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 La$_{0.5}$Ba$_{0.5}$CoO$_{2.8}$ have been studied by the X-ray and neutron diffraction methods at high pressures up to 30 and 6 GPa, respectively. A structure transition from the cubic phase with symmetry Pm$\bar{3}$m to the tetragonal phase with symmetry P4/mmm has been observed in this compound at $P\approx3\,$. It is accompanied by the change of the ground antiferromagnetic state of the G type to the ferromagnetic state because of a change in the electronic configuration of Co$^{3+}$ ions. Possible mechanisms of the observed magnetic phase transition, as well as the role of the superexchange interactions between Co$^{3+}$ ions in various spin states in the formation of magnetic properties, have been discussed. Pressure-induced change in the magnetic state of La$_{0.5}$Ba$_{0.5}$CoO$_{2.8}$ significantly differs from related stoichiometric compounds La$_{0.5}$A$_{0.5}$CoO$_{3.0}$ (A = Sr, Ca), where the ground ferromagnetic state is formed and the sign of the pressure dependence of the Curie temperature depends on the element A.