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A. INTRODUCTION

a. historical

The idea of identifying crystalline substances by ratios of cell edges and by interedge angles intrigued pre-x-ray crystallographers. The chief stumbling block was the necessity to agree on conventional rules to insure uniqueness of crystal setting. We are indebted to crystal morphologists for such determinative schemes as Fedorov’s Krystallreich (1920) and Barker’s Index (Porter and Spiller, 1951). For the past fifteen years, Hanawalt’s determinative method based on powder x-ray diffraction data (Hanawalt et al., 1938) has ruled supreme, to the extent that some crystallographers, in order to get powder data, have not hesitated to smash good crystals to smithereens, while others, refusing to turn crystalloclasts, have invented ingenious devices to rock the single crystal in the x-ray beam (Mathews and Mcintosh, 1948).

Recent developments in single-crystal techniques and x-ray instrumentation, particularly M. J. Buerger’s precession method (1944) and the commercial availability of his precession camera, make it easy and fast to obtain cell dimensions and space group, especially when precession and Weissenberg methods are used jointly. In favorable cases the total amount of work required is not any greater than that involved in measuring the many interplanar spacings from a powder pattern and estimating the relative intensities of the lines, to say nothing of their indexing. Since identification by powder diffraction data cannot, in all rigor, be considered reliable until all the lines have been indexed, or matched with those of a previously indexed standard, the knowledge of the cell dimensions and diffraction aspect actually is…

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