Theoretical Study on BiCoO ₃ : Pressure‐Tuned Transformation of Structure, Magnetism, Charge, and Spin State

Abstract The atomic structures, electronic structures, magnetic properties, and spin states of BiCoO 3 multiferroic material under hydrostatic pressure are comprehensively investigated by using first‐principles calculation based on density functional theory. Firstly, we found the pressure‐tuned structural symmetry transition from tetragonal P 4 mm (No. 99) to cubic Pm m (No. 221) space group. The structural transformation prompts the Co─O polyhedral coordination transfer from a CoO 5 pyramidal to a CoO 6 distorted octahedron. Secondly, the pressure‐induced magnetic change from C‐type antiferromagnetic state to ferromagnetic state and then to paramagnetic configuration occurred after continuous compression of the volume. Essentially, the fundamental reason for the transition of the magnetic structure lies in the change of the internal atomic structure, such as the pressure induced abrupt changes of Co─O bond length and <Co─O─Co> bond angle. Meanwhile, an obvious volume collapse was observed when the pressure was applied to 51–56 GPa. Finally, as the pressure increases, the magnetism changes from C‐type antiferromagnetic state to paramagnetic configuration, accompanying the transformation from high spin state with b 2 g 2 e g 2 a 1 g 1 b 1 g 1 configuration in pyramid structure to low spin state with t 2 g 6 e g 0 in octahedral field. Therefore, our results demonstrate that applied hydrostatic pressure in BiCoO 3 can trigger the transformation of structure, magnetism, charge, and spin state.