Abstract

Traditional solid state synthetic methods were used to prepare a polycrystalline sample of the n = 3 Ruddlesden–Popper phase Ca₄Mn₂TiO_9.93. The crystal structure (space group Pbca, a = 5.31411(5), b = 5.31148(5), c = 26.9138(2) Å) was determined by the simultaneous analysis of neutron and X-ray diffraction data, with near-edge anomalous scattering being used to provide contrast between Mn and Ti cations. The latter show a small preference for the octahedral sites at the centre of the three-layer perovskite blocks within the structure. Neutron diffraction data collected at 5 K show no evidence for long-range magnetic ordering, although an enhanced magnetisation with a weak remanence is observed at low temperature; this is ascribed to the presence of antisymmetric exchange interactions. Ca₄Mn₂TiO_9.93 is a semiconductor with a temperature-dependent activation energy of ∼100 meV. Only weak (ρ_B/ρ₀ > 0.9 in 14 T at 75 K) magnetoresistance was observed.