Tiêu đề 33. X-RAY DIFFRACTION.pdf ✅

PHARMD GURU Page 2  X-ray diffraction peaks are produced by constructive interference of a monochromatic beam of X-rays scattered at specific angles from each set of lattice planes in a sample.  The peak intensities are determined by the distribution of atoms within the lattice. Consequently, the X-ray diffraction pattern is the fingerprint of periodic atomic arrangements in a given material. DIFFRACTION: Diffraction is a wave phenomenon in which the apparent bending and spreading of waves when they meet an obstruction. Diffraction occurs with electromagnetic waves, such as light and radio waves, and also in sound waves and water waves. The most conceptually simple example of diffraction is double-slit diffraction, that's why firstly we remember light diffraction. LIGHT DIFFRACTION: Light diffraction is caused by light bending around the edge of an object. The interference pattern of bright and dark lines from the diffraction experiment can only be explained by the additive nature of waves; wave peaks can add together to make a brighter light, or a peak and a through will cancel each other out and result in darkness.  Constructive interference is the result of synchronized light waves that add together to increase the light intensity.  Destructive interference results when two out-of- phase light waves cancel each other out, resulting in darkness. DIFFRACTION OF WAVES BY CRYSTALS:  Solid State Physics deals how the waves are propagated through such periodic structures. Crystal structure can be studied through the diffraction of photons (X- ray), neutrons and electrons.  The diffraction depends on the crystal structure and on the wavelength. At optical wavelengths such as 5000 Å the superposition of the waves scattered elastically by the individual atoms of a crystal results in ordinary optical refraction.
PHARMD GURU Page 3  When the wavelength of the radiation is comparable with or smaller than the lattice constant, one can find diffracted beams in directions quite different from the incident radiation.  The structure of a crystal can be determined by studying the diffraction pattern of a beam of radiation incident on the crystal. Beam diffraction takes place only in certain specific directions, much as light is diffracted by a grating.  By measuring the directions of the diffraction and the corresponding intensities, one obtains information concerning the crystal structure responsible for diffraction. PRINCIPLE: X-ray diffraction is based on constructive interference of monochromatic X- rays and a crystalline sample. These X-rays are generated by a cathode ray tube, filtered to produce monochromatic radiation, collimated to concentrate, and directed toward the sample. The interaction of the incident rays with the sample produces constructive interference (and a diffracted ray) when conditions satisfy Bragg's Law (nλ = 2d sinθ). This law relates the wavelength of electromagnetic radiation to the diffraction angle and the lattice spacing in a crystalline sample. These diffracted X-rays are then detected, processed and counted. By scanning the sample through a range of 2θ angles, all possible diffraction directions of the lattice should be attained due to the random orientation of the powdered material. Conversion of the diffraction peaks to d-spacings allows identification of the mineral because each mineral has a set of unique d-spacings. Typically, this is achieved by comparison of d-spacings with standard reference patterns. All diffraction methods are based on generation of X-rays in an X-ray tube. These X-rays are directed at the sample, and the diffracted rays are collected. A key component of all diffraction is the angle between the incident and diffracted rays.
PHARMD GURU Page 4 BRAGG’S LAW: W.L Bragg in 1913 showed that scattered radiation from a crystal behaves as if the diffracted beam were reflected from a plane passing through points of the crystal lattice in a manner that makes these crystal lattice planes analogous to mirrors. The conditions leading to diffraction are given by Bragg's law. Consider the reflection of two parallel rays of the same wavelength by two adjacent planes of a lattice. One ray strikes point D on the upper plane but the other ray must travel an additional distance AB before striking the plane immediately below. Similarly, the reflected rays will differ in path length by a distance BC.