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Abstract:

Recent studies have shown that structural instabilities play a vital role in segregation of charge carriers in the perovskites. We have used extended x- ray absorption fine structure (EXAFS) to investigate the instability of the CuO6 octahedra of the Lai.85Sr0.i5CuO4 (LSCO) system showing high Tc superconductivity and the MnO6 octhahedra of the Lao.7sCao.25MnO3 (LCMO) system showing GMR (giant-magneto resistance) properties. Temperature dependent Cu K-edge EXAFS spectra on the LSCO and Mn Kedge on the LCMO systems are reported. The results show a similarity in the lattice instability of the two systems providing evidence for a similar phase segregation of localized and itinerant charge carriers in the two families of perovskite systems. © Springer-Verlag 1997.

Abstract:

In mercury superconducting cuprates synthesized at high pressure, partial carbon substitution on the mercury site occurs when the precursors are not absolutely carbon free. Hg-12(n-1)n samples made from carbonate-containing precursors show lower Tcs and smaller c parameter. A model of the structural arrangement of the CO3-2 groups is proposed from neutron diffraction data and crystal-chemistry considerations. The third oxygen of the oxyanion is situated in the (Hg,C)O∂ layer and hinders the incorporation of extra-oxygen atoms. The (Hg1-xCx)Ba2CuO4+∂ phase diagram shows three different phases, namely HgBa2CuO4+∂, Hg0.5C0.5Ba2CuO4+∂ and CBa2CuO4+∂. The latter two phases are not superconducting due to the insufficient hole concentration on the superconducting layer. Sulphur can also partially replace mercury in Hg1-xSxBa2CuO4+∂ and forms a solid solution up to x=0.15. Neutron diffraction shows that the sulphur atoms arrange as S04-2 oxyanions. The larger S-O distances induce steric limitations to the presence of other sulphate anions in the same mesh.

Abstract:

The crystal structure and the high temperature behaviour of the β′ phase in the Mg-Zr-O-N system have been investigated by powder neutron diffraction. It crystallizes isostructurally to Zr5Sc2O13 (rhombohedrally distorted fluorite-type structure) and can be described by a stacking of Bevan clusters (A7X12) and A7X14 units along the hexagonal c-axis. In agreement with Pauling's 2nd rule, there are clear indications of N/O ordering with nitrogen occupying mainly the anion position coordinating the central atom of the Bevan cluster. At >960°C, the β′ phase shows a first order transition, basically of an order-disorder type of the anion vacancies, to a fluorite-like structure. Detailed analysis of the Debye-Waller factors and the diffuse scattering show many similarities to cation doped zirconia materials. The local short range order of the high temperature phase is related to the structural features of the β′ structure. From this study, nitrogen containing zirconia seems to be a promising material for a high temperature super-ionic conductor. © 1997 Elsevier Science Ltd.

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) Ã…). 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.