Spin and structural transitions in AlFeO 3 and FeAlO 3 perovskite and post-perovskite

Abstract

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 Fe 3+ as cations. We look in particular at their relative stability and at their magnetic properties. We distinguish two crystallographic cases: AlFeO 3 and FeAlO 3, 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 FeAlO 3 perovskite structure, i.e. with Fe 3+ 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 AlFeO 3, i.e. with Fe 3+ 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 + Fe 3+ 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.