  A novel atomistic effective Hamiltonian scheme, incorporating an original and
simple bilinear energetic coupling, is developed and used to investigate the
temperature dependent physical properties of the prototype antiferroelectric
PbZrO3 (PZO) system. This scheme reproduces very well the known experimental
hallmarks of the complex Pbam orthorhombic phase at low temperatures and the
cubic paraelectric state of Pm 3m symmetry at high temperatures. Unexpectedly,
it further predicts a novel intermediate state also of Pbam symmetry, but in
which anti-phase oxygen octahedral tiltings have vanished with respect to the
Pbam ground state. Interestingly, such new state exhibits a large dielectric
response and thermal expansion that remarkably agree with previous experimental
observations and the x-ray experiments we performed. We also conducted direct
first-principles calculations at 0K which further support such low energy
phase. Within this fresh framework, a re-examination of the properties of PZO
is thus called for.
