Browsing by Author "Hudspeth, JM"

Now showing 1 - 5 of 5
  • No Thumbnail Available
    Item
    Crystal and magnetic structures in perovskite-related La1-xCaxFeO3-δ (x=0.2, 0.33)
    (Elsevier, 2011-12) Hudspeth, JM; Stewart, GA; Studer, AJ; Goossens, DJ
    Using sol gel synthesis, single phase perovskite-related compounds in the family La1-xCaxFeO3-δ have been formed for x=0.2 and x=0.33, but not for x=0.5. The x=0.2 and x=0.33 compounds are isostructural with LaFeO3 (Pnma). The magnetic structure of La 0.8 Ca0.2FeO3-δ has been studied through Mössbauer spectroscopy and neutron powder diffraction. La 0.8 Ca 0.2FeO3-δ is a G-type antiferromagnet with a magnetic moment magnitude of 3.0±0.2μB at room temperature. The reduction in the magnitude of the antiferromagnetic moment compared to that published for LaFeO3 is explained by the measurement being taken at room temperature rather than 4 K and by the presence of Fe4 ions which have weaker exchange interactions than Fe3, causing a strong reduction in TN. Room temperature Mössbauer shows a broad magnetic hyperfine field distribution on the Fe sites in both La0.8 Ca0.2FeO3-δ and La0.67 Ca 0.33FeO3-δ. On cooling, disproportionation of Fe14 into Fe3 and Fe5 is apparent, and the resulting Fe5 sextet measured at low temperature gives a reliable measure of the Fe4 fraction. This in turn shows that creation of high-oxidation-state Fe is the dominant charge balance mechanism on doping Ca2 into the Ln3 site indicating a disordered distribution of Fe3 and Fe4. The lack of broadening of the Fe 5 sextet suggests that there may be ordering in the distribution of Fe5. © 2011 Elsevier Ltd.
  • No Thumbnail Available
    Item
    The crystal and magnetic structures of LaCa2Fe3-xMxO8 (M=Al, Ga, In)
    (Elsevier, 2012-06-27) Goossens, DJ; Henderson, LSF; Trevena, S; Hudspeth, JM; Avdeev, M; Hester, JR
    LaCa2Fe3O8 (A3B3O8) is an example of a layered structure in that it consists of pairs of octahedral, perovskite-like layers alternating with a single tetrahedral layer. This work explores the doping of non-magnetic group 13 elements, M=Al, Ga and In, onto the B-site of LaCa2Fe3−xMxO8 as a function of x. The structural and magnetic effects are examined using a combination of neutron and X-ray diffraction. Solubility limits are established. It is found that for M=Ga the solubility limit occurs between x=1.0 and x=1.25, for the synthesis conditions used, while there is evidence for low ( x < 0.25 ) but non-zero substitution of Al. Structural refinements at x=1 suggest that Ga prefers neither the tetrahedral nor octahedral sites. The magnetic structure of LaCa2Fe2GaO8 has been examined using neutron diffraction at 3.2 K and room temperature. At low temperature the staggered moment per Fe3+ is 3.8 ( 1 ) μ B in LaCa2Fe3O8 and 4.8 ( 1 ) μ B in LaCa2Fe2GaO8. The magnetic space group (P2b21′ma′) and moment direction (along c) does not appear to change with Ga substitution. © 2012, Elsevier Ltd.
  • No Thumbnail Available
    Item
    The crystal and magnetic structures of LaCa2Fe3O8 and NdCa2Fe3O8
    (Institute of Physics, 2009-03-25) Hudspeth, JM; Goossens, DJ; Studer, AJ; Withers, RL; Norén, L
    The crystal and magnetic structures of LaCa2Fe3O8 and NdCa2Fe3O8 have been established using a combination of x-ray, neutron and electron diffraction. It was already considered likely that LaCa2Fe3O8 and NdCa2Fe3O8 were made up of stacked perovskite-like layers of FeO6 octahedra, with every third layer being replaced by a layer of tetrahedrally coordinated Fe, rather like a variation on the Brownmillerite (Ca2Fe2O5) structure type. We have gone further and determined a likely space group for this Grenier phase and determined the magnetic structure of the compounds at room temperature. The space group is found to be P2(1)ma (b axis as the long axis), and the crystal structure has been refined, subject to the stacking faulting along the long axis that is apparent in electron diffraction patterns. The magnetic structure of LaCa2Fe3O8 is shown to consist of antiferromagnetically ordered Fe3+ ions on a collinear G-type antiferromagnetic structure, with the magnetic moments most likely (anti) parallel with the c axis, and of magnitude 3.4 +/- 0.2 mu(B) (3.6 +/- 0.2 mu(B) for NdCa2Fe3O8). The result is reasonable given the magnetic structures of the end members of the La1-xCaxFeO3 series, LaFeO3 (x = 0) and Ca2Fe2O5 (x = 1). © 2009, Institute of Physics
  • Thumbnail Image
    Item
    Investigating short-range order in triglycine sulphate using x-ray and neutron diffuse scattering
    (Australian Institute of Physics, 2012-02-02) Hudspeth, JM; Goossens, DJ; Gutmann, MJ; Studer, AJ; Welberry, TR
    Triglycine sulphate (TGS) [(NH2CH2COOH)3H2SO4] is a hydrogen-bonded ferroelectric with a phase transition temperature of 322K [1]. The phase transition is reversible and second order, order disorder type, making TGS of fundamental interest in the field of phase transitions [2]. Above the critical temperature, one of the glycine molecules is disordered across a mirror plane. Below the critical temperature, it chooses a side, breaking the symmetry. The ferroelectric state is obtained through the ordering of the glycine orientations on neighbouring sites, but the mechanism for the phase transition is not well understood. We have investigated the short-range order in TGS by collecting single crystal x-ray and neutron diffuse scattering data on hydrogenous and fully deuterated TGS respectively. Data was collected at temperatures from well below to well above TC. We have also developed a model for the short-range order using the program ZMC [3]. This has given us some new insight into the behavior of the disordered glycine in TGS. For example, above TC, the orientations of the disordered glycine appear to be correlated over short range rather than being completely random as suggested by the average structure.
  • No Thumbnail Available
    Item
    A neutron diffraction study of the phase transition of fully deuterated triglycine sulphate (ND2CD2COOD)(3 center dot) D2SO4
    (Wiley-V CH Verlag GMBH., 2013-03-01) Hudspeth, JM; Goossens, DJ; Gutmann, MJ; Studer, AJ
    Using neutron single crystal and powder diffraction, the first thorough investigation of the structure of fully deuterated triglycine sulphate, (ND2CD2COOD)3.D2SO4 is presented, including its evolution with T, through its structural phase transition. This includes new precise structural parameters determined at several key temperatures above and below TC using single crystal diffraction, and for the first time a parametric study has been undertaken over a wide temperature range — from 4 to 500 K in 2 K steps. It was found that fully deuterated TGS shows a structure consistent with hydrogenous TGS and partially deuterated TGS. The evolution of several key hydrogen bond lengths suggests that weakening of the H-bond network with T is crucial in decoupling the polarising glycine molecules from the other glycines and allowing the long-range ferroelectric order to break down. A new parameterisation of the phase transition is demonstrated. Contrary to results of physical properties measurements, there is no evidence of a second low temperature phase transition in TGS – no low temperature anomalies were observed in the crystal structure. © 2013, Wiley-VCH Verlag.