Polycrystalline samples of n = 1 Ruddlesden–Popper manganites Nd1−xCa1+xMnO4 (0.55 ≦ x ≦ 1.00) were synthesized by a solid-state reaction. X-ray diffraction (XRD) and electron diffraction (ED) measurements confirmed that the fundamental crystal structure at room temperature consists of three distorted K2NiF4-types: orthorhombic Bmab (64) phase in 0.55 ≦ x < 0.73, orthorhombic Acam (64) phase in 0.73 ≦ x < 0.85 and tetragonal I41/acd (142) phase in 0.85 ≦ x ≦ 1.00. Furthermore, in a whole range of 0.55 ≦ x ≦ 0.75, low-temperature magnetic and ED measurements revealed charge-orbital ordering (COO) states, which are accompanied by suppression of magnetization and structural modulations with q = (1 − x)a*. The COO transition temperatures are high with a maximum of ∼330 K at x = 0.67, and then higher than those in non-distorted n = 1 Ruddlesden–Popper manganites. The observations suggest that COO states are much stabilized by the distortion in the fundamental structures.

The crystal structure of our newly discovered Sr–Co–O phase is investigated in detail through high-resolution electron microscopy (HREM) techniques. Electron diffraction (ED) measurement together with energy dispersive X-ray spectroscopy (EDS) analysis show that an ampoule-synthesized sample contains an unknown Sr–Co–O ternary phase with monoclinic symmetry and the cation ratio of Sr/Co=1. From HREM images a layered structure with a regular stacking of a CdI2-type CoO2 sheet and a rock-salt-type Sr2O2 double-layered block is observed, which confirms that the phase is the parent of the more complex “misfit-layered (ML)” cobalt oxides of [MmA2Om+2]qCoO2 with the formula of [Sr2O2]qCoO2, i.e. m=0m=0. It is revealed that the misfit parameter q is 0.5, i.e. the two sublattices of the CoO2 sheet and the Sr2O2 block coexist to form a commensurate composite structure. We propose a structural model with monoclinic P21/m symmetry, which is supported by simulations of ED patterns and HREM images based on dynamical diffraction theory.A crystal structure model proposed for the parent misfit-layered cobalt oxide [Sr2O2]qCoO2 (q=0.5q=0.5) with monoclinic P21/m (no. 11) symmetry.

Crystal structures of a homologous series of Co-based layered cuprates, CoSr2(Y,Ce)sCu2O5+2s (s=1–3), have been investigated by high-resolution electron microscopy (HREM) and electron diffraction (ED) techniques. For all the three phases ED patterns showed double periodicity along a direction parallel to the CoO layers, indicating a regular alternation of two types of CoO4-tetrahedra chains within the layers. Also seen was ordering of the chains along the layer-stacking direction for the s=1 phase (Co-1212); ED patterns simulated based on the proposed superstructure model well reproduced the observed patterns. For the s=2 (Co-1222) and s=3 (Co-1232) phases in which an additional fluorite-type layer-block is inserted between two CuO2 planes, HREM and ED analysis revealed complete disorder of the CoO4 chains along the layer-stacking direction. This implies that the interlayer ordering is mainly controlled by the distance between the neighboring CoO layers.

•We investigate a Ag-sheathed Bi-2223 wire by low-voltage aberration-corrected STEM.•Atomic displacements enable each layer to be continuous at edge grain boundaries.•The grains are terminated with deficient (Bi,Pb)–O layers at the other boundaries.•EELS mapping visualizes atomic columns in the intergrowth of Bi-2212 and 2234 phases.•HAADF analysis shows modulation of the occupancy of the (Bi,Pb) sites along [1 1 0].Aberration correction in scanning transmission electron microscopy (STEM) enables an atomic-scale probe size of ∼0.1 nm at a low accelerating voltage of 80 kV that avoids knock-on damage in materials including light elements such as oxygen. We used this advanced method of microscopy to directly observe atomic columns in a (Bi,Pb)2Sr2Ca2Cu3O10+δ (Bi-2223) superconducting wire produced by a powder-in-tube method. Using the atomic-number (Z) contrast mechanism, incoherent high-angle annular dark-field (HAADF) imaging clearly showed the atomic columns. Atomic displacements toward the boundary with a maximum magnitude of ∼0.26 nm enable each atomic layer to be continuous at edge grain boundaries (EGBs). The grains tend to be terminated with deficient (Bi,Pb)–O single layers at c-axis twist boundaries (TWBs) and small-angle asymmetrical tilt boundaries (ATBs); a quantitative HAADF analysis showed that the occupancies of the (Bi,Pb) sites around these boundaries are ∼0.66 and ∼0.72, respectively. Electron energy-loss spectroscopy (EELS) mapping successfully visualized atomic columns in the half-unit cell intergrowth of (Bi,Pb)2Sr2CaCu2O8+δ (Bi-2212) and (Bi,Pb)2Sr2Ca3Cu4O12+δ (Bi-2234) phases. Furthermore, the HAADF analysis indicated that the occupancy of the (Bi,Pb) sites is modulated between ∼0.88 and 1.0 along the diagonal direction of the primitive perovskite cell with the same period as the structural modulation.