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 LaCa2Fe3O9 oxide with neutron powder diffraction. This A-site layer ordered triple perovskite LaCa2Fe3O9 undergoes charge disproportionation on cooling and shows two different charge ordering patterns. At 230 K, Fe3.67+ disproportionates into a 2:1 ratio of Fe3+ : Fe5+, which order in a layered manner along the < 010 > direction of the pseudocubic unit cell. At lower temperatures (T < 170 K), the charge ordering pattern changes to a layered arrangement along the < 111 > direction. Neutron powder diffraction data show that in the intermediate temperature range (170 K < T < 230 K) the spins order into a cycloidal structure on the ac plane for the Fe3+ cations while the Fe5+ cations remain paramagnetic. For the lowest temperature range (2K < T < 190 K), the spin structure follows the charge ordering and evolves to a < 111 > layered magnetic structure.