  We have successfully calculated the electronic and structural properties of
chromia (Cr_2O_3) in the Local Spin Density Approximation (LSDA). We predict a
transformation from the corundum to the Rh_2O_3(II) structure around 15 GPa in
the anti-ferromagnetic (AFM) phase as well as in the paramagnetic (PM)
insulating state which occurs above the Neel 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.
