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Introduction to Spectrum Analysis


After obtaining the diffraction spectrum on the X-ray diffractometer, it is important for beginners to understand how to interpret the spectrum and the significance of each feature in the spectrum. First, let's examine a simulated X-ray spectrum and use it to explain the meaning of spectral peaks.

Figure 1: Interpretation of spectral peaks

Peak position, peak width, and peak intensity can be referred to as the three main elements of an X-ray diffraction spectrum. For the analysis of peak position, we primarily rely on Bragg's Law: 2dsinθ=nλ. For conventional diffractometers, copper targets are commonly used, with a wavelength approximately equal to 0.154 nm. The wavelength varies with the target material, and each target material has corresponding characteristics for X-ray wavelength and excitation voltage, as outlined below:

Figure 2: The wavelength and excitation voltage of characteristic X-rays

The d-value represents the interplanar spacing of crystal planes. Each crystalline substance has a unit cell, and various crystal planes constitute a three-dimensional crystalline structure. Consequently, each crystalline substance exhibits a characteristic diffraction spectrum. By analyzing the spectrum obtained from X-ray diffractometry, we can determine the composition of a material. When external factors, such as doping, solid solution, stress changes, relaxation, defects, etc., alter the unit cell and cause changes in crystal plane spacing, the positions of diffraction peaks shift accordingly. Analyzing such peak shifts allows us to draw conclusions and verify whether material modification has been successful.

Peak width is generally measured using the full width at half maximum (FWHM). In software, it is commonly abbreviated as FWHM. Peak width can be used to analyze material crystallinity, grain size, etc. For example, in the testing of lithium iron phosphate materials in the field of batteries, the FWHM of peaks is a key parameter for assessing material quality. In the semiconductor field, for epitaxially grown single crystal films, we can obtain the FWHM parameter by measuring the rocking curve, thereby evaluating the crystalline quality of the single crystal film. When a broadening, also known as a "breadloaf" peak, appears, as indicated by the pink portion in the figure above, we can determine the presence of amorphous components or uncrystallized substances in the material.

Peak intensity generally refers to the area under the peak and is used to analyze the content of materials. After performing full-profile fitting and structural refinement on the tested spectrum, we can obtain the content of each phase in a mixed-phase material. This quantitative analysis method is more reliable than semi-quantitative methods such as the RIR method. By correlating the integrated intensity of the peak area with material content, we can not only perform quantitative analysis but also analyze crystallinity.

Through the analysis of diffraction peak position, width, and intensity, we can thoroughly examine materials. Future updates will continue to explore the applications of X-ray diffraction in various fields and analyze materials from a fundamental perspective.