 | Acta Crystallographica Section E Acta Crystallographica Section E CRYSTALLOGRAPHIC COMMUNICATIONS |
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![[Figure 5]](oi2031fig5mag.jpg)
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Figure 5
The Fourier analysis graph for the 111 reflection of the MX rock salt structure in the upper graph explains the finding of W. L. Bragg that this reflection was observed for NaCl, but not for KCl. As seen, all cations (orange) match with the positive maxima and all chlorine anions (dark green) coincide with the negative maxima. As M:X is 1:1 in this structure, it can be concluded that positive and negative contributions of M and X cancel each other out if the scattering amplitudes of the cations and anions have approximately the same magnitude (the same number of electrons in their shells). This is the case for KCl, where K+ and Cl− are isoelectronic ions (see above figure). However, in the case of NaCl, the individual contributions of the ions are different, which leads to a net value for the A coefficient. This, in turn, results in the appearance of the 111 reflections in NaCl. As illustrated in the lower graph, for the higher-order 222 reflection all positive maxima are coincident with the positions of the ions, so this reflection is present in the diffractograms of NaCl and KCl. |
![[Figure 5]](oi2031fig5.jpg)
| CRYSTALLOGRAPHIC COMMUNICATIONS |
ISSN: 2056-9890
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