Pressure-induced structural and magnetic phase transitions in ${\mathrm{La}}_{0.75}{\mathrm{Ba}}_{0.25}{\mathrm{CoO}}_{2.9}$ 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.

Figure 1

(a) Temperature-dependent ZFC and FC magnetizations for ${\mathrm{La}}_{0.75}{\mathrm{Ba}}_{0.25}{\mathrm{CoO}}_{2.9}$, measured in an applied magnetic field of 200 Oe. (b) Field-dependent ZFC magnetization measured at 5 K. Inset: The same plot at lower fields showing the finite coercive field of about 0.4 T. (c) Temperature-dependent resistivity measurements in zero field for ${\mathrm{La}}_{0.75}{\mathrm{Ba}}_{0.25}{\mathrm{CoO}}_{2.9}$ and ${\mathrm{LaCoO}}_3$ samples.

Figure 2

The Rietveld refinement of the neutron-diffraction data for ${\mathrm{La}}_{0.75}{\mathrm{Ba}}_{0.25}{\mathrm{CoO}}_{2.9}$ measured at 2 K (a) with $\lambda$ = 1.59 Å and at 300 K (b) with $\lambda$ = 1.05 Å. The experimental data are in open circles, the calculated pattern is in blue, and the difference curve is in green. The tick marks indicate Bragg peak positions for $Pm\overline{3}m$ (gray), $R\overline{3}c$ (green), FM (blue), and AFM (red) phases. (c) The magnetic moment of ${\mathrm{Co}}^{3+}$ for separate AFM and FM phases as a function of temperature.

Figure 3

The temperature dependence of (a) Co-O bond length and (b) MSD of Co-O bond, both obtained from the Rietveld refinement of the neutron powder diffraction data for ${\mathrm{La}}_{0.75}{\mathrm{Ba}}_{0.25}{\mathrm{CoO}}_{2.9}$ and ${\mathrm{LaCoO}}_3$ samples.

Figure 4

The Rietveld refinement of the synchrotron x-ray diffraction data for ${\mathrm{La}}_{0.75}{\mathrm{Ba}}_{0.25}{\mathrm{CoO}}_{2.9}$ measured at 50 K and 16 GPa, with experimental data in open circles, the calculated pattern in blue, and the difference curve in green. The tick marks indicate Bragg peak positions for $R\overline{3}c$ (black) and $Pm\overline{3}m$ (red) phases.

Figure 5

The pressure dependence of the unit-cell volume for the rhombohedral (a) and cubic (b) phases at various temperatures. Corresponding Co-O bond length for the rhombohedral (c) and cubic (d) phases at the same temperatures. The lines are guides for the eye.

Figure 6

The volume of the cubic phase fraction as a function of applied pressure at various temperatures.

Figure 7

(a) The energy of different magnetic configurations for as a function of the unit-cell volume. (b) The dependence of the difference $\mathrm{\Delta }E$ of total energies on the volume between $Pm\overline{3}m$ (AFM, FM) and $R\overline{3}c$ (AFM) magnetic states. The vertical dashed lines correspond to the phase transitions.

Figure 8

Total and partial densities of states for (a), (b) $Pm\overline{3}m$ (NM) at V = 8.96 ${\AA }^3$/atom (LS state), for (c), (d) $Pm\overline{3}m$ (AFM) with V = 11.51 ${\AA }^3$/atom (HS state), and for (e), (f) $Pm\overline{3}m$ (FM) at $V$ = 10.18 ${\AA }^3$/atom (IS state). Zero energy corresponds to the Fermi energy. Positive and negative values of DOS correspond to spin-up and spin-down.