Shiyou Pei

La2CuO4+δ and La2NiO4+δ: Phase separation resulting from excess oxygen defects.☆

Abstract The structural, superconducting and magnetic properties of La 2 CuO 4+δ and La 2 NiO 4+δ were studied by neutron powder diffraction, thermogravimetric analysis, resistivity and magnetic susceptibility measurements. La 2 CuO 4+δ was found to phase separate around 320K into antiferromagnetic La 2 CuO 4 and superconducting La 2 CuO 4.08 . La 2 NiO 4+δ forms a compound isostructural to La 2 CuO 4+δ , for 0.02≤δwere studied by the system separates into two phases with different defect concentrations and structures. The structure of the interstitial oxygen defect was determined for La 2 NiO 4.18 .

Crystal structure and Mössbauer spectroscopy of Y2SrFeCuO6.5, a double layer perovskite intergrowth phase

The crystal structure of Y2SrFeCuO6.5 was determined from single-crystal X-ray and neutron powder diffraction studies. Mr = 488.81, orthorhombic, Ibam, a = 5.4036(8)[5.4149(1)] A, b = 10.702(1)[10.7244(1)] A, c = 20.250(2)[20.2799(2)] A; values in square brackets are neutron data. V = 1171.0(4), Z = 8, Dx = 5.544 g cm−3, λ = 0.71069 A, μ = 345.1 cm−1, R = 0.048 for 567 observed reflections. The FeCu atoms occupy randomly the approximate center of oxygen pyramids. The pyramids share the apical oxygen and articulate laterally by corner sharing of oxygen to form a double pyramidal layer perpendicular to c. The pyramidal slabs are separated by double layers of Y that are in 7-fold coordination to oxygen, forming a defect fluorite unit. Mossbauer spectra indicate a unique iron environment and magnetic ordering at about 265 K. The paramagnetic phase coexists with the magnetic phase over an approximate temperature range 300-263 K, characteristic of magnetic ordering in 2-D magnetic structures. The isomer shift, 0.26, and quadrupole splitting, 0.56 mm sec−1, are consistent with Fe3+ in 5-fold coordination and Hint values also indicate classic high spin Fe3+. The average YO bond length is 2.331(6) A and Sr is in a dodecahedral environment in which, however, two oxygen atoms at the corners of the cube are missing. The average SrO bond length is 2.793(10) A. The structure is derived from the Ruddlesden-Popper phase Srn+1TinO3n+1 with n = 2.

Neutron diffraction study of Tl2Ba2CuO6+y under high pressure

The crystal structure of Tl2Ba2CuO6+y has been investigated under high pressure and at low temperature using neutron powder diffraction method. A gas pressure cell is made of aluminum and helium gas is used to generate pressure up to 0.609GPa. The crystal structure at 0.609GPa and 60 K depends systematically on the path in pressure-temperature space by which these conditions are achieved. Lattice parameters obtained by applying pressure at room temperature and then cooling are different from those obtained by cooling first and then applying pressure. Our results support those of Sieburger and Schilling1) who reported that the TC at high pressure for this compound depends markedly on whether the sample is pressurized at room temperature or low temperature.