Spin order in the charge disproportionated phases of the A-site layer ordered triple perovskite LaCa2Fe3O9

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 $\ensuremath{\langle}010\ensuremath{\rangle}$ direction of the pseudocubic unit cell. At lower temperatures $(Tl170\phantom{\rule{0.16em}{0ex}}\mathrm{K})$, the charge ordering pattern changes to a layered arrangement along the $\ensuremath{\langle}111\ensuremath{\rangle}$ direction. Neutron powder diffraction data show that in the intermediate temperature range ($170\phantom{\rule{0.16em}{0ex}}\mathrm{K}lTl230\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) 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 ($2\phantom{\rule{0.16em}{0ex}}\mathrm{K}lTl190\phantom{\rule{0.16em}{0ex}}\mathrm{K}$), the spin structure follows the charge ordering and evolves to a $\ensuremath{\langle}111\ensuremath{\rangle}$ layered magnetic structure.