Past exam of the materials science course of the University of Cambridge / 2022 / Paper 1 / 7 / c / Solution

Within each cubic phase, increasing pressure decreases $a$, so $d_{hkl}=a/\sqrt{h^2+k^2+l^2}$ decreases and Bragg's law moves a given reflection to higher $2\theta$. Near $2.55\mathrm{GPa}$, peaks of the rock salt structure disappear as those of the CsCl structure appear, potentially with a coexistence interval. The former has an FCC translation selection rule, with $h,k,l$ all even or all odd; mixed-parity reflections vanish kinematically. The latter has primitive-cubic translations and basis factor $f_K+(-1{)}^{h+k+l}{f}_{Cl}$, so it has no FCC extinction rule. Odd-sum reflections may nevertheless be weak because the two ionic scattering factors are similar. Peak spacings, relative intensities and the cell volume per formula unit change discontinuously. A simple uniform shift of the entire original pattern cannot describe the structural transition.