Spin and structural transitions in AlFeO3 and FeAlO3 perovskite and post-perovskite

We extend the field of investigation of the perovskite to post-perovskite phase transition from the Earth's lowermost mantle with a study of the double substitution Mg + Si = Al + Fe. For this we use an advanced formalism within the density functional theory, the planar augmented wavefunction method, to investigate the perovskite and post-perovskite structures containing only Al and Fe3+ as cations. We look in particular at their relative stability and at their magnetic properties. We distinguish two crystallographic cases: AlFeO3 and FeAlO3, corresponding respectively to two ordered cases: one with Fe and then one with Al in octahedral coordination. For each case we investigate several spin configurations. We observe that up to 90 GPa the FeAlO3 perovskite structure, i.e. with Fe3+ in the interoctahedral space, with antiferromagnetic configuration and large local magnetic moment is the most stable one. Beyond 90 GPa the post-perovskite structure of AlFeO3, i.e. with Fe3+ in octahedral coordination, with antiferromagnetic configuration and small local magnetic moment is the most stable one. The perovskite to post-perovskite phase transition at 90 GPa is associated with a site exchange that triggers a partial collapse of the magnetic moment. The local magnetic moment vanishes beyond 150 GPa in post-perovskite. Our calculations suggest that the presence of Al + Fe3+ in perovskite/post-perovskite renders the phase transition sluggish, induces a large density jump at the transition and contributes into maintaining a residual magnetic spin down to the base of the mantle.