Anomalous enhancement of tetragonality in ${\mathrm{PbTiO}}_3$ induced by negative pressure

Using a first-principles approach based on density-functional theory, we find that a large tetragonal strain can be induced in ${\mathrm{PbTiO}}_3$ by application of a negative hydrostatic pressure. The structural parameters and the dielectric and dynamical properties are found to change abruptly near a crossover pressure, displaying a “kinky” behavior suggestive of proximity to a phase transition. Analogous calculations for ${\mathrm{BaTiO}}_3$ show that the same effect is also present there, but at much higher negative pressure. We investigate this unexpected behavior of ${\mathrm{PbTiO}}_3$ and discuss an interpretation involving a phenomenological description in terms of a reduced set of relevant degrees of freedom.