  We have studied ab initio the phase transition in PbZrO3, a perovskite oxide
usually presented as the prototypic anti-ferroelectric material. Our work
reveals the crucial role that anti-ferrodistortive modes -- involving concerted
rotations of the oxygen octahedra in the structure -- play in the
transformation, as they select the observed anti-ferroelectric phase, among
competing structural variants, via a cooperative trilinear coupling.
