Debye-Scherrer technique

Owen et al. reported x-ray powder diffraction data for procaine and 16 other anesthetics, as obtained using the Debye-Scherrer technique [55]. 

Wyckoff and Crittenden (7), using the Debye-Scherrer technique, have investigated FesCh, and Fe304 promoted with FeOAUOa and with K2OAI2O3. Their results indicate that the promoters do not produce separate phases, but form solid solution with magnetite. 

Much of the early work was done before the need for high resolution diffraction equipment was generally recognized, and the Debye-Scherrer techniques available at that time may well have failed to reveal the fine detail at present being recorded. The very ease with which powder patterns can be recorded, and indexed for phases of high symmetry in terms of a particular unit cell, does not in itself mean that a structure is satisfactorily determined by analogy with others of similar composition and crystallographic constants. It is of equal importance... 

Phase equilibria have been established in the ternary Nd-Ag-Ge system over the whole concentration region for the isothermal sections at 870 K and 1070 K by Salamakha et al. (1996f) and Zaplatynsky et al. (1996) (figs. 67a,b). The existence of four and five ternary compounds respectively have been observed. Samples were melted from pieces of high purity components (Nd 99.85 mass%, Ag 99.99 mass%, Ge 99.99 mass%) under argon atmosphere in an arc furnace with water-cooled copper hearth. The ingots were subsequently annealed at 870 K (1070 K) and quenched in cold water. The isothermal sections were constructed using X-ray powder diflraction film data obtained by the Debye-Scherrer technique with non-filtered CrK radiation. 

By using Debye-Scherrer technique for polycrystals, lattice parameter can be determined. The lines (arcs) can be indexed by adopting the following... 

Vin] X-ray diffraction (Debye-Scherrer technique) Mo5Fc95 ,B , (15 < X < 25)... 

The preceding setup allows both X-ray diffraction (32) and absorption experiments (33, 34). The capillary geometry was used until about 30 years ago for ex situ XRD studies in connection with the placement of Lindemann tubes in powder Debye-Scherrer cameras. At that time, films were used to detect the diffracted X-rays. Today, this cumbersome technique has been almost completely replaced as modern detectors are used. 

Powder photographs were taken with a 57.3-mm radius Debye-Scherrer camera and nickel-filtered Cu Ka radiation (Xmean = 1.5418 A). For intensity work, the multiple-film technique was used. The lattice constant, derived from 23 reflections from 42.56° to 73.47° 20 and corrected for film shrinkage with a parallel film of a reference substance, was Oo = 15.02 A with an estimated standard deviation of 0.01 A. 

There are several experimental techniques for realizing the diffraction conditions, the most powerful of which depends on having a single crystal of the substance to be studied see Exp. 46. In the present experiment we are concerned only with the Debye-Scherrer method (often called the powder method), which does not make use of a single crystal but rather of a powder obtained by grinding up crystalline or microcrystalline material. This powder eontains crystal particles of a few micrometers in size. 

There is an optimum specimen thickness for the transmission method, because the diffracted beams will be very weak or entirely absent if the specimen is either too thin (insufficient volume of diffracting material) or too thick (excessive absorption). As will be shown in Sec. 9-8, the specimen thickness which produces the maximum diffracted intensity is given by l/ i, where /t is the linear absorption coefficient of the specimen. Inspection of Eq. (1-10) shows that this condition can also be stated as follows a transmission specimen is of optimum thickness when the intensity of the beam transmitted through the specimen is 1 /e, or about j, of the intensity of the incident beam. Normally this optimum thickness is of the order of a few thousandths of an inch (0.1 mm). There is one way, however, in which a partial transmission pattern can be obtained from a thick specimen and that is by diffraction from an edge (Fig. 6-13). Only the upper half of the pattern is recorded on the film, but that is all that is necessary in many applications. The same technique has also been used in some Debye-Scherrer cameras. 

