Pressure-induced antiferromagnet-ferromagnet transition and a change in the spin state of Co in La0.5Ba0.5CoO2.8

The crystal and magnetic structures of anion-deficit cobaltite La0.5Ba0.5CoO2.8 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 to the tetragonal phase with symmetry P4/mmm has been observed in this compound at P a parts per thousand 3 GPa. 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 Co3+ ions. Possible mechanisms of the observed magnetic phase transition, as well as the role of the superexchange interactions 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 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.