Spin order in the charge disproportionated phases of the $A$-site layer ordered triple perovskite $\mathrm{LaC}{\mathrm{a}}_2\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_9$

The coupling between spins and charge disproportionation states has been investigated in the $\mathrm{LaC}{\mathrm{a}}_2\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_9$ oxide with neutron powder diffraction. This $A$-site layer ordered triple perovskite $\mathrm{LaC}{\mathrm{a}}_2\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_9$ undergoes charge disproportionation on cooling and shows two different charge ordering patterns. At 230 K, $\mathrm{F}{\mathrm{e}}^{3.67+}$ disproportionates into a 2:1 ratio of $\mathrm{F}{\mathrm{e}}^{3+}:\mathrm{F}{\mathrm{e}}^{5+}$, which order in a layered manner along the $\langle 010\rangle$ direction of the pseudocubic unit cell. At lower temperatures $(T<170\mathrm{K})$, the charge ordering pattern changes to a layered arrangement along the $\langle 111\rangle$ direction. Neutron powder diffraction data show that in the intermediate temperature range ($170\mathrm{K}<T<230\mathrm{K}$) the spins order into a cycloidal structure on the $ac$ plane for the $\mathrm{F}{\mathrm{e}}^{3+}$ cations while the $\mathrm{F}{\mathrm{e}}^{5+}$ cations remain paramagnetic. For the lowest temperature range ($2\mathrm{K}<T<190\mathrm{K}$), the spin structure follows the charge ordering and evolves to a $\langle 111\rangle$ layered magnetic structure.

Figure 1

(a) Thermal evolution of the NPD patterns between 150 and 250 K on warming. Magnetic Bragg reflections are marked with arrows; the colored patterns show the transition temperatures observed by magnetometry at 230 and 170 K (red and blue arrows, respectively). (b) Temperature dependence of the lattice parameters and the volume from NPD data of $\mathrm{LaC}{\mathrm{a}}_2\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_9$ with both CD/magnetic transitions marked by dashed lines. The transition temperatures appear at 190 and 235 K here. They differ from the previously reported 170 and 230 K values due to possible differences in warming rate or thermal equilibration in the previous experiments.

Figure 2

$\mathrm{LaC}{\mathrm{a}}_2\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_9$ Rietveld refinement plots of neutron powder-diffraction data collected on WISH@ISIS at 240, 210, and 1.5 K. The observed (red dots), calculated (black lines), and difference (blue lines) patterns are shown. The allowed Bragg reflections (green vertical lines) for nuclear (top) and magnetic (bottom) phases are shown. Small asterisks mark magnetic diffraction from a secondary $\mathrm{CaFe}{\mathrm{O}}_3$ phase.

Figure 3

(a) Magnetic structure of $\mathrm{LaC}{\mathrm{a}}_2\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_9$ in the intermediate phase, $170\mathrm{K}<T<240\mathrm{K}$. $\mathrm{F}{\mathrm{e}}^{5+}$ spins remain idle while $\mathrm{F}{\mathrm{e}}^{3+}$ spins order with a propagation vector $k_{1^{\prime }}=(1/3,0,0)$ on a Pnma cell (only half of the cell is shown). (b) Low-temperature magnetic structure of $\mathrm{LaC}{\mathrm{a}}_2\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_9$ ($1.5\mathrm{K}<T<200\mathrm{K}$) and magnetic structure of $\mathrm{L}{\mathrm{a}}_{1/3}\mathrm{C}{\mathrm{a}}_{2/3}\mathrm{Fe}{\mathrm{O}}_3$ ($T_{\mathrm{N}}=217\mathrm{K}$). Both $\mathrm{F}{\mathrm{e}}^{3+}$ and $\mathrm{F}{\mathrm{e}}^{5+}$ order on a triple layered antiferromagnetic structure with a propagation vector $k_{2^{\prime }}=(0,0,\frac{1}{2})$ on a $\surd 2a_p\times \surd 2a_p\times \surd 3a_p$ hexagonal cell shown with lines. Dashed lines show the $3\surd 2a_p\times 3\surd 2a_p\times 6a_p$ supercell needed to account for the CD 2:1 $\mathrm{F}{\mathrm{e}}^{3+}:\mathrm{F}{\mathrm{e}}^{5+}$, layered $A$-site cation ordering and octahedral tilting. (c) Magnetic moment as function of temperature obtained from Rietveld refinement on $\mathrm{LaC}{\mathrm{a}}_2\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_9$ and the disordered $\mathrm{L}{\mathrm{a}}_{1/3}\mathrm{C}{\mathrm{a}}_{2/3}\mathrm{Fe}{\mathrm{O}}_3$.