 Solution

Use mass density $\rho =ZM/(N_A{a}^3)$, with $M=74.55\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$. The rock salt structure has $Z=4$ formula units per conventional cell; the CsCl structure has $Z=1$. In pressure order the densities are approximatelyThe increase across the transition is approximately $14.5\%$, much greater than the smooth compression within either branch. Comparing $a^3$ directly without correcting the different $Z$ would create a fictitious volume discontinuity.