 Solid-phase Clapeyron slope from cell volumes

Molar crystal volume follows from $V_m=N_A{V}_{\mathrm{cell}}/Z$ with $Z$ atoms per cell for an elemental solid. Combining two such values with their molar entropy difference gives $\mathrm{d}P/\mathrm{d}T=\Delta S_m/\Delta V_m$. Ambient values supply only an approximation to a high-pressure coexistence slope, not a phase-boundary intercept.