S. Funahashi

Preparation and crystal structure of Sr2CuO2(CO3)

Abstract Sr 2 CuO 2 (CO 3 ) was prepared at 1273 K and 0.01 MPa CO 2 partial pressure in a flowing gas of O 2 CO 2 using a mixture of SrCO 3 and CuO powders as a starting material. The compound has a tetragonal structure with lattice constants a = 7.8045(1), and c = 14.993(1) A , and its space group is 14. The formula per unit cell is 8 Sr 2 CuO 2 (CO 3 ), and measured and calculated densities are D m = 4.71 g/cm 3 , and D x = 4.81 g/cm 3 , respectively. The crystal structure was refined by Rietveld analysis on X-ray powder diffraction and neutron powder diffraction data. The final residuals ( R F ) were 4.31 and 4.27% for the X-ray and neutron data, respectively. The structure consists of deformed [CuO 6 ] octahedrons and layers of ordered triangular CO 3 groups. Sr atoms having eight near oxygen neighbors are between the [CuO 6 ] octahedrons and the CO 3 layers.

The crystal structure of (C0.4Cu0.6)Sr2(Y0.86Sr0.14)Cu2O7

Abstract The crystal structure of a new compound, (C0.4Cu0.6)Sr2(Y0.86Sr0.14)Cu2O7, is derived from the structure of YBa2Cu3O7. Forty percent of CuO chains in the YBa2Cu3O7 structure are replaced by CO3 groups. This new compound has a superstructure along the a-axis and the c-axis. Diffuse superlattice reflections having periods of a∗/2-a∗/3 and c∗/2 were observed in electron diffraction patterns. Locally ordered distributions of C and Cu atoms were seen high-resolution images taken by transmission electron microscopy with an incident beam parallel to [010]. The basic structure of this superstructure was determined by neutron powder diffraction, assuming orthorhombic symmetry with the space group, Pmmm (lattice constants: a=3.8278(2), b=3.8506(2) and c=11.1854(5) A ).

Magnetization and Spin Correlation of Two-Dimensional Triangular Antiferromagnet LuFe2O4

Magnetization, neutron diffraction and Mossbauer measurements have been performed on a two dimensional (2D) double-layered triangular antiferromagnet LuFe 3+ Fe 2+ O 4 single crystal. Two magnetic rods (0, 1, l ) and (1/3, 1/3, l ) with broad widths across them reveal that 2D magnetic ordering is not of long range and no 3D one occurs down to 4.2 K in contrast to an ordinary magnetic system. Thermoremanent magnetization measurements suggest that the system consists of various size ferrimagnetic clusters. The characteristic profile of l -scan in (0, 1, l ) can be realized by taking account of correlation among clusters for intra double-layers and inter double-layers. The temperature variations of intensity in magnetic scattering can be explained qualitatively by assuming unequal exchange interactions among three sublattice.

High‐resolution neutron powder diffraction study on nitrogenated Nd2Fe17

High‐resolution neutron powder diffraction measurements with λ=1.8232 A and collimation of 6’‐20’‐6’ on Nd2Fe17Nx with x=0, 2.85, and 2.91 were carried out at room temperature. Structural parameters and magnetic moments were determined by the Rietveld profile‐fitting calculation. The magnetic moments of iron atoms increased with increasing nitrogen concentration; e.g., the magnetic moments of iron located at 6c, 9d, 18f, and 18h sites increase from less than 0.7μB in the x=0 sample to about 2.1μB in the x=2.91 sample. It is noticed that a local atomic group composed of two Fe(1) atoms at the 6c site plus six Fe(3) atoms at the 18f site keeps its shape against the nitrogen uptake.

