Orbital ordering, ferroelasticity, and the large pressure-induced volume collapse in PbCrO₃

We predict a tetragonal ground state for perovskite-structured PbCrO ₃ from density functional theory (DFT) + U calculations, and explain its anomalously large volume. The predicted structure is stabilized due to orbital ordering of Cr d in the presence of a large tetragonal crystal field, mainly due to off-centering of the Pb atom. At higher pressures (smaller volumes) there is a first-order transition to a cubic phase where the Cr-d orbitals are orbitally liquid. This phase transition is accompanied by a ∼11.5% volume collapse, one of the largest known for transition-metal oxides. The large ferroelasticity and its strong coupling to the orbital degrees of freedom could be exploited to form potentially useful magnetostrictive materials.