Suppression of the antiferromagnetic state in La$_{0.82}$Ba$_{0.18}$CoO$_{3}$ cobaltite at high pressure

The crystal and magnetic structure of La$_{0.82}$Ba$_{0.18}$CoO$_{3}$ complex cobalt oxide is studied by the neutron diffraction technique at high pressure (up to $4$ GPa) within the temperature range of $10$–$290$ K. At normal pressure, the crystal structure of La$_{0.82}$Ba$_{0.18}$CoO$_{3}$ corresponds to the rhombohedral symmetry described by the space group $R\bar{3}c$. At temperatures below $T_N=100$ K, the onset of a noncollinear antiferromagnetic (AFM) phase is observed. This phase is characterized by the wave vector $k=(0, -0.5, 0.5)$. The applied high pressure leads to the rapid suppression of the AFM phase, which completely vanishes at $P>2$ GPa. In La$_{0.82}$Ba$_{0.18}$CoO$_{3}$, the degree of instability of the AFM phase with respect to the high pressure is appreciably higher than that in the related compounds with the ferromagnetic ground state. The correlation between the instability of the AFM state in La$_{0.82}$Ba$_{0.18}$CoO$_{3}$ at high pressure and the changes in the electron configuration of Co$^{3+}$ ions is also analyzed.