Jessica Hudspeth

The crystal and magnetic structures of LaCa2Fe3O8 and NdCa2Fe3O8

The crystal and magnetic structures of LaCa(2)Fe(3)O(8) and NdCa(2)Fe(3)O(8) have been established using a combination of x-ray, neutron and electron diffraction. It was already considered likely that LaCa(2)Fe(3)O(8) and NdCa(2)Fe(3)O(8) were made up of stacked perovskite-like layers of FeO(6) octahedra, with every third layer being replaced by a layer of tetrahedrally coordinated Fe, rather like a variation on the Brownmillerite (Ca(2)Fe(2)O(5)) 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 P 2(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 LaCa(2)Fe(3)O(8) is shown to consist of antiferromagnetically ordered Fe(3+) 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 ± 0.2μ(B) (3.6 ± 0.2μ(B) for NdCa(2)Fe(3)O(8)). The result is reasonable given the magnetic structures of the end members of the La(1-x)Ca(x)FeO(3) series, LaFeO(3) (x = 0) and Ca(2)Fe(2)O(5) (x = 1).

Diffuse scattering and the mechanism for the phase transition in triglycine sulphate

Despite the order/disorder nature of its ferroelectric phase transition, and evidence for the evolution of local order as being important for understanding the transition, no comprehensive diffuse scattering study of the short-range order in triglycine sulphate, (TGS), (NH2CH2COOH)3H2SO4 has been undertaken. Diffuse scattering from single crystals is sensitive to two-body correlations, and can act as a probe of local structure, which in a second order phase transition acts as a precursor to the low temperature phase. The role of hydrogen bonding and dipolar interactions in the ferroelectric phase transition in TGS has been a long matter of conjecture. Using neutron and X-ray single crystal diffuse scattering this study shows that hydrogen bond mediated interactions between polarising glycine molecules cause local one-dimensional polarised domains to develop, oriented parallel to the b axis. These domains interact via dipolar interactions, and the three-dimensional ferroelectric order arises. This provides a real-space, interaction-based model for the phase transition in TGS, showing in detail how the local chemistry and physics give rise to the polarised state.

Approaches to modelling thermal diffuse scattering in triglycine sulfate, (NH2CH2COOH)3·H2SO4

The thermal diffuse scattering in triglycine sulfate, (NH2CH2COOH)3·H2SO4, has been modelled by treating the intermolecular interactions that give rise to the correlated atomic displacements like Hookes law springs. To limit the number of variables in the model, the force constants for the interactions were parameterized in a number of ways, the most successful of which was an empirical interaction potential using an exponential function of the interatomic separations.

Disordered structures in lead-free piezoelectrics

Patrick Kin Man Tung1, Marton Major2, Jessica Hudspeth3, Christina Hoffmann4, John Daniels5 1School Of Materials Science And Engineering, University Of New South Wales, Sydney, Australia, 2Institute of Materials Science, Technical University of Darmstadt, Darmstadt, Germany, 3Beamline ID15, European Synchrotron Radiation Facility, Grenoble, France, 4TOPAZ beamline, Spallation Neutron Source, Knoxville, United States, 5School Of Materials Science And Engineering, University Of New South Wales, Kensington, Australia E-mail: patrick.tung@unsw.edu.au

Diffuse scattering in the polymorphs of p(N-methylbenzylidene)-p-methylaniline

Polymorphism refers to the ability of a solid to exist in more than one crystal structure. Apart from being of scientific interest, it is of practical importance in the pharmaceutical and chemical manufacturing industries. In pharmaceuticals the polymorphic form of the substance can affect the ease of manufacture or the rate of uptake by the human body [1]. There is consequently a great need to be able to understand, predict and control polymorphism. This work is part of a larger study using diffuse scattering methods to investigate the role of molecular flexibility and disorder in polymorphism. Diffuse scattering is sensitive to two-body correlations so can provide information about the intermolecular interactions that cannot be obtained from the Bragg peaks, such as how the displacement or orientation of a molecule is correlated with that of its neighbours. p-(N-methylbenzylidene)-p-methylaniline (MeMe) is a model system for studying polymorphic behaviour. The system is trimorphic with all three polymorphs exhibiting highly structured diffuse scattering patterns [2]. The short-range order has been modelled using the program ZMC in which the molecules are allowed to interact via Hooke’s law springs and brought to thermal equilibrium using a Monte Carlo algorithm [3]. Here we present a comparison of the diffuse scattering in the different forms of MeMe and assess how successfully different models for the intermolecular interactions reproduce the observed diffuse scattering data.

The crystal and magnetic structures of LaCa{sub 2}Fe{sub 3-x}M{sub x}O{sub 8} (M=Al, Ga, In)

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)μB in LaCa2Fe3O8 and 4.8(1)μB in LaCa2Fe2GaO8. The magnetic space group (P2b21′ma′) and moment direction (along c) does not appear to change with Ga substitution.

The crystal and magnetic structures of LaCa(2)Fe(3)O(8) and NdCa(2)Fe(3)O(8).

The crystal and magnetic structures of LaCa(2)Fe(3)O(8) and NdCa(2)Fe(3)O(8) have been established using a combination of x-ray, neutron and electron diffraction. It was already considered likely that LaCa(2)Fe(3)O(8) and NdCa(2)Fe(3)O(8) were made up of stacked perovskite-like layers of FeO(6) octahedra, with every third layer being replaced by a layer of tetrahedrally coordinated Fe, rather like a variation on the Brownmillerite (Ca(2)Fe(2)O(5)) 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 P 2(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 LaCa(2)Fe(3)O(8) is shown to consist of antiferromagnetically ordered Fe(3+) 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 ± 0.2μ(B) (3.6 ± 0.2μ(B) for NdCa(2)Fe(3)O(8)). The result is reasonable given the magnetic structures of the end members of the La(1-x)Ca(x)FeO(3) series, LaFeO(3) (x = 0) and Ca(2)Fe(2)O(5) (x = 1).

The crystal and magnetic structures of LaCa2Fe3O8and NdCa2Fe3O8

The crystal and magnetic structures of LaCa(2)Fe(3)O(8) and NdCa(2)Fe(3)O(8) have been established using a combination of x-ray, neutron and electron diffraction. It was already considered likely that LaCa(2)Fe(3)O(8) and NdCa(2)Fe(3)O(8) were made up of stacked perovskite-like layers of FeO(6) octahedra, with every third layer being replaced by a layer of tetrahedrally coordinated Fe, rather like a variation on the Brownmillerite (Ca(2)Fe(2)O(5)) 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 P 2(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 LaCa(2)Fe(3)O(8) is shown to consist of antiferromagnetically ordered Fe(3+) 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 ± 0.2μ(B) (3.6 ± 0.2μ(B) for NdCa(2)Fe(3)O(8)). The result is reasonable given the magnetic structures of the end members of the La(1-x)Ca(x)FeO(3) series, LaFeO(3) (x = 0) and Ca(2)Fe(2)O(5) (x = 1).