Ambiguous Unknowns for X-Ray Diffraction Identification X-ray diffraction analysis is a time-honored and usually dependable analytical tool for the identification of crystalline materials. But i t is not necessarily infallible! While teaching instrumental methods of analysis some years ago, I would give out several (usually cubic) inorganic salts along with a list of the d-spacings and relative intensities of many compounds. The object, of course, was to identify the unknown by its lattice constants and/or d-spacings determined from a Debye-Schemer photograph. Potassium bromide and ruhidium chloride were among the unknowns. Their lattice constants are 6.5966 and 6.5810 A, respectively; both having the NaCl type lattiee. Since these compounds are also is-electronic, their diffraction patterns are nearly identical with regard to both d-spacings and intensities as shown in the table. tensity variances in the patterns are essentially due to the differences in the shapes of the atomic scattering factor curves. ASTM Powder Diffraction Data for Rubidium Chloride and Potassium Bromide a dA
Ill,
dA
RbCl
lih
d-4
lI11
dA
IiIl
dA
KBr
liIl
dA
Ilh
Only d-spacings >1.0A aregiven
This caused considerable consternation among the students. Thus, i t provided a perfect pedagogical example illustrating haw goad experimental technique, precise measurement, and careful interpretation of the data are of the utmost importance. In addition, the identical symmetries and the iso-electronic nature of these compounds was used to illustrate, from an operational point of view, that X-ray diffraction really senses only the periodic distribution of electrons in a crystalline lattice.
E. L. Lippert, Jr. Owens-Illinois, Inc. Toledo, Ohio 43666
Volume 50, Number 11, November 1973
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