Most solids are rather incompressible.
That is, when squeezed, their volume changes only a little, even if a structural phase transition takes place during the process. The physical mechanisms responsible for these transitions have been debated since discovery of the Cerium phenomenon over 50 years ago. In both cases the strongly correlated nature of the 4f electrons is fundamentally implicated. These theories are at a somewhat unsatisfactory stage. We published a comprehensive review of the central experimental and theoretical issues.