Pressure-induced structural and magnetic phase transitions in La 0.75 Ba 0.25 CoO 2.9 studied with scattering methods and first-principle calculations

We studied structural and magnetic phase transitions under applied pressure for the doped cobaltite ${\mathrm{La}}_{0.75}{\mathrm{Ba}}_{0.25}{\mathrm{CoO}}_{2.9}$. Neutron and x-ray diffraction experiments established the coexistence of rhombohedral and cubic phases in the sample. The magnetic state at 2 K is best described as a long-range ordered antiferromagnet (AFM) with small ferromagnetic (FM) clusters. With application of pressure, the rhombohedral phase gradually transforms into a cubic one. At room temperature and the highest applied pressure of 16 GPa, the cubic phase accounts for 70% of the sample volume. Quantum mechanical modeling confirmed the experimental findings and provided more insights into the structural and magnetic phase transitions at pressures exceeding 16 GPa. While the cubic crystal structure was preserved above 10 GPa, the AFM to FM phase transition was found at around 16 GPa. Further increase of the pressure resulted in suppression of magnetic order above 45 GPa. Using density functional theory (DFT)$+\mathrm{U}$ calculations, we were able to relate macroscopic magnetic properties induced by pressure with corresponding spin-state transitions in Co ions.