Introduction
X-Ray Diffraction (XRD) is an essential analytical technique used to determine the atomic and molecular structure of crystalline materials. When a beam of X-rays strikes a crystal, the atoms act as scattering centres, reflecting the X-rays in specific directions. These reflected beams produce a diffraction pattern that serves as a “fingerprint” for the crystalline substance. By analyzing these diffraction patterns, scientists can determine unit cell dimensions, crystal symmetry, phase composition, and structural properties of materials.
XRD plays a critical role in pharmaceutical sciences because many drugs exist in crystalline forms or can adopt different polymorphic structures, each with unique solubility, stability, and bioavailability. The ability to study the arrangement of atoms and detect structural changes is essential for drug design, identification, and quality control.
Definition and Principle
X-ray diffraction is a technique based on the constructive interference of monochromatic X-rays and crystalline materials. When X-rays of known wavelength strike a crystal, they interact with electrons in the atoms, producing scattered waves. These scattered waves interfere with one another, and constructive interference occurs only at specific angles.
This fundamental principle is described by Bragg’s Law:
nλ = 2d sin θ
where:
- n = order of reflection
- λ = wavelength of incident X-rays
- d = interplanar spacing in the crystal
- θ = angle of incidence
Bragg’s law establishes a direct relationship between the angle of diffraction and the spacing between crystal planes. By measuring the diffraction angles, we can determine the internal structure of a crystal.
Generation of X-Rays
X-rays are produced by bombarding a metal target with high-energy electrons in a vacuum tube. When electrons rapidly decelerate after striking the target, they emit X-ray radiation. The resulting spectrum consists of:
- Continuous X-rays (Bremsstrahlung radiation)
- Characteristic X-rays emitted when inner-shell electrons are ejected and replaced
Common targets include copper (Cu), iron (Fe), molybdenum (Mo), and chromium (Cr). Copper Kα radiation is most widely used because of its suitable wavelength for studying organic and pharmaceutical crystals.
Crystal Lattices and Unit Cells
A crystal lattice is a 3-dimensional arrangement of atoms, ions, or molecules. The smallest repeating unit in a crystal is called the unit cell. A unit cell is characterized by:
- Three edge lengths: a, b, c
- Three interaxial angles: α, β, γ
Seven Crystal Systems
Based on unit cell dimensions, crystals are classified into:
- Cubic
- Tetragonal
- Orthorhombic
- Hexagonal
- Trigonal (Rhombohedral)
- Monoclinic
- Triclinic
Bravais Lattices
There are 14 Bravais lattices representing all possible 3D lattice types. These lattices explain how points are arranged in space and determine the symmetry of the crystal.
Powder X-Ray Diffraction
Powder diffraction is the most widely used XRD method. In this technique, the sample is finely powdered so that crystals are randomly oriented. As a result, all possible diffraction angles appear simultaneously.
The diffraction pattern appears as a series of peaks (called 2θ peaks), each corresponding to a specific set of crystal planes. By comparing these peaks with standard reference patterns, the identity and purity of crystalline materials can be easily confirmed.
Instrumentation for X-Ray Diffraction
A typical XRD system consists of the following components:
1) X-Ray Source
Usually a sealed X-ray tube or rotating anode containing a target metal. X-rays are produced when high-voltage electrons strike the metal target. The tube is shielded except for a small window that emits X-rays toward the sample.
2) Collimators and Monochromators
Collimators narrow the X-ray beam into a straight path, improving resolution. Monochromators filter out unwanted wavelengths, allowing only monochromatic radiation to reach the sample.
3) Sample Holder
The powdered sample is placed on a flat holder, ensuring an even and compact surface. Proper sample packing is essential for high-quality diffraction peaks.
4) Goniometer
The goniometer controls the precise movement of the X-ray tube, sample, and detector. It ensures that the angle between the X-ray source and detector changes systematically during scanning.
5) X-Ray Detectors
Detectors measure the intensity of diffracted beams. Common types include:
- Geiger-Müller counters
- Scintillation counters
- Proportional counters
- Solid-state detectors
Modern instruments use fast solid-state detectors for high sensitivity and rapid data collection.
6) Readout/Computer System
The diffracted beam intensities are converted into digital data, processed, and displayed as an XRD pattern. Advanced software helps identify crystal phases, calculate lattice parameters, and perform structural analysis.
Applications of X-Ray Diffraction
- Identification of crystalline compounds by comparing diffraction patterns
- Determination of unit cell dimensions and crystal structures
- Analysis of polymorphism in pharmaceutical substances
- Measurement of crystallinity in polymers and solids
- Qualitative and quantitative phase analysis in mixtures
- Structural studies of minerals and inorganic materials
- Investigation of chemical reactions involving solid phases
- Detection of impurities through peak variations
Advantages
- Non-destructive and rapid technique
- Provides precise structural information
- Minimal sample preparation required
- Useful for qualitative and quantitative analysis
- High reproducibility
Disadvantages
- Only crystalline materials can be studied
- Poor sensitivity for amorphous substances
- High-cost instrumentation
- Requires trained personnel for interpretation
Detailed Notes:
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