= 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 $\sqrt2-1$ for sixfold and $\sqrt3-1$ for eightfold coordination. The high-pressure change is therefore consistent with $r_K/r_{Cl}$ increasing under compression. Since
$$
\frac{d}{dP}\ln\frac{r_K}{r_{Cl}}=\kappa_{r,Cl}-\kappa_{r,K},\qquad
\kappa_r=-\frac{d\ln r}{dP},
$$
\b[the 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.
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