  Understanding ferroelectricity is of both fundamental and technological
importance to further stimulate the development of new materials designs and
manipulations. Here, we perform an in-depth first-principle study on the
well-known ferroelectric barium titanate BaTiO$_{3}$ under a hydrostatic
negative pressure, showing an isosymmetric phase transition to a
supertetragonal phase with high $c/a$ ratio of $\sim1.3$. The microscopic
origin and driving mechanisms of this phase transition are identified as a
drastic change of the covalently $\pi$-bonded electrons. These findings provide
guidance in the search for new supertetragonal phases, with great opportunities
for novel multiferroic materials; and can be generalized in the understanding
of other isosymmetric phase transitions.
