process of qualitative and semiquantitative identification is considerably simpler 

 than if only single-crystal rotation patterns were available. For identification of a 

 specimen from a powder-diffraction pattern, the radii, or diameters, of all lines 

 in the pattern are measured, and the interplanar spacings calculated. The 

 details of the procedure to be followed depend on the nature of the unknown 

 and on the amount of other data available, such as optical and physical proper- 

 ties and chemical analyses. 



If the unknown represents a pure compound or a mixture composed 

 essentially of one constituent with only minor amounts of other ingredients 

 and nothing is known concerning the identity of the compound or the principal 

 ingredient, the Hanawalt method of identification is used. 



The Hanawalt method, recommended by the American Society for Testing 

 Materials, is based upon a card-file index system catalogued according to the 

 three strongest lines in the pattern (A.S.T.M., E43-42T, 1942). After the 

 pattern of the unknown has been measured, converted into interplanar spacings, 

 the intensity of the lines estimated, and at least the three strongest lines (more 

 if the three strongest lines are not outstanding) identified, the group of cards 

 representing materials for which the strongest line corresponds to the same 

 interplanar spacing as does the strongest line in the pattern is selected from 

 the index. The subgroup for which the second-most-intense line corresponds to 

 the same interplanar spacing as does the second-strongest line in the pattern 

 of the unknown is then examined for correspondence between the third line of 

 the cards and the third-strongest line in the pattern. Finally, the entire pattern 

 of the unknown is checked against the pattern selected from the card index. This 

 procedure is illustrated in Figure 9-9. However, because of differences between 

 the techniques used in obtaining the data for the card index and that used by 

 the operator in obtaining the pattern of the unknown, or because of variations 

 found in the patterns of some types of materials (to be discussed later), the 

 operator should regard correspondence within ± 0.05 A as a satisfactory match 

 for interplanar spacing in comparing his patterns with those recorded in the 

 index. This same possible variation should be allowed in selecting the groups 

 of cards for comparison. 



The A.S.T.M. data cards described above are available in three different 

 types, namely, plain cards, punch system cards, and I.B.M. machine cards. 



Should the foregoing procedure be unsuccessful or if the specimen to be 

 identified is known to be a mixture of several ingredients all in only small or 

 moderate concentration, a somewhat different method of identification must be 

 used. In mixtures, each of the three strongest lines may belong to patterns of 

 different constituents so that the above procedure could not be used. For 

 relatively simple mixtures, the procedure above may work if more of the strong- 

 est lines (10 or 12) are used in searching the card index. In general, however, 



165 



