Oxide electronics

Abstract:

The crystal structures of Sr2HoMn2O7 and Sr2YMn2O7 have been determined at 290 and 1.7 K from neutron and X-ray powder diffraction data. Both are distorted Ruddlesden-Popper structures formed by the intergrowth of rock-salt-like layers and perovskite-like blocks of tilted MnO6 octahedra (space group P42/mnm; for Ho at 290 Ka = 5.40388(5), c = 19.9050-(2) Å). The majority of the lanthanide cations are located in the rock-salt layers. Neither structure changes significantly on cooling to 1.7 K. There is evidence for neither charge ordering of Mn3+ and Mn4+ cations nor for long-range magnetic ordering. The magnetic susceptibility maximum observed previously is thus confirmed as signifying a transition to a spin-glass phase. The behavior of these compounds is contrasted with that of other Mn oxides which show long-range magnetic order and colossal magnetoresistance.

Abstract:

A detailed investigation into the preparation of powder samples of the n = 2 Ruddlesden-Popper (RP) oxide Sr1.8Nd1.2Mn2O7 is presented. This material is of interest as it displays colossal magntoresistance (CMR) without three-dimensional ferromagnetic long-range order. It is shown that Rietveld refinement of high-resolution powder X-ray and neutron diffraction data is essential to assess the outcomes of syntheses because of the potential coexistence of two very similar Ruddlesden-Popper phases. Phase coexistence or significant temperature-dependent anisotropic strain can be indicated by the broadening of the {0 0 10} reflection at 5 K. This suggests that a more subtle form of the previously reported phase separation at x = 0 is also difficult to avoid at the x = 0.2 composition. Precise attention to reaction temperature and time is required to prepare samples containing only one n = 2 Ruddlesden-Popper phase, and contamination by small quantities of the n = ∞ perovskite is a pervasive problem. Neutron powder diffraction structural analysis of the highest quality sample allows a comparison with the three-dimensional ferromagnet Sr1.8La1.2Mn2O7. The structure suggests that only slight changes in the Mn-O bond lengths are required to radically alter the electronic structure of n = 2 Ruddlesden-Popper phases.

Abstract:

Local structure of the CuO2 plane in Bi2Sr2CaCu2O8+δ (Bi2212) superconductor has been solved by joint Cu K-edge extended X-ray absorption fine structure (EXAFS) and anomalous X-ray diffraction. The Cu-O pair distribution function (PDF) has been measured by EXAFS and diffraction. The displacement of the Cu ions along the c-axis shows largest amplitude and anharmonic character. The results show flat stripes of the CuO2 lattice of width L = 15 ± 0.5 Å that are separated by stripes of bent CuO2 plane. The modulation of the CuO2 plane measured by EXAFS and anomalous diffraction has been discussed.

Abstract:

The evolution of the structural properties of (Formula presented) was determined as a function of temperature, average (Formula presented)-site radius (Formula presented) and applied pressure for the “optimal” doping range (Formula presented) 0.30, by using high-resolution neutron powder diffraction. The metal-insulator transition, which can be induced both as a function of temperature and of (Formula presented) was found to be accompanied by significant structural changes. Both the paramagnetic charge-localized phase, which exists at high temperatures for all values of (Formula presented) and the spin-canted ferromagnetic charge-ordered phase, which is found at low temperatures for low values of (Formula presented) are characterized by large metric distortions of the (Formula presented) octahedra. These structural distortions are mainly incoherent with respect to the space-group symmetry, with a significant coherent component only at low (Formula presented) These distortions decrease abruptly at the transition into the ferromagnetic metal phase. These observations are consistent with the hypothesis that, in the insulating phases, lattice distortions of the Jahn-Teller type, in addition to spin scattering, provide a charge-localization mechanism. The evolution of the average structural parameters indicates that the variation of the electronic bandwidth is the driving force for the evolution of the insulator-to-metal transition at (Formula presented) as a function of “chemical” and applied pressure. © 1997 The American Physical Society.

Abstract:

The structure of HgBa2CuO4+∂ is studied as function of temperature by neutron diffraction and Cu-Kedge EXAFS on the same powder sample with Tc=97K. Both techniques reveal the presence of an anomalous behaviour of the Cu-O in-plane bond length starting at ≈ 140K.