Browsing by Author "Howard, CJ"

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    (Ca0.37Sr0.63)TiO3 perovskite - an example of an unusual class of tilted perovskites
    (Institute of Physics, 2008-04-02) Howard, CJ; Withers, RL; Knight, KS; Zhang, Z
    The structure of Ca0.37Sr0.63TiO3 perovskite has been carefully investigated using electron and neutron diffraction. Electron diffraction supports the supercell and Pbcm space group previously proposed, while high resolution neutron powder diffraction data provide the basis for a new structure refinement. The distortions of the resultant structure relative to the ideal parent perovskite structure type have been decomposed into normal modes. It is found that the primary modes of distortion are octahedral tilting modes associated with the R point (k = [1/2, 1/2, 1/2]∗) and the ξ = 1/4 point on the T line of symmetry (k = [1/2, 1/2, ξ]∗). There is a smaller secondary, probably induced, mode of distortion involving antiferroelectric displacements of the Ti cations associated with the ξ = 1/4 point on the T line of symmetry (k = [0, 0, ξ]∗). This compound provides one of the very few currently known examples of a tilted perovskite structure that shows tilting modes associated with the ubiquitous k = (1/2, 1/2, ξ)∗ modulation wavevectors for a value of ξ different from 0 and 1/2. © 2008, Institute of Physics
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    Crystal structure of Ln(1/3)NbO(3) (Ln = Nd, Pr) and phase transition in Nd1/3NbO3
    (Elsevier, 2007-06) Zhang, Z; Howard, CJ; Kennedy, BJ; Knight, KS; Zhou, QD
    The crystal structure of the A-site deficient perovskite Ln1/3NbO3 (Ln=Nd, Pr) at room temperature has been determined, for the first time, as orthorhombic in space group Cmmm using high-resolution neutron powder diffraction. Pertinent features are the alternation of unoccupied layers of A-sites and layers partly occupied by Ln cations, as well as out-of-phase tilting of the NbO6 octahedra around an axis perpendicular to the direction of the cation/vacancy ordering. The phase transition behaviour of Nd1/3NbO3 has also been studied in situ. This compound undergoes a continuous phase transition at around 650°C to a tetragonal structure in space group P4/mmm due to the disappearance of the octahedral tilting. The analysis of spontaneous strains shows that this phase transition is tricritical in nature. © 2007, Elsevier Ltd.
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    Crystal structures and phase transition in (Sr0.8Ce0.2)(Mn(1-y)Coy)O-3 (y=0 and 0.2): the influence of Jahn-Teller distortion
    (Institute of Physics, 2009-03-25) Zhang, Z; Howard, CJ; Kennedy, BJ; Matsuda, M; Miyake, M
    We have studied the crystal structures of (Sr0.8Ce0.2)(Mn1-yCoy)O-3 (y = 0 and 0.2) using neutron diffraction. Both (Sr0.8Ce0.2)MnO3 and (Sr0.8Ce0.2)(Mn0.8Co0.2)O-3 have a tetragonal structure in space group I4/mcm at room temperature, and the octahedral tilt angle around the c-axis is nearly the same. The only significant difference is the shape of the Mn(Co)O-6 octahedron: it is elongated in (Sr0.8Ce0.2)MnO3 due to the cooperative Jahn-Teller (JT) effect, but essentially regular in (Sr0.8Ce0.2)(Mn0.8Co0.2) O3 due to the absence of JT-active Mn3+ ions. With increasing temperature, both compounds undergo a continuous phase transition at around 400°C to a cubic structure in Pm (3) over barm, with no indication of a distinct transition in (Sr0.8Ce0.2) MnO3 from the removal of the static JT distortion. In addition, the temperature dependence of the octahedral tilt angle is very similar in the two samples, implying that the JT distortion has minimal effect on the octahedral tilting and the phase transition to cubic. X-ray absorption near-edge structure (XANES) analysis indicates that the Ce oxidation state is predominantly 4+ in both samples. The electrical conductivity is higher in (Sr0.8Ce0.2)MnO3 than in (Sr0.8Ce0.2)(Mn0.8Co0.2)O-3 in the temperature range studied (100-900°C). © 2009, Institute of Physics
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    Hydration as a trigger for new properties in inorganic materials
    (Australian Institute of Nuclear Science and Engineering, 2016-11-29) Duyker, SG; Peterson, VK; Kearley, GJ; Studer, AJ; Kepert, CJ; Hill, JA; Howard, CJ; Goodwin, AL
    The humble water molecule binds to metal ions strongly enough that it can have a significant distortive influence on the coordination geometry, yet weakly enough that it can be readily removed, thus providing scope for reversible chemical switching between structural forms. When this principle is applied in 3D coordination frameworks, the unique topological constraints of the framework can lead to new behaviours. Examples from our work will be presented, including anomalous mechanical properties enabled by unsaturated coordination spheres,[1] and new kinds of symmetry breaking transformations triggered by (de)hydration[2] (see figure).
