Browsing by Author "Morales, L"

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    Magneto-structural study of La0.57-x,,CexCa0.33Mn03 (X = 0, 0.05, 0.10, 0.20)
    (The Bragg Institute, Australian Nuclear Science and Technology Organisation, 2005-11-27) Alejandro, G; Morales, L; Steren, L; James, M
    A comparative study of the nuclear and magnetic structure of La0.57-x,,CexCa0.33Mn03 (X = 0, 0.05, 0.10, 0.20) perovskite (x =0, 0.05, 0.10, and 0.20) is reported. When the lanthanum is partially substituted by yttrium or a rare earth such as Pr or Tb, the structural cation disorder has been shown to lead to different and interesting phenomena, depending on the nature of the substitute cation (in general isovalent one in Ce-doped manganites, Ce3+ and Ce4+ species are able to coexist adding an extra source of disorder. This statement is supported by previous x-ray absorption (XAS) experiments[1]. Magnetic measurements made on these compounds have shown that Ce-doping introduces strong magnetic frustration in the spin system[2]. Furthermore, the magnetic behavior of La0.47CeO.2OCa0.33Mn03.OO observed[2] has been to be closely correlated with the electronic transport properties. To achieve a better understanding the nature of the magnetic phase in these Ce-doped manganites, we have carried out systematic neutron of powder diffraction experiments. Both nuclear and magnetic structure are studied as a function of temperature and cerium content. © 2005 The Authors
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    Neutron powder diffraction study of the nuclear and magnetic structures of LaMn1-xCrxO3.00 (x = 0.05, and 0.15)
    (The Bragg Institute, Australian Nuclear Science and Technology Organisation, 2006-11-15) Morales, L; Caneiro, A; James, M
    A comparative study of the nuclear and magnetic structures of LaMn1-xCrxO3.00 perovskite (x=0.05, and 0.15) is reported. Cr3+ is isoelectronic with Mn4+(3d3) and is also a non-Jahn-Teller ion. Both magnetic ions increase the hole (empty eg levels) concentration and decrease the Jahn-Teller distortion. To characterize the interaction established between Mn3+ and Cr3+ and the effect of Cr3+ on the magnetic structure, samples without Mn4+ were synthesized. X-ray powder diffraction, at room temperature, reveals the existence of single phase materials with orthorhombic symmetry (Pnma) for all the studied samples. Magnetic data shows an increase of the FM interaction with Cr content. Magnetic phases change from A-type antiferromagnetic order to a system with magnetic clusters through a spin-glass-like phase. In order to fully understand the magnetic phase diagram, neutron powder diffraction was carried out. Both, nuclear and magnetic structures are studied as a function of temperature and Cr content. These results reveal the presence of more than one magnetic phase and a clear change of the spin orientation with temperature and Cr content. The mixture of magnetization and neutron powder diffraction data allow us to describe the role of the Cr3+ on the magnetic properties. © The Authors
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    Neutron powder diffraction study of the nuclear and magnetic structures of LaMn1-xCrxO3.00 (x=0.05, and 0.15)
    (Elsevier B. V., 2006-11-15) Morales, L; Caneiro, A; James, M
    A comparative study of the nuclear and magnetic structures of LaMn1-xCrxO3.00 perovskite (x=0.05, and 0.15) is reported. Cr3+ is isoelectronic with Mn4+(3d3) and is also a non-Jahn-Teller ion. Both magnetic ions increase the hole (empty eg levels) concentration and decrease the Jahn-Teller distortion. To characterize the interaction established between Mn3+ and Cr3+ and the effect of Cr3+ on the magnetic structure, samples without Mn4+ were synthesized. X-ray powder diffraction, at room temperature, reveals the existence of single phase materials with orthorhombic symmetry (Pnma) for all the studied samples. Magnetic data shows an increase of the FM interaction with Cr content. Magnetic phases change from A-type antiferromagnetic order to a system with magnetic clusters through a spin-glass-like phase. In order to fully understand the magnetic phase diagram, neutron powder diffraction was carried out. Both, nuclear and magnetic structures are studied as a function of temperature and Cr content. These results reveal the presence of more than one magnetic phase and a clear change of the spin orientation with temperature and Cr content. The mixture of magnetization and neutron powder diffraction data allow us to describe the role of the Cr3+ on the magnetic properties. © 2006 Elsevier B.V.
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    Orthorhombic superstructures within the rare earth strontium-doped cobaltate perovskites: Ln1−xSrxCoO3−δ (Ln=Y3+, Dy3+–Yb3+; 0.750≤x≤0.875)
    (Elsevier, 2007-08) James, M; Avdeev, M; Barnes, PRF; Morales, L; Wallwork, KS; Withers, RL
    A combination of electron, synchrotron X-ray and neutron powder diffraction reveals a new orthorhombic structure type within the Sr-doped rare earth perovskite cobaltates Ln1−xSrxCoO3−δ (Ln=Y3+, Dy3+, Ho3+, Er3+, Tm3+and Yb3+). Electron diffraction shows a C-centred cell based on a 2√2ap×4ap×4√2ap superstructure of the basic perovskite unit. Not all of these very weak satellite reflections are evident in the synchrotron X-ray and neutron powder diffraction data and the average structure of each member of this series could only be refined based on Cmma symmetry and a 2√2ap×4ap×2√2ap cell. The nature of structural and magnetic ordering in these phases relies on both oxygen vacancy and cation distribution. A small range of solid solution exists where this orthorhombic structure type is observed, centred roughly around the compositions Ln0.2Sr0.8CoO3−δ. In the case of Yb3+ the pure orthorhombic phase was only observed for 0.850≤x≤0.875. Tetragonal (I4/mmm; 2ap×2ap×4ap) superstructures were observed for compositions having higher or lower Sr-doping levels, or for compounds with rare earth ions larger than Dy3+. These orthorhombic phases show mixed valence (3+/4+) cobalt oxidation states between 3.2+ and 3.3+. DC magnetic susceptibility measurements show an additional magnetic transition for these orthorhombic phases compared to the associated tetragonal compounds with critical temperatures > 330K. © 2007, Elsevier Ltd.
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    Structure and magnetism in rare earth strontium-doped cobaltates
    (Elsevier, 2006-11-15) James, M; Morales, L; Wallwork, KS; Avdeev, M; Withers, RL; Goossens, DJ
    Substantial interest has recently been generated by rare earth cobaltate compounds as cathode materials for solid oxide fuel cells. We have synthesised a wide range of single–phase perovskite-based rare earth cobaltates (Ln1−xSrxCoO3−δ) (Ln=La3+–Yb3+). A combination of electron and X-ray diffraction of these phases reveals a complex family of tetragonal and orthorhombic superstructures. The nature of structural and magnetic ordering relies on both cation and oxygen vacancy distribution. Phase boundaries exists between compounds containing large, medium and small rare earths (between Nd3+ and Sm3+, and also between Gd3+ and Dy3+) and also at different Sr-doping levels. Powder neutron diffraction has been used in conjunction with the other techniques to reveal cation and oxygen vacancy ordering within these materials. These phases show mixed valence (3+/4+) cobalt oxidation states that increases with Sr content. A range of magnetic behaviours has been observed, including ordered antiferromagnetism at elevated temperatures (>300 K) in Ho0.2Sr0.8CoO2.75. © 2006 Elsevier B.V.