Xray generators-The Synchrotron Schematic llustration angement for using bragg of an X-ray Diffractometer amples is illustrated determined by the r energy and Since it is usually difficult rajectory. When the particle the electrons(or positrons)are detector is moved through an accelerated toward the center of Radiation Facility Grenoble, France ectromagnetic radiation, an to mitted includes high energy x' particular planes can be of Common Mechanical Schematic Illustration of Xray Diffraction Movement in powder Diffractometers Matched = characteristic xrays Matched filters ar beam to optimize the fraction of energy which is in the Ka line. 0-20 geometry The Bragg- Brentano diffractometer is the dominant laue stationary single crvstal tube moves (and the specimen is fixed), this is called 0 Monochromatic Powder: specimen is polycrystalline, etry. The essential characteristics are (1)The relationship between e (the angle pecimen surface and the incident xray beam) and 20 e Jong Bouman' single crystal rota tes the incident b m and the illates about chosen axis in path of iving slit detector)is maintained throughout I Precession; chosen axis of single crystal (2)r, and r, are fixed and equal and define a diffractometer circle in which the specimen is always at the center4 X-ray Generators ¾ The Synchrotron European Synchrotron Radiation Facility Grenoble, France Electrons (or positrons) are released from a particle accelerator into a storage ring. The trajectory of the particles is determined by their energy and the local magnetic field. Magnets of various types are used to manipulate the particle trajectory. When the particle beam is “bent”by the magnets, the electrons (or positrons) are accelerated toward the center of the ring. Charged particles moving under the influence of an accelerating field emit electromagnetic radiation, and when they are moving at close to relativistic speeds, the radiation emitted includes high energy x￾ray radiation.. Schematic Illustration of an X-ray Diffractometer X-RAY SOURCE CRYSTAL DETECTOR PATH OF DETECTOR An experimental arrangement for using Bragg diffraction to determine the structure of single-crystal samples is illustrated schematically left Since it is usually difficult to move the X-ray source the sample itself is rotated with respect to the source and when the sample is moved through an angle q the detector is moved through an angle 2q. The wavelength of the x-ray source is well known so by measuring the angles at which strong diffraction peaks (i.e. strong detector signal) occur the spacing of particular planes can be determined. Type Tube Specimen Receiving Slit r1 r2 Brag -Brentano q:2q Fixed Varies as q Varies as 2q Fixed =r1 Brag -Brentano q:q Varies as q Fixed Varies as q Fixed =r1 Seeman-Bohlin Fixed Fixed Varies as 2q Fixed variabl e Texture Sensitive (Ladel) Fixed Varies as q processes about a Varies as 2q Fixed variabl e * Generally fixed, but can rotate about a or rock about goniometer axis. Common Mechanical Movement in Powder Diffractometers Schematic Illustration of X-ray Diffraction To obtain nearly monochromatic x-rays, an x-ray tube is used to produce characteristic x-rays. Matched filters are used in the x-ray beam to optimize the fraction of the energy which is in the Ka line. q-2q geometry The Bragg-Brentano diffractometer is the dominant geometry found in most laboratories. In this system, if the tube is fixed, this is called q-2 q geometry. If the tube moves (and the specimen is fixed), this is called q : q geometry. The essential characteristics are: (1) The relationship between q (the angle between the specimen surface and the incident x-ray beam) and 2q (the angle between the incident beam and the receiving slit detector) is maintained throughout the analysis. (2) r1 and r2 are fixed and equal and define a diffractometer circle in which the specimen is always at the center. Radiation Method White Laue: stationary single crystal Monochromatic Powder: specimen is polycrystalline, and therefore all orientations are simultaneously presented to the beam Rotation, Weissenberg: oscillation De Jong-Bouman: single crystal rota tes or oscillates about chosen axis in path of beam Precession: chosen axis of single crystal precesses about beam direction