Magnetostructural effects and phase transition in Cr₂O₃ under pressure

We have successfully calculated the electronic and structural properties of chromia (Cr₂O₃) in the local spin density approximation. We predict a transformation from the corundum to the Rh₂O₃ (II) structure around 15 GPa in the antiferromagnetic (AFM) phase as well as in the paramagnetic (PM) insulating state which occurs above the Néel temperature (T_N). This transition is relevant to interpreting the optical anomalies observed in the absorption spectrum of ruby under pressure. We have modeled the structural properties of the PM state using a Landau-like expansion of the magnetostriction energy. This treatment correctly describes the structural anomalies across T_N in the corundum phase and indicates that the AFM and PM insulating states should have distinct compressive behaviors.