Debye-Scherrer powder diffraction lines

Figure 5.8 A Debye-Scherrer powder camera for X-ray diffraction. The camera (a) consists of a long strip of photographic film fitted inside a disk. The sample (usually contained within a quartz capillary tube) is mounted vertically at the center of the camera and rotated slowly around its vertical axis. X-rays enter from the left, are scattered by the sample, and the undeflected part of the beam exits at the right. After about 24 hours the film is removed (b), and, following development, shows the diffraction pattern as a series of pairs of dark lines, symmetric about the exit slit. The diffraction angle (20) is measured from the film, and used to calculate the d spacings of the crystal from Bragg s law.

The Debye-Scherrer powder X-ray diffraction data for dilithiomethane, reported by the same authors unfortunately consist of 12 lines only, several of which overlap and except for one, all peaks are of low intensity. The low-symmetry crystal system and the limited number of lines made solving the crystal structure impossible. In an attempt to analyse the 12-line diffraction pattern a triclinic cell was obtained, which, however, cannot be considered as definitive. 

When a specimen has no particular orientation (for example, a randomly oriented mass of small crystals, as in a powder), each reflection is spread out into a right circular cone of radiation whose axis is the x-ray beam. The intersections of these cones with the photographic films are the Debye-Scherrer powder lines. If, however, the crystals are not randomly distributed but lie in preferred orientations, the powder rings become nonuniform in density, indicating orientation. One of the outstanding studies of orientation in the field of catalysis was that of Beeck (1) who showed by electron diffraction that nickel films deposited under certain conditions showed unusual catalytic activity and that this activity was accompanied by a particular orientation of the nickel crystals. 

X-ray diffraction patterns have been worked out by Fankuchen (8) with the Debye-Scherrer powder method, which, because of the cubic nature of the crystals were quite helpful in some respects for the interpretation of the structure. Fankuchen obtained patterns reconcilable with a face-centered cubic unit cell, and the pattern was the same both in ferritin and in apoferritin. The distances of the lines were the same, but there was a difference in the intensities of some lines, those of ferritin being more intense than those of apoferritin. This seems to show, once... 

The second sample was analyzed similarly as BaU03 Q6 X-ray powder diffraction Debye-Scherrer films were indexed as simple cubic with a0 - 4.4155 0.0005 A for Ba0.99U03 20 and a0 = 4.4007 0.0020 A for BaU03 06 Several weak non-cubic lines were found on the Ba8 9gU03 2o film. 

The deformation of the lattice as a result of the mechanical working is seen from the broadening of the lines in the X-ray diffraction picture, which are narrow under normal circumstances (Debye-Scherrer or powder diagram). 

Powder X-ray diffraction (XRD) is performed on a Siemens D5005 diffractometer with Cu Ka radiation. The particle size is calculated from the X-Ray line broadening, using the Debye-Scherrer equation. DRS is measured by Perkin-Elmer Lambda 20 UV-visible spectophotometer at room temperature in the wavelength region between 200 and 800 nm. Raman spectra are recorded on a Broker RFS 100 with 2 cm resolution. 

Basically, a diffractometer is designed somewhat like a Debye-Scherrer camera, except that a movable counter replaces the strip of film. In both instruments, essentially monochromatic radiation is used and the x-ray detector (film or counter) is placed on the circumference of a circle centered on the powder specimen. The essential features of a diffractometer are shown in Fig. 7-1. A powder specimen C, in the form of a flat plate, is supported on a table H, which can be rotated about an axis O perpendicular to the plane of the drawing. The x-ray source is S, the line focal spot on the target T of the x-ray tube S is also normal to the plane of the drawing and therefore parallel to the diffractometer axis O. X-rays diverge from this source and are diffracted by the specimen to form a convergent diffracted beam which comes to a focus at the slit F and then enters the counter G. A and... 

This is a very recent development. It involves a side-window position-sensitive proportional counter (Sec. 7-5), a multichannel analyzer, and the rpeasurement of the angular positions of many diffraction lines simultaneously. The anode wire of the counter, which is long and curved, coincides with a segment of the diffractometer circle and is connected, through appropriate circuits, to an MCA. The powder specimen is in the form of a thin rod centered on the diffractometer axis. The geometry of the apparatus therefore resembles that of a Debye-Scherrer camera (Fig. 6-2), except that the curved film strip is replaced by a curved counter. 

The powder pattern of the unknown is obtained with a Debye-Scherrer camera or a diffractometer, the object being to cover as wide an angular range of 20 as possible. A camera such as the Seemann-Bohlin, which records diffraction lines over only a limited angular range, is of very little use in structure analysis. The specimen preparation must ensure random orientation of the individual particles of powder, if the observed relative intensities of the diffraction lines are to have any meaning in terms of crystal structure. After the pattern is obtained, the value of sin 9 is calculated for each diffraction line this set of sin 9 values is the raw material for the determination of cell size and shape. Or one can calculate the d value of each line and work from this set of numbers. 

The classical photographic method for recording powder diffraction patterns is still used, particularly when the amount of sample is small. The most common instrument forthis purpose is the Debye-Scherrer pov/det camera, which is shown schematically in Figure 12-17a. Here, the beam from an X-ray tube is filtered to produce a nearly monochromatic beam (often the copper or molybdenum Ka line), which is collimated by passage through a narrow tube. 

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