The PB optics provides accurate measurement of diffracted X-ray positions unaffected by sample shape. During the diffraction process the X rays are refocused at the detector slit. Divergent X rays from the source hit the sample at different points on its surface. The incident- and diffracted-beam slits move on a circle that is centered on the sample.Sample surface is kept on the tangent plane of the focusing circle defined by three spots at sample, X-ray source and receiving slit.The incident angle ω between X-ray source and the sample is always 1/2 of the detector angle 2 θ: 1) ω:2 θ or θ:2 θ scan: with the X-ray tube fixed, the sample rotates at θ/min and the detector always at 2 θ/min and 2) θ: θ scan: the sample is fixed and the tube rotates at the same rate as the detector at θ/min.Most of powder diffractometers use Bragg-Brentano parafocusing geometry, offering high-resolution and high beam-intensity analysis at the cost of very precise alignment requirements and carefully prepared samples. In most diffractometers the X-ray wavelength λ is fixed and the diffraction angle θ is measured by goniometer, therefore the crystal lattice constants can be decided by above equation. When a crystal with an interplanar spacing d (crystal lattice constant) is irradiated by X-ray beam with a comparable wavelength λ, the X-ray diffraction, or the constructive interference between elastically scattered X-ray beams can be observed at specific angles 2 θ when the the Bragg’s Law is satisfied
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