We propose a Landau theory for phase change materials (PCMs), which describes stress-induced amorphization in vacancy-free, ordered PCMs as a condensation of defects in analogy to equilibrium gas-liquid transitions. Three dimensionless parameters suffice to deduce a highly-accurate equation of state for both phases from it. The reference data for our model alloy GexSb1−x is produced from molecular dynamics simulations and synchrotron X-ray diffraction (XRD). Raman spectroscopy is used to estimate the density of tetrahedrally coordinated germanium atoms, which we relate to the order parameter. All methods provide consistent support for the reversibility of the transition.