Crystal structure and electronic properties of Ca4Mn2TiO9.93, an n=3 Ruddlesden-Popper compound

Traditional solid state synthetic methods were used to prepare a polycrysta lline sample of the n = 3 Ruddlesden-Popper phase Ca4Mn2TiO9.93. The crysta l structure (space group Pbca, a = 5.31411(5), b = 5.31148(5), c = 26.9138( 2)Angstrom) was determined by the simultaneous analysis of neutron and X-ra y diffraction data, with near-edge anomalous scattering being used to provi de contrast between Mn and Ti cations. The latter show a small preference f or the octahedral sites at the centre of the three-layer perovskite blocks within the structure. Neutron diffraction data collected at 5 K show no evi dence for long-range magnetic ordering, although an enhanced magnetisation with a weak remanence is observed at low temperature; this is ascribed to t he presence of antisymmetric exchange interactions. Ca4Mn2TiO9.93 is a semi conductor with a temperature-dependent activation energy of similar to 100 meV. Only weak (rho (B)/rho (0) > 0.9 in 14 T at 75 K) magnetoresistance wa s observed.