Ray Powder Methods

Counter tube detectors were introduced in commercial X-ray apparatus at the end of World War II. This development launched a vast increase in the use of X-ray methods. Although film techniques are still widely used, the bulk of X-ray powder work is now done with counter tube diffractometers. 

By present standards, it is possible to define a reasonable standard of a good X-ray diffractometer pattern against which performance can be judged. Such a pattern is one of the most important single pieces 

The relative amounts of two (or more) phases present in a mixture may also be obtained from powder X-ray data. The accuracy is 

The lattice parameters are the averages of millions of unit cell dimensions in the specimen and are important in characterizing materials. A precision of 1-0.1 % can be obtained rapidly using routine methods. Higher precision measurements of the order of 0.01 % or better are required to determine the coefficients of thermal expansion and in a variety of other studies. The indexing of complex powder patterns requires precise measurements of many reflections in the low-angle region. 

Relative lattice parameter values may be determined with much higher precision. Bond has described a relatively simple method for single-crystal plates in which a precision of a few parts per million was attained. The Kossel technique has been reported to have an accuracy better than one part in 50,000 provided certain precautions are taken. The elegant centroid method is limited at present to high-symmetry substances of small unit cell dimensions and by the need for better X-ray spectral data. 

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Jenkins, R. (2002). X-ray powder methods. In McGraw-Hill Encyclopedia of Science and Technology 19, New York, McGraw-Hill, pp. 668-673. 

Six comments are appropriate at this point. Firstly, it is the experience of the reviewer that chemically-similar compounds with very similar infrared and Raman spectra in factor-group split regions are isomorphous. This method is probably at least as reliable as X-ray powder methods when there is a significant change in scattering factors between the two compounds studied (e.g. bromo-derivatives of... 

Schematic diagram illustrating x-ray powder method (Straumanis arrangement). Note splitting of hack-reflection lines.

H. P. King andL. E. Alexander, X-Ray Diffraction Procedures , 2nd edition, John Whey Sons, New York, 1974, A very comprehensive treatment. X-ray powder methods including linebroadening analysis, texture determination, stress measurement and studies of amorphous materials. 

The product is a yellow solid, extremely sensitive to air and moisture, characterized by X-ray powder methods to give the following distances (pm) K—O, 267 and 278 K—K, 365 C—O, 128 C—C, 121. Treatment with water results in the formation of glycolic acid. The salt obtained from molten cesium and CO also gives glycolic acid on hydrolysis ... 

There is some controversy concerning the existence of the a isomers of these tris complexes, Israily reported a purple complex to be or-[Cr(gly)3] however, subsequent workers have shown that this substance most probably was the dihydroxy dimer [Cr2(gly)4(OH)2]. Careful chromatography, on potato starch, of solutions from chromium(III)/glycine reactions yielded red and purple fractions,the electronic spectra of which were consistent with jS and a isomers respectively. Solutions of the a complex were unstable even in the dark and cold. Hoggard has recently claimed the preparation of the a isomer of the glycine complex by a fractional crystallization. The complex was anhydrous, unlike its cobalt(III) analogue. X-Ray powder methods could hence not be used to confirm the identity of the complex the luminescence spectra were held to be consistent with meridional coordination. There have been a number of studies of the physical properties of /S-[Cr(gly)3], summarized in Table 99. 

Mayer and Tendy (1970) investigated the phase equilibria along the concentration section Pr5 Gd Si3 by means of X-ray powder methods. For sample preparation, see LajSij. Ge. Among the two competing structure types the MujSij-type of structure (Pbj/mcm) was claimed to be stable in the range 2 < jc < 5, whereas the Cr Pj-type with space group 14/mcm occurred for 0 < jc < 1.5. For lattice parameters versus a , see fig. 28. The discontinuities of V and a, however, do not seem to coincide ... 

By X-ray powder methods Bodak et al. (1978) observed the compound Tm2Rc3Si5 to crystallize with the Sc2Fe3Si5-type of structure [P4/mnc, a = 10.83(1), c = 5.474(5)]. Samples were arc melted, but no detailed conditions of heat treatment were reported. Segre (1981) confirmed the structure type but measured different lattice parameters a — 10.89(1) and c — 5.456(8). Samples were arc melted = 1.2 K. 

Y2Mn3Si5 was reported with the ScjFejSij-type of structure, P4/mnc, a = 10.64(1), c = 5.462(8) (Segre, 1981 X-ray powder methods) samples were prepared by arc melting (no annealing). The location of Y2Mn3Si5 (high-temperature phase ) has been marked in fig. 56 by an asterisk. 

X-ray powder methods, like IR spectra, are exceptionally useful for a cheap, quick, nondestructive positive fingerprint identification of phosphorus compounds - as indeed they are for chemical compounds in general. Reference powder data are available in published x-ray powder diffraction data files. 

The X-ray powder method showed that the compound, containing almost no water (less than 0.08 weight%), is isostructural with the clathrate compound [CdBi X2] 0.678 0.33H20 described above, the latter melting in-congruent ly at 136.4 0.5°C. 

Alloys were synthesized by arc melting pure metals (99.9 mass% R 99.99 mass% In and Ge) followed by annealing for 700 h at 870 K in evacuated quartz tubes. Crystal structure was investigated by the single-crystal method for the LajlnGea compound, for other phases the X-ray powder method was employed. 

Oleksyn (1990) investigated the crystal structure of Gdii7Fe52Gen2 by the X-ray powder method (Tbii7Fe52Gen2 type, a = 2.8656). The alloy was synthesized by arc melting followed by annealing at 1070 K for 350 h. The starting materials were Gd 99.83 mass%, Fe 99.9 mass%, Ge 99.99 mass%. 

Rossi et al. (1978b) reported an investigation of the erystal structure of the ErMu2Ge2 compound (CeGa2Al2-type structure, a=0.3963, c= 1.0835 X-ray powder method). For the experimental details, see TbMn2Ge2 xmder Tb-Mn-Ge. 

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