Hanawalt method

The Hanawalt method is relatively straightforward for unknowns which are nearly single phase. For multiphase mixtures the identification becomes tedious. Consider the data for a white powder given in Table 4 (7). Again the search is begun with the two most intense lines, 2.41 A and 3.04 A, but no match is found for this line-pair. Now a new line-pair must be chosen and the first and third most intense lines seem likely. Searching the Hanawalt manual with the pair 2.41 A and 1.70 A yields the possibilities 7-TaH, CaO, and NaYOa. Only the four lines of the CaO standard pattern lines match the lines in the unknown pattern. 

Extracted from the numerical search manual (Inorganic Compounds, Hanawalt Method). The three strongest lines appear in bold-face type. Intensities (rounded off) are shown by the... 

In theory, the Hanawalt method should lead to the positive identification of any substance whose diffraction pattern is included in the card file. In practice, various difficulties arise, and these are usually due either to errors in the diffraction pattern of the unknown or to errors in the card file. 

These remarks on abnormal intensities are not meant to suggest that successful use of the Hanawalt method requires extremely accurate intensity measurements. If reasonable care has been taken to minimize preferred orientation, then it is often enough to be able to list the diffraction lines in the correct order of decreasing intensity. 

The Hanawalt method fails completely, of course, when the unknown is a substance not listed in the card file, or when the unknown is a mixture and the component to be identified is not present in sufficient quantity to yield a good diffraction pattern. The latter effect can be quite troublesome, and, as mentioned in Sec. 12-4, mixtures may be encountered which contain more than 50 percent of a particular component without the pattern of that component being visible in the pattern of the mixture. 

How then can an exact match be made In this case there must be some outside knowledge supplied and an examination of the X-ray fluorescence spectra would indicate the presence of only Co and O in the unknown powder. When this information is used, the unknown is identified as Co304, card 9-418. However, when the card is examined the weak line 2.13 A remains unidentified in the unknown pattern. X-ray intensity is proportional to the amount of the diffracting material present, so it is natural to suspect that a contaminant phase will only display its most intense lines. The problem in this case is that there is only one line remaining so that the Hanawalt manual is useless to help in the identification. The intuitive method must be used. The key question to be answered is, what contaminants might be found in a sample containing only Co and O The two possibilities that come to mind are elemental Co or another oxide of Co. When these two possibilities are checked, it is quickly found that the most intense line of CoO, card 9-402, is indeed 2.13 A, and the identification is complete. 

The x-ray powder diffraction method dates back to Debye and Scherrer who were the first to observe diffraction from LiF powder and succeeded in solving its crystal structure. Later, HulF suggested and Hanawalt, Rinn and FreveP formalized the approach enabling one to identify crystalline substances based on their powder diffraction patterns. Since that time the powder diffraction method has enjoyed enormous respect in both academia and industry as a technique that allows one to readily identify the substance both in a pure form and in a mixture in addition to its ability to provide information about the crystal structure (or the absence of crystallinity) of an unknown powder. 

Throughout the long history of the technique, its emphasis underwent several evolutionary and revolutionary transformations. Remarkably, none of the new developments have taken away nor diminished the value of earlier applications of the powder diffraction method on the contrary, they enhanced and made them more precise and dependable. A noteworthy example is phase identification from powder diffraction data, which dates back to the late 1930 s (Hanawalt, Rinn, and Frevel). Over the years, this application evolved into the Powder Diffraction File containing reliable patterns of some 300,000 crystalline materials in a readily searchable database format (Powder Diffraction File is maintained and distributed by the International Centre for Diffraction Data, http //www.icdd.com). 

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