Crystal structure and critical temperature of RBa2Cu4O8 (R = Tm, Er, Ho, Y, Dy and Gd)

Abstract Bulk superconductors RBa 2 Cu 4 O 8 (R = rare earth element; Tm, Er, Ho, Y, Dy and Gd) were synthesized by hot isostatic pressing (HIP) technique. The highest T c ( = 84 K) was obtained for the substitution of Er. The T c decreases systematically with the increase of the ionic radius of these rare earth ions, and reaches 77 K for the largest ion Gd. The crystal structure of the compounds with R = Tm, Er, Ho and Y has been refined by Rietveld analysis of neutron powder diffraction data at room temperature. Structural changes due to the substitution of these rare earth elements are compared with those of YBa 2 Cu 4 O 8 under high pressure, and they are discussed in connection with the variation of T c . The results indicate that l (Cu(2)-O(2)) and l (Cu(2)-O(3)), the bond lengths between Cu and O in the CuO 2 plane, are the most important parameters to decide the T c of the present systems.

Two-Dimensional Spin Correlation in YFe2O4

Neutron and X-ray diffraction studies were carried out on polycrystalline specimens of stoichiometric (S-) and non-stoichiometric (N-) YFe 2 O 4 . An asymmetric diffraction line was observed above the Neel point ( T N ≃235 K) in S-YFe 2 O 4 and both above and below T N ( T N ≃210 K) in N-YFe 2 O 4 . At above T N , the line profile was quantitatively interpreted as due to two-dimensional critical scattering, on an assumption that the scattering cross section is a Lorentzian in a h - k plane, independent of l . The correlation length, κ, and the susceptibility at (1/3, 1/3, l ), χ, obey an exponential law: κ∝( T - T N ′ ) v and χ∝( T - T N ′ ) -γ , respectively, with ν=1.5 ±0.05, γ=3.1 ±0.1 and T N ′ =233.0 ±0.5 K in S-YFe 2 O 4 . It was concluded that the spin correlation is two-dimensional in the hexagonal YFe 2 O 4 , both in the S- and N-compounds, but a crystal deformation induces a three-dimensional spin ordering in S-YFe 2 O 4 .

Magnetic Structure of Rhombohedral Cr2Se3

Rhombohedral Cr 2 Se 3 is an antiferromagnetic compound with antiferromagnetic AF(L) phase below 38 K (= T t ) and the antiferromagnetic AF(H) one between 38 K and 45 K (= T N ). Neutron diffraction studies for rhombohedral Cr 2 Se 3 are carried out at temperatures from 6 K to 77 K. The results show that the magnetic structures of AF(L) and AF(H) phase are a non-collinear and collinear antiferromagnetic structures, respectively, with the monoclinic magnetic unit cell. It was found that Cr moments at the a-sites are frustrated in the AF(H) phase.

Two‐dimensional neutron diffraction of YFe2O4 and CoCr2O4

A new wide‐angle neutron diffraction instrument has been used to study CoCr2O4 and YFe2O4. The instrument is capable of measuring the scattered intensity over a wide region of reciprocal space in a short time, and this feature was used to measure the magnetic diffraction pattern of these materials at temperatures below the magnetic ordering temperature.

Structure of High-Tc Superconductor YBa2Cu3O6.6 at Low Temperatures

A single-phase YBa2Cu3O6.6 shows a sharp superconductive transition at 94 K. The crystal structure of this compound has been determined by the Rietveld profile analysis of neutron powder diffraction data measured at 10 K and 120 K. The space group Pmmm is confirmed, and the positions and occupations of oxygen atoms are established. No phase transitions or other structural anomalies were observed in relation to the superconductive transition.

Structure analysis of oxygen-deficient TlSr2CuOy by neutron diffraction and high-resolution electron microscopy

Abstract The crystal structure of orthorhombic TlSr 2 CuO y was determined by Rietveld analysis of neutron powder diffraction data with the aid of electron diffraction and high-resolution electron microscopy. We found that oxygen vacancies are formed along the a -direction in a CuO 2− z sheet, which is consistent with a superstructure of twice the subcell along the b -direction. CuO 6 octahedra and CuO 4 square planes alternate along the b -direction by sharing their corners in accordance with the ordered oxygen vacancies in the CuO 2− z sheet. An apical oxygen atom in the CuO 6 octahedron is nearer to a TlO layer than an oxygen atom shared by a TlO 6 octahedron and the CuO 4 plane. A crystal-structure image obtained by high-resolution electron microscopy could be successfully reproduced by computer simulation using structure parameters obtained by neutron powder diffraction.