Presentation1.pdf

X-Ray Diffraction (XRD) and Crystal Structure Electron Diffraction ‫ قسم الفيزياء‬- ‫كلية العلوم‬ The crystalline structure can be studied using electrons and neutrons, depending on the dual nature of matt...

X-Ray Diffraction (XRD) and Crystal Structure Electron Diffraction ‫ قسم الفيزياء‬- ‫كلية العلوم‬ The crystalline structure can be studied using electrons and neutrons, depending on the dual nature of matter governed by the law of De Broglie. The λ wavelength associated with electrons or neutrons can be determined in terms ℎ of both the Planck ℎ and the particle moving states (momentum,𝑝) where (𝜆 = ) and 𝑝 the accompanying wavelength must be suitable for the dimensions of the crystal so that its crystal structure can be studied. Electron diffraction and interference effects owing to the wavelike nature of a beam of electrons occur when passing near matter. According to the proposal of De Broglie, electrons and other particles have wavelengths that are ℎ inversely proportional to their momentum (𝜆 = ). 𝑝 Chapter (3) PHYS 471  X-Ray Diffraction (XRD) and Crystal Structure Electron Diffraction ‫ قسم الفيزياء‬- ‫كلية العلوم‬ Consequently, high-speed electrons have short wavelengths, a range of which are comparable to the spacings between atomic layers in crystals. A beam of such high-speed electrons should undergo diffraction, a characteristic wave effect, when directed through thin sheets of material or when reflected from the faces of crystals. The wavelike nature of electron beams was thereby experimentally established. As an analytic method, electron diffraction is used to identify a substance chemically or to locate the position of atoms in a substance. This information can be read from the patterns that are formed when various portions of the diffracted electron beam cross each other and by interference make a regular arrangement of impact positions, some where many electrons reach and some where few or no electrons reach. This is very similar to the constructive interference of x-ray which appears as bright spots. Chapter (3) PHYS 471  X-Ray Diffraction (XRD) and Crystal Structure Neutron Diffraction ‫ قسم الفيزياء‬- ‫كلية العلوم‬ The measurement principle of neutron diffraction is based on the Bragg Equation. Bragg diffraction occurs when electromagnetic radiation with wavelength comparable to atomic spacing are incident upon a crystalline sample, scattered by the atoms of the sample and undergo constructive interference in accordance to Bragg's law The technique requires a source of neutrons. Neutrons are usually produced in a nuclear reactor. Neutrons are diffracted by solids, especially crystals. Neutron Diffraction is a useful tool to identify structures. Chapter (3) PHYS 471  X-Ray Diffraction (XRD) and Crystal Structure Neutron Diffraction ‫ قسم الفيزياء‬- ‫كلية العلوم‬ A sample to be examined is placed in a beam of thermal, hot or cold neutrons to obtain a diffraction pattern that provides information of the structure of the material. The technique is similar to X-ray diffraction but due to the different scattering properties of neutrons versus x-rays, complementary information can be obtained. In particular, neutron diffraction is advantageous for the localization of light atoms and the determination of magnetic ordering. Neutron diffraction has some unique characteristics when compared to other types of radiation. This makes it very useful for the study of the atomic structure of liquids, amorphous materials and crystalline materials. Chapter (3) PHYS 471  X-Ray Diffraction (XRD) and Crystal Structure Neutron Diffraction ‫ قسم الفيزياء‬- ‫كلية العلوم‬ Diffraction methods can be divided into two interactions, Nuclear diffraction: diffraction due to the interaction between neutrons and atomic nuclei. Magnetic diffraction: diffraction due to the interaction between the magnetic moments of neutrons and magnetic moments of atoms. Chapter (3) PHYS 471