Synthesis, structure, and electronic properties of LaCa2Mn2O7

Synthesis of the Ruddlesden-Popper phase, LaCa2Mn2O7 is presented. The difficulty in obtaining a single-phase material, free from a perovskite impurity, is highlighted. Powder X-ray diffraction indicates an orthorhombic symmetry. Powder neutron diffraction reveals a subtle rotation of the MnO6 octahedra and reduction of symmetry to a C-centered orthorhombic cell (Cmcm). Cation ordering occurs with the larger La ions adopting the 12-coordinated site within the perovskite block and the smaller Ca ions situated in the 9-coordinated site in the rock salt layer. Magnetization measurements indicate a Freak ferromagnetic transition at around 250 K, which is assigned to a perovskite impurity of 11.4%, as determined by neutron diffraction. Low-temperature neutron diffraction measurements confirms ferromagnetic ordering in the perovskite impurity and show no indication of long-range magnetic ordering in the majority Ruddlesden-Popper phase. Magnetotransport measurements show a metal-insulator transition at around 150 K and magnetoresistance up to 320 K, However, the perovskite impurity limits confidence in the assignment of the origin of the magnetoresistance effect.