Abstract
We present an ab initio based method of constructing the phase diagram of the second-order phase transitions with a significant volume dependence of the soft mode. This approach is based on the assumption that in the investigated crystal the phonon frequencies are mainly dependent on crystal volume and geometry. A critical volume for the transition at $T=0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is derived from the crystal symmetry breaking point. The quasiharmonic approximation is used to find the free energy and the equation of state at finite temperature. The phase transition line is then the locus of points in the $p\text{--}T$ diagram corresponding to the critical volume. As an example we determine the $p\text{--}T$ phase diagram for $\mathrm{Ca}{\mathrm{Cl}}_{2}$, which undergoes a rutile type---orthorhombic second-order phase transition.
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