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4. Interpretation of Powder Photographs

We must first have the values of $\theta$ for all lines on a powder photograph, and, if the number is large, assigning indices hkl to them all may be difficult. Fortunately, many metal and alloy structures are so simple that their powder pattern can be recognized at a glance. Our first task is thus to familiarize ourselves with these patterns.

The three most common structures are called face-centred cubic (f.c.c.), body-centred cubic (b.c.c.) and hexagonal close-packed (h.c.p.). They are illustrated in Fig. 4. The sequence of lines - in position and intensity - is characteristic of each structure and the scale of the pattern gives the dimensions of the unit cell; the smaller the scale of the pattern, the larger is the unit cell. Therefore there are more lines for a larger unit cell.

 

Figure 4: Powder patterns of three common types of simple crystal structures. (a) Face-centred cubic, (b) body-centred cubic, (c) hexagonal close-packed. For (c) the exact sequence will depend upon the axial ratio. Relative intensities are not represented.

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