Bunn chart

Another graphical method of indexing tetragonal patterns has been devised by Bunn [10.2]. Like the Hull-Davey chart, a Bunn chart consists of a network of curves, one for each value of hkl, but the curves are based on somewhat different functions of hkl and cja than those used by Hull and Davey, with the result that... 

Rhombohedral crystals are also characterized by unit cells having two parameters, in this case a and a. No new chart is needed, however, to index the patterns of rhombohedral substances, because, as mentioned in Sec. 2-4, any rhombohedral crystal may be referred to hexagonal axes. A hexagonal Hull-Davey or Bunn chart may therefore be used to index the pattern of a rhombohedral crystal. The indices so found will, of course, refer to a hexagonal cell, and the method of converting them to rhombohedral indices is described in Appendix 4. 

We can conclude that the pattern of any two-parameter crystal (tetragonal, hexagonal, or rhombohedral) can be indexed on the appropriate Hull-Davey or Bunn chart. If the structure is known, the procedure is quite straightforward. The best method is to calculate the cja ratio from the known parameters, lay a straightedge on the chart to discover the proper line sequence for this value of c/a, calculate the value of sin 6 for each line from the indices found on the chart, and then determine the indices of the observed lines by a comparison of calculated and observed sin 6 values. 

Since a rhombohedral lattice may be referred to hexagonal axes, it follows that the powder pattern of a rhombohedral substance can be indexed on a hexagonal Hull-Davey or Bunn chart. How then can we recognize the true nature of the lattice From the equations given above, it follows that... 

The value of these charts, in showing what adjustments of atomic parameters will increase or decrease the structure amplitude, has already been mentioned. To this little need be added except a suggestion on procedure in dealing with several independent reference atoms simultaneously. This is best explained by an actual example. In attempting to find the x and y coordinates of the atoms in the hkO projection of polychloroprene (Bunn, 1942 a) a particular set of postulated coordinates... 

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