the angle of incidence and diffraction.
  • Sample Preparation: Sample preparation is crucial for obtaining high-quality XRD data. Methods include powdering, pressing into pellets, or using single crystals, depending on the nature of the sample.
X-Ray Diffraction (XRD) Experiment

Objective: To demonstrate the principles of X-ray diffraction and the use of an XRD instrument to identify an unknown crystalline material.

Materials:
  • X-ray diffractometer
  • Sample of crystalline material (e.g., NaCl, a metal powder - specify a known and an unknown sample)
  • Sample holder
  • Computer with XRD analysis software
  • Standard sample for calibration (e.g., silicon powder)
Procedure:
  1. Calibration: Prepare the diffractometer by using a standard sample (e.g., silicon) to calibrate the instrument's 2θ scale and ensure accurate measurements. This involves scanning the standard and comparing the obtained diffraction peaks with known values.
  2. Sample Preparation: Prepare the unknown crystalline sample by finely grinding it to a homogeneous powder. This ensures uniform scattering of X-rays. Mount a small amount of the powder onto a sample holder ensuring a flat, even surface.
  3. Instrument Setup: Place the prepared sample into the diffractometer's sample holder. Ensure correct positioning in the X-ray beam path.
  4. Data Acquisition: Set the scan parameters (e.g., 2θ range, step size, scan speed, and count time) using the XRD software. Begin the scan. The diffractometer will measure the intensity of diffracted X-rays as a function of the scattering angle (2θ).
  5. Data Analysis: Once the scan is complete, use the XRD software to analyze the data. Identify the diffraction peaks, determine their 2θ values and relative intensities. Compare the obtained diffraction pattern with known patterns in a database (e.g., JCPDS-International Centre for Diffraction Data database) to identify the crystalline material(s) present in the sample.
Key Considerations:
  • Sample preparation is crucial. A poorly prepared sample can lead to broad peaks and inaccurate results.
  • Careful selection of scan parameters is necessary to obtain sufficient data resolution and signal-to-noise ratio.
  • Appropriate background correction and peak fitting techniques should be applied to ensure accurate data interpretation.
  • Understanding the limitations of the XRD technique is important in analyzing the results. For example, XRD may not be able to differentiate between polymorphs of the same material or detect amorphous phases.
Significance:

XRD is a powerful non-destructive technique for characterizing crystalline materials. It can be used to identify unknown materials, determine crystal structures, calculate lattice parameters, assess crystallite size and strain, quantify phase compositions in mixtures, and study the effects of various treatments on materials. The experiment demonstrates this technique and allows for practical experience in handling instrumentation and interpreting diffraction patterns.

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