Question

5. The (111) peak (reflection) in the diffraction pattern of KCI has zero intensity, but in the diffraction pattern of KI it is fairly strong. Explain (hint: use the figure at right showing the 111 planes with black dots representing the cation and white dots representing anion, and consider the relative scattering capabilities). 2 (111 111) 1111) (111)

Answer 1

In KCl the K+ reflection is out of phase to that of Cl and the K+ and Cl are isoelectronic. Due to this scattering factor are almost same and they cancel each other leading to zero intensity of the (111) peak. While the I and K+ are not isoelectronic and their scattering factors are not similar and hence their scattering intensity cannot cancel each other resulting in the strong (111) peak for KI.