For characterization, common X-ray techniques (Debye-Scherrer or, better, Guinier patterns) are useful. The compound CsPr2Cl7 crystallizes in the ortho-... 

R. Brill has carried out a series of experiments which indicate very clearly the influence of particle size on the sharpness of the Debye-Scherrer lines for iron powder. There is quite a powerful effect, the evaluation of which would be very profitable. Scherrer has already established in the work previously mentioned that the particle sizes of colloidal gold calculated by the use of his formula agree very satisfactorily with those obtained by the Zsigmondy method of direct counting. Later, Brill compared the different relations available for the computation with one another and obtained the data contained in Table 72. This includes a comparison of the relation originally given by Scherrer with the Laue formula modified by Brill and Pelzer. Various preparations of very fine-grained iron, which are of importance in the technique of catalysis in the ammonia process, served as material. 

While much of his reputation was based on nonpolymeric accompHshments, such as demonstrated by the Debye-Huckel theory, the Debye-Scherrer x-ray diffraction technique, the Debye-Sears effect in liquids, the Debye temperature, the Debye shielding distance, the Debye frequency and the Debye unit of electric moment, his development of the hght scattering technique for the determination of the molecular weight of polymers resulted in his also being recognized as a world class polymer scientist. 

People often use the x-ray diffraction (XRD) technique to estimate the crystal size based on the crystallite facets. For Pt, the prominent crystalline surfaces are the (111), (200), and (220) and (311), respectively. Based on the broadening of the x-ray diffraction peaks, the crystallite sizes are calculated according to the Debye-Scherrer equation. 

The discussion of this technique has been kept short because the diffraction spectra are very similar to Debye - Scherrer patterns the method is becoming very important for molecular structure analysis. An electron beam in a high vacuum (0.1-10 Pa) collides with a molecular beam, and the electrons are diffracted by the molecules. The film reveals washed-out Debye-Scherrer rings, from which a radial electron density distribution function for the molecule can be derived. Together with spectroscopic data, the distribution makes it possible to infer the molecular structure 129]. 

Crystalline materials can be identified by rapid computerized powder diffraction techniques. The principle of this technique [6,7] is that the crystallites within a sample, placed in a collimated x-ray beam, reflect x-rays at specific angles and intensities. The diffraction pattern can be recorded photographically, using a camera, e.g. a Debye-Scherrer camera, or using a powder diffractometer. Chemical analysis depends on the fact that each chemical composition and crystallographic structure produces a unique angular distribution of diffracted intensity. Analysis is based on comparison of the diffractometer scan with known standards. Typical applications of the powder diffraction technique to polymers would be the identification of mineral fillers in engineering resins, the nature of crystalline contaminants and determination of crystalline phases in a material. 

The Guinier camera (e.g. [1]) is altogether more sophisticated than the pinhole and filtration techniques used in Debye-Scherrer type cameras, and will enable the user to make accurate determinations of lattice spacings up to 15 nm with exposure times of the order of a few tens of minutes for an average sample on a standard X-ray generator. 

Experimental Techniques Orientation of the crystalline phase in polyethylene films has been mainly characterized by WAXD [82-98]. In the first studies, the Debye-Scherrer method was employed with a flat-film camera [82-84]. This technique was still used in further work because of its rapidity and its simplicity [87,88,93, 95-98]. 

In the range of 10 kbar, a gas-filled bomb with beryllium windows suffices. Since 1949, a number of miniature pressure vessels have been designed in which both the bomb and sample are bathed in the X-ray beam. Polycrystalline beryllium, single-crystal beryllium, and singlecrystal diamond containers have been used for X-ray. transparency. More recently, diamond and WC anvils have permitted pressures of 200-500 kbar, respectively. Details of such instrumentation have been reviewed by Klement and Jayaraman. These techniques employ the Debye-Scherrer geometry and use mostly film recording, though counter tubes have been used in some of the anvil procedures. Cell dimension accuracy is limited to about 0.5 % under these conditions and structure refinements are hampered by the difficulties associated with intensity measurement. 

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