Origin of supertetragonality in BaTiO3

Understanding ferroelectricity is of both fundamental and technological importance to further stimulate the development of new materials designs and manipulations. In this respect, supertetragonal (ST) phases with high $c/a$ ratio of $\ensuremath{\sim}1.3$ have recently resulted in outstanding ferroelectric polarization values, which urges for a microscopic origin of these novel structural phases. Here, we perform an in-depth first-principle study on the well-known ferroelectric barium titanate ${\mathrm{BaTiO}}_{3}$ under a hydrostatic negative pressure, showing an isosymmetric phase transition to such a ST phase. The chemical origin and driving mechanisms of this phase transition are identified as a drastic change of the covalently $\ensuremath{\pi}$-bonded electrons. These findings provide guidance in the search for new ST phases, with great opportunities for novel multiferroic materials, and can be generalized in the understanding of other isosymmetric phase transitions.