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    Impact of Jahn-Teller active Mn3+ on strain effects and phase transitions in Sr0.65Pr0.35MnO3
    (American Physical Society, 2012-03-19) Tan, TY; Kennedy, BJ; Zhou, QD; Ling, CD; Miiller, W; Howard, CJ; Carpenter, MA; Knight, KS
    The mixed-valence manganite Sr(0.65)Pr(0.35)MnO(3) has been prepared and its crystal and magnetic structure investigated between 7 and 1200 K using high-resolution powder neutron diffraction. The structural and lattice parameter data have been used to determine the octahedral tilting and spontaneous strains associated with the structural, electronic, and magnetic phase transitions. At room temperature, the structure is tetragonal and is characterized by cooperative out-of-phase tilts of the MnO(6) octahedra about the c axis and a large Jahn-Teller-type distortion due to the presence of Mn(3+). The sample exhibits a reversible phase transition from the cubic Pm3m perovskite to a tetragonal I4/mcm structure at 750 K. The Pm (3) over barm <-> I4/mcm phase transition is continuous, and the tetragonal strain, which is dominated by the Jahn-Teller-type distortion of the MnO(6) octahedra, exhibits an unusual e(0.5)(t)alpha(T(c)-T) temperature dependence. At low temperatures, a C-type antiferromagnetic structure develops with a Neel temperature T(N) of 250 K. TheMnmagnetic moment at 7 K is 2.99(2) mu(B)/Mn. The magnetic ordering introduces additional tetragonal strain, and this strain shows the expected quadratic dependence on the magnetic moment at low temperatures. An increase in the octahedral tilt angle at TN demonstrates an effective coupling between the magnetic ordering process and octahedral tilting. © 2012, American Physical Society
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    Influence of cation ordering on octahedral tilting in perovskites
    (The Bragg Institute, Australian Nuclear Science and Technology Organisation, 2005-11-27) Zhang, Z; Howard, CJ; Carpenter, MA; Knight, KS
    The compound LaO.6SrO.1Ti03, taken to represent the ideal Lag/3TiO3, adopts a structure in which the A-site layers are alternately fully and partly occupied by the La(Sr)ions. At room temperature the structure also shows a tilting of the TiO5 octahedra around a single axis perpendicular to the layering direction. Quenching the material during preparation can produce a variant in which the cations are disordered on the A-site, offering the opportunity to study the effect of cation ordering on octahedral tilting. Slow-cooled and quenched specimens were studied over a range of temperatures using high-resolution neutron powder diffraction. Inspection of the relevant diffraction peaks confirmed the expected difference in cation ordering. At room temperature, the cell volume was 0.1% less in the ordered sample than in the disordered one. The extent of octahedral tilting was much the same in the two samples, with similar transition temperatures for removal of the tilting. The orthorhombic distortion that accompanied this tilting in the ordered sample was, surprisingly, not evident in the disordered sample——that is the effect of disordering the cations seems to switch off the strain (tilt) order parameter coupling in this perovskite. © 2005 The Authors
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    The influence of composition and temperature on the phases in Sr1−xBaxZrO3 perovskites: a high-resolution powder diffraction study
    (Elsevier, 2001-10) Kennedy, BJ; Howard, CJ; Thorogood, GJ; Hester, JR
    High-resolution synchrotron X-ray diffraction measurements on a series of polycrystalline samples of Sr1−xBaxZrO3 at room temperature show that the symmetry increases with increasing Ba content, the sequence of phases being orthorhombic Pnma, orthorhombic Imma, tetragonal I4/mcm, and cubic Pmm. The effect of temperature has also been studied for a number of compositions and indicates that the same series of phases is present. The transition temperatures decrease with increasing Ba content. © 2001 Academic Press
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    Neutron beam research in the Pacific Rim region - an Australian perspective
    (The Institution of Engineers Australia, 1994-05-01) Sabine, TM; Howard, CJ
    A program of research in neutron scattering was established in Australia in the early 1960's, shortly after the reactor HIFAR commenced full power operation. The program has included elements of regional cooperation from the very early days. A report is given on the present status of Australian neutron scattering research, and the activities (including several major new developments) in other Pacific Rim countries are briefly reviewed. Given the increasing sophistication of neutron scattering facilities and techniques, the case for regional cooperation is strongly advocated.
