Debye-Scherrer pattern, indexing

Three black nitrides have been reported to crystallize from Sr metal. SrN was reported to have the rocksalt structure [140]. A composition of Sr3N2 was deduced from elemental analysis [138. 141. 142], and a Debye-Scherrer pattern was collected [122] and indexed on a centered monodinic cell (actual composition Sr5N8) [143]. The structure of Sr,N was determined to be anti-CdCl, using single-crystal X-ray diffraction data [144],... 

The arrangement of helices in the solid and liquid crystalline states of poly(a-phenylethyl isocyanide) were determined by X-ray and electron diffraction. Well-defined diffraction patterns were obtained from oriented films using selected area electron diffraction. Intermolecular and intramolecular patterns were calculated from the five Debye-Scherrer rings. All the reflections were indexed in terms of a pseudo-hexagonal triclinic unit cell, with... 

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... 

For reasons to be discussed in Chap. 11, the observed values of sin 6 always contain small systematic errors. These errors are not large enough to cause any difficulty in indexing patterns of cubic crystals, but they can seriously interfere with the determination of some noncubic structures. The best method of removing such errors from the data is to calibrate the camera or diffractometer with a substance of known lattice parameter, mixed with the unknown. The difference between the observed and calculated values of sin 6 for the standard substance gives the error in sin 9, and this error can be plotted as a function of the observed values of sin 6. Figure 10-1 shows a correction curve of this kind, obtained with a particular specimen and a particular Debye-Scherrer camera. The errors represented by the ordinates of such a curve can then be applied to each of the observed values of sin 0 for the diffraction lines of the unknown substance. For the particular determination represented by Fig. 10-1, the errors shown are to be subtracted from the observed values. 

The films were deposited onto NaCl or KBr discs to facilitate examination by infrared transmission. Substrates of platinum, aluminum, ALOn, and fused SiO approximately one cm. in diameter were prepared for the rate studies. The index of refraction and isotropic character of film specimens obtained from the substrates and/or gas inlet tube were determined with a petrographic microscope. Debye-Scherrer x-ray diffraction powder patterns were made to establish whether the films were amorphous or crystalline. 

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