X-ray diffraction (XRD) is a commonly used testing method, yet many students remain unfamiliar with its principles and applications. In this article, Drawell will provide insights into XRD, addressing various aspects of this technique.
1. Understanding the Utility of XRD
X-ray diffraction involves scattering X-rays when they interact with a material. When X-rays strike crystalline substances, they undergo coherent scattering, which is a diffraction phenomenon. This means that the incident X-ray beam changes direction without changing its wavelength upon exiting the material. This unique behavior is specific to crystalline substances.
Most solid-state materials are either crystalline, microcrystalline, or quasi-crystalline and can exhibit X-ray diffraction. The crystal's microstructure exhibits a periodic, long-range ordered arrangement. The X-ray diffraction pattern is a representation of the three-dimensional structure of the crystal microstructure, containing critical information about the crystal's composition and structure. XRD is currently the most powerful method for studying crystal structures, including atomic positions, unit cell dimensions, and more.
XRD is particularly well-suited for phase analysis of crystalline substances. Different phases or structures within crystalline substances exhibit variations in the number of diffraction peaks, their angles, relative intensities, and peak shapes. By comparing the X-ray diffraction pattern of an unknown sample with that of known crystalline substances, one can qualitatively identify the phase composition and structure of the sample. Additionally, quantitative analysis of the sample's phase composition is possible by analyzing the diffraction intensity data.
2. Distinguishing Amorphous, Quasi-Crystalline, and Crystalline Structures
Differentiating between amorphous, quasi-crystalline, and crystalline structures in XRD patterns is not always straightforward. In XRD patterns obtained from crystalline materials, you typically observe sharp, distinct peaks with narrow 2Θ widths at half-height (usually 0.1° to 0.2°). Broadened peaks suggest that the crystals in the sample have smaller particle sizes, often less than 300 nm, known as "microcrystals." The Scherrer formula can estimate the grain size based on spectral line broadening.
In contrast, amorphous materials exhibit a gentle, continuous change in X-ray scattering intensity over a wide angle range (2θ 1° to several tens of degrees), often with one or more maxima. This phenomenon occurs due to extremely fine grain sizes leading to broadened, overlapping, and blurred diffraction peaks. Quasi-crystalline materials represent a transitional state between crystalline and amorphous, characterized by unique diffraction patterns.
3. Impact of Different X-ray Targets
The choice of X-ray target, such as copper or chromium, affects the characteristic wavelengths used in XRD experiments. The diffraction angle, determined by the Bragg equation, changes with varying wavelengths. Consequently, the positions of diffraction peaks on XRD patterns obtained from different X-ray tubes will differ systematically. However, a crystal's set of d values, inherent to its structure, remains consistent, independent of the X-ray target.
The relative intensities of diffraction peaks may vary slightly when using different targets for the same sample. This variation is attributed to the absorption properties of the sample in relation to the incident X-ray wavelength and the target material.
4. Determining Crystal Planes Corresponding to Diffraction Angles
To determine the crystal planes corresponding to different diffraction angles, consult powder diffraction data cards that provide diffraction index information for each diffraction line. For unknown crystal structures, the process of determining diffraction indices for each line is known as "indexing the diffraction pattern." This step requires a foundation in crystallography and proficiency in indexing software, such as treaor90.
5. Obtaining Atomic Coordinates in Crystals
To acquire atomic coordinates within a crystal, perform single-crystal X-ray diffraction. This technique, in addition to using CCD detectors, provides precise atomic position data.
6. Calculating Grain Size, Lattice Constants, and Distortion
Grain size, lattice constants, and distortion can be calculated from X-ray diffraction data, specifically from peak shape information. When the broadening of diffraction peaks is solely due to crystal grain fineness, the Scherrer formula can estimate grain size based on the extent of peak broadening.
The above is the arrangement of the common problems of XRD. If you need an XRD test or the XRD instrument, you can contact Drawell.