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    Structural characterization of the perovskite series La1−xSrxCr1−xTixO3
    (Elsevier, 2000-12) Kennedy, BJ; Howard, CJ; Thorogood, GJ; Mestre, MAT; Hester, JR
    The perovskite-type compounds La1−xSrxCr1−xTixO3, x=0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, have been synthesized by alkoxide methods, and the room temperature structures characterized using X-ray synchrotron powder diffraction. The x=0 endmember, LaCrO3, and the x=0.1 compound are orthorhombic in space group Pbnm, the compounds with x=0.2 to x=0.7 inclusive are rhombohedral in Rc, and the x=0.8 and x=0.9 compounds are, like the x=1 endmember SrTiO3, cubic in Pmm. This sequence differs from that reported recently in this journal (R. H. Mitchell and A. R. Chakhmouradian, J. Solid State Chem.144, 81–85, 1999), but is as expected by comparison with the high-temperature behavior of the end member LaCrO3. © 2000 Academic Press
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    Structures and phase diagram for the system CaTiO3-La2/3TiO3
    (Elsevier, 2007-03) Zhang, ZM; Lumpkin, GR; Howard, CJ; Knight, KS; Whittle, KR; Osaka, K
    High-resolution neutron and synchrotron X-ray powder diffraction have been used to examine various compositions across the system (1−x)CaTiO3−xLa2/3TiO3. The structures at room temperature were determined according to composition: in Pbnm for 0≤x≤0.5, Ibmm for 0.50.5 in the current study based on our high-resolution neutron diffraction data. From in situ measurements to identify structures above room temperature, an approximate composition–temperature phase diagram has been constructed, involving four temperature-induced phase transitions: Pbnm→I4/mcm, Ibmm→I4/mcm, I4/mcm→Pm3¯m and Cmmm→P4/mmm. © 2007, Elsevier Ltd.
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    Suppression of strain coupling in perovskite La0.6Sr0.1TiO3 by cation disorder
    (American Physical Society, 2007-08) Howard, CJ; Zhang, Z; Carpenter, MA; Knight, KS
    An octahedral tilting transition has been investigated in two samples of La0.6Sr0.1TiO3 by powder neutron diffraction. One (slowly cooled) sample had a degree of cation/vacancy ordering on the A sites and was tetragonal, P4/mmm, at high temperatures. The second (quenched) sample had disordered cations and cubic, Pm3¯m, symmetry at high temperatures. On cooling, both underwent the same R-point tilting transition at ~560–570 K, to give symmetry changes P4/mmm↔Cmmm and Pm3¯m↔I4/mcm, respectively. From the evolution of the tilt angles, the transition appears to be close to tricritical in both cases. As expected from a Landau expansion in the two order parameters, the transition temperature was found to be only weakly dependent on the degree of cation order. In contrast with the expectation of standard patterns of strain to order parameter coupling, however, the tilted (tetragonal) form of the disordered sample remained metrically cubic, implying that coupling of the tetragonal shear strain with the octahedral tilting was suppressed. It is proposed that cation disordering causes the development of local strain heterogeneities in crystals of La0.6Sr0.1TiO3 which prevent a coherent, long-range shear strain from developing. Manipulation of the degree of cation order in perovskites with carefully selected compositions might therefore provide a means by which the magnitudes of ferroelastic strains could be engineered according to the requirement of particular applications. © 2007, American Physical Society