Entropic stabilization of perovskite structures*

Starting with a theoretical picture of the Coulomb Floppy Network, previously developed for empty perovskite materials, e.g. ScF₃, we demonstrate a prominent role played by the entropic effects in a broader context of the entire perovskite family.  In the case of ScF_3, those effects are essential for understanding a number of anomalous properties: negative thermal expansion,  entropic elasticity, thermal stabilization of high symmetry cubic phase, strong anharmonic and colossal pressure-induced softening. A conceptually similar theoretical approach is applicable to regular perovskites, as well. In particular, we show how intrinsically unstable cubic structure gets stabilized at finite temperature thanks to a combination of thermal fluctuations and steric constraints. The same entropic effect also manifests itself in the strong anharmonic of soft phonon modes.