Indexing patterns of noncubic crystals analytical methods

Indexing patterns of noncubic crystals (analytical methods) 337 

Every mark on the paper strip must coincide with a curve on the chart, except for extraneous lines. A structure which accounts for only a portion of the observed lines is not correct all the lines in the pattern must be accounted for, either as due to the structure of the substance involved or as extraneous lines. 

There need not be a mark on the paper strip for every curve on the chart, because some lines may have zero intensity or be too weak to be observed. 

Substances with these low-symmetry structures yield powder patterns which are almost impossible to index by graphical methods, although the patterns of some orthorhombic crystals have been indexed by a combination of graphical and analytical methods. The essential difficulty is the large number of variable parameters involved. In the orthorhombic system there are three such parameters (flf, b, c), in the monoclinic four (a, b, c, P), and in the triclinic six a, b, c, oc, p, y). If the structure is known, patterns of substances in these crystal systems can be indexed by comparison of the observed sin 0 values with those calculated for all possible values of hkl. 

10-5 INDEXING PATTERNS OF NONCUBIC CRYSTALS (ANALYTICAL METHODS) 

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