= 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.
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