International
Tables for Crystallography Volume H Powder diffraction Edited by C. J. Gilmore, J. A. Kaduk and H. Schenk © International Union of Crystallography 2018 
International Tables for Crystallography (2018). Vol. H, ch. 2.5, pp. 145147
Section 2.5.4.4.2. Crystallite size^{a}Bruker AXS Inc., 5465 E. Cheryl Parkway, Madison, WI 53711, USA 
The size of the crystallites in a polycrystalline material has a significant effect on many of its properties, such as its thermal, mechanical, electrical, magnetic and chemical properties. Xray diffraction has been used for crystallitesize measurement for many years. Most methods are based on diffractionline broadening and lineprofile analysis (Wilson, 1971; Klug & Alexander, 1974; Ungár, 2000). Another approach to crystallitesize measurement is based on the spotty diffraction rings collected with twodimensional detectors when a small Xray beam is used (Cullity, 1978; He, 2009). Lineprofile analysis is based on the diffraction profile in the 2θ direction, while crystallitesize analysis with a spotty 2D diffraction pattern is based on the diffraction profile in the γ direction. The latter may be referred to as γprofile analysis.
Fig. 2.5.29(a) shows a diffraction profile collected from gold nanoparticles and regular gold metal. The 2θ profile from the gold nanoparticles is significantly broader than the profile from regular gold metal. The crystallite size can be calculated by measuring the broadening and using the Scherrer equation:where λ is the Xray wavelength (in Å), B is the full width at half maximum (FWHM) of the peak (in radians) corrected for instrumental broadening and strain broadening, θ is the Bragg angle, C is a factor, typically from 0.9 to 1.0, depending on the crystallite shape (Klug & Alexander, 1974), and t is the crystallite size (also in Å). This equation shows an inverse relationship between crystallite size and peakprofile width. The wider the peak is, the smaller the crystallites. The 2θ diffraction profiles can be obtained either by using a conventional diffractometer with a point or line detector, or by γ integration from a diffraction pattern collected with 2D detector. When a 2D detector is used, a long sampletodetector distance should be used to maximize the resolution. A small beam size and low convergence should also be used to reduce instrument broadening.

Crystallitesize analysis: (a) 2θ profile of a gold nanoparticle (grey) and regular gold metal (black); (b) γ profile of LaB_{6}; (c) measurement range. 
Fig. 2.5.29(b) shows a frame collected from an SRM660a (LaB_{6}) sample with a 2DXRD system. The spotty diffraction rings are observed with average crystallite size of 3.5 µm. The number of spots in each diffraction ring is determined by the crystallite size and diffraction volume. Introducing a scaling factor covering all the numeric constants, the incidentbeam divergence and the calibration factor for the instrument, we obtain an equation for the crystallite size as measured in reflection mode:where d is the diameter of the crystallite particles, p_{hkl} is the multiplicity of the diffracting planes, b is the size of the incident beam (i.e. its diameter), Δγ is the γ range of the diffraction ring, μ is the linear absorption coefficient and N_{s} is the number of spots within Δγ. For transmission mode, we havewhere t is the sample thickness. In transmission mode with the incident beam perpendicular to the sample surface, the linear absorption coefficient affects the relative scattering intensity, but not the actual sampling volume. In other words, all the sample volume irradiated by the incident beam contributes to the diffraction. Therefore, it is reasonable to ignore the absorption effect for crystallitesize analysis as long as the sample is thin enough for transmissionmode diffraction. The effective sampling volume reaches a maximum for transmissionmode diffraction when .
For both reflection and transmission,where β is the divergence of the incident beam. Without knowing the precise instrumental broadening, k can be treated as a calibration factor determined from the 2D diffraction pattern of a known standard. Since only a limited number of spots along the diffraction ring can be resolved, it can be seen from equation (2.5.94) that a smaller Xray beam size and lowmultiplicity peak should be used if a smaller crystallite size is to be determined.
Fig. 2.5.29(c) shows the measurement ranges of 2θprofile and γprofile analysis. The 2θprofile analysis is suitable for crystallite sizes below 100 nm (1000 Å), while γprofile analysis is suitable for crystallite sizes from as large as tens of µm down to 100 nm with a small Xray beam size. By increasing the effective diffraction volume by translating the sample during data collection or multiple sample integration (or integrating data from multiple samples), the measurement range can be increased up to millimetres. Multiple sample integration can deal with large crystallite sizes without recalibration. The new calibration factor is given aswhere n is the number of targets that are integrated. The number of crystallites can be counted by the number of intersections of the γ profile with a threshold line. Every two intersections of the γ profile with this horizontal line represents a crystallite. In order to cancel out the effects of preferred orientation and other material and instrumental factors on the overall intensity fluctuation along the γ profile, one can use a trend line fitted to the γ profile by a secondorder polynomial. It is always necessary to calibrate the system with a known standard, preferably with a comparable sample geometry and crystallite size. For reflection mode, it is critical to have a standard with a comparable linear absorption coefficient so as to have similar penetration.
References
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