Pressure-induced antiferromagnet-ferromagnet transition and a change in the spin state of Co in $La_{0.5}Ba_{0.5}CoO_{2.8}$

The crystal and magnetic structures of anion-deficit cobaltite La0.5Ba0.5CoO2.8 have been studied by theX-ray and neutron diffraction methods at high pressures up to 30 and 6 GPa, respectively. A structure transi-tion from the cubic phase with symmetry Pm m to the tetragonal phase with symmetry P4/mmm has beenobserved in this compound at P ≈ 3 GPa. It is accompanied by the change of the ground antiferromagneticstate of the G type to the ferromagnetic state because of a change in the electronic configuration of Co3+ ions.Possible mechanisms of the observed magnetic phase transition, as well as the role of the superexchange inter-actions between Co3+ ions in various spin states in the formation of magnetic properties, have been discussed.Pressure induced change in the magnetic state of La0.5Ba0.5CoO2.8 significantly differs from related stoichi-ometric compounds La0.5A0.5CoO3.0 (A = Sr, Ca), where the ground ferromagnetic state is formed and thesign of the pressure dependence of the Curie temperature depends on the element A