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

The coordination number rises from six in the rock salt structure to eight in the CsCl structure. In the elementary hard-sphere ionic bonding model, accommodating eight opposite neighbours favours a larger cation/anion radius ratio: the geometrical limiting ratios are $\sqrt{2}-1$ for sixfold and $\sqrt{3}-1$ for eightfold coordination. The high-pressure change is therefore consistent with $r_K/r_{Cl}$ increasing under compression. Sincethe chloride ion is inferred to be more compressible than the potassium ion in this radius-ratio interpretation. This is a qualitative structural argument; real ionic radii depend on coordination and electronic structure, and the transition does not uniquely determine separate microscopic compressibilities.