X射线衍射
Q1: What is Bragg's Law and how does it relate to X-ray diffraction?
Bragg's Law relates the wavelength of X-rays, the incident angle, and the lattice spacing of a crystal. When X-rays irradiate a crystal at a specific angle theta, they diffract only at angles satisfying Bragg's Law, where n is the harmonic order. This relationship creates a unique signature for each crystalline structure, allowing researchers to identify materials from their diffraction patterns.
Q2: How do Miller indices and unit cells describe crystal structure?
Miller indices are three integers that identify specific directions and planes within crystals. The unit cell is the smallest repeating structure in a crystal, defined by angles and lengths that determine its symmetry. Different unit cell parameters produce seven distinct crystal systems, including cubic, tetragonal, and hexagonal, each with unique structural properties.
Q3: What causes variations in X-ray diffraction peak intensities?
Peak intensities result from two factors: non-structural contributions like material absorption and experimental geometry, and structural contributions from the material itself. Each diffraction peak represents the sum of scattered amplitudes from all unique atoms in a unit cell. Constructive interference produces strong peaks, while destructive interference weakens them, directly reflecting the crystal's atomic arrangement.
Q4: How is sample preparation critical for X-ray diffraction experiments?
Samples must be sufficiently thick, at least three times the X-ray attenuation length, to ensure adequate diffraction signal. The irradiated sample surface must be parallel to the sample stage, and the X-ray beam footprint must remain smaller than the sample width at the smallest incident angle. Proper preparation ensures accurate data collection and reliable structural analysis.
Q5: What information can X-ray diffraction reveal about material structure?
X-ray diffraction determines atomic and molecular arrangements, identifies crystallographic phases, and reveals internal stress and crystal defects. The technique also measures how actual structures deviate from ideal arrangements. Beyond materials science, XRD is used in biology to determine protein and nucleic acid structures, and in geochemistry for mineral analysis and planetary exploration.
Q6: How does the data acquisition process work in X-ray diffraction?
The sample is rotated while a detector records scattered X-ray intensity at varying angles. An initial broad scan from 15 to 90 degrees identifies major peaks. Subsequent focused scans use smaller angle steps around specific peaks for higher resolution data. The resulting intensity versus angle graph reveals characteristic peaks that correspond to specific crystal symmetries and planes.
Q7: How does X-ray diffraction identify unknown materials?
Software compares observed diffraction peaks against database libraries, matching each peak to specific crystal symmetries. The combination of peak positions and intensities uniquely identifies the crystal structure type. For example, nickel powder shows peaks corresponding to face-centered cubic symmetry, allowing definitive material identification and phase composition analysis.