Lasse Noren

Oxygen/fluorine ordering, structured diffuse scattering and the local crystal chemistry of K3MoO3F3

Abstract Bond valence sum calculations are used to investigate the crystal chemistry of the elpasolite-related oxyfluoride K3MoO3F3 in order to obtain insight into the type/s of structural distortion (away from an ideal, high symmetry, elpasolite type parent structure) responsible for a characteristic, highly structured, three-dimensional diffuse intensity distribution. The first required type of local structural distortion corresponds to large amplitude MoO3F3 octahedral rotations while the second is associated with O/F ordering and associated induced Mo ion shifts. Monte Carlo modelling is used to show how the latter when coupled with an appropriate local crystal chemical constraint can give rise to the observed structured diffuse scattering. The study is part of a wider search for diffraction evidence of oxygen/fluorine ordering in metal oxyfluoride systems.

A combined diffraction (XRD, electron and neutron) and electrical study of Na3MoO3F3

Abstract Na3MoO3F3, a member of the A2BMVIO3F3 family of elpasolite-related oxyfluorides, has been prepared by the reaction of NaF with MoO3 at 650°C. It is shown by a combined X-ray, electron and neutron diffraction study, that the true symmetry of Na3MoO3F3 is not monoclinic (pseudo-orthorhombic) as previously reported but instead triclinic (metrically rhombohedral) P1. The superstructure unit cell is given by a=−aR+bR, b=cR, c=(aR+bR+cR), when expressed with respect to the underlying rhombohedral parent structure. Neutron diffraction refinement of the rhombohedral sub-structure shows O and F to be fully ordered with alternate (111)R planes being occupied by O and then F. The nature of the distortion away from the high temperature Fm 3 m parent structure is consistent with a combination of φφφ type rotations of the octahedral framework together with ppp type displacements of the octahedral cations, which is not observed in any of the closely related A2BMVIO3F3 family members. Bond valence arguments are used to give a plausible explanation for this difference in behavior.

A correlated electron diffraction, in situ neutron diffraction and dielectric properties investigation of poled (1-x)Bi0.5Na0.5TiO3-xBaTiO3 ceramics

J.W., Y.L., R.L.W., Q.L., and Y.P.G. appreciate the support of the Australian Research Council (ARC) in the form of a Discovery Grant. Y.L. also appreciates support from the ARC Future Fellowships program.

Synthesis, electron diffraction, XRD and DSC study of the new elpasolite-related oxyfluoride, Tl3MoO3F3

Abstract Tl3MoO3F3, a previously unreported member of the A2BMVIO3F3 family of elpasolite-related oxyfluorides, has been prepared by the reaction of TlF with MoO3 at 655°C. DSC shows two major polymorphic phase transitions at 42°C and 130°C, respectively. Electron diffraction and XRD studies of the complex room temperature polymorphic form of this material indicate that the spacegroup symmetry is monoclinic P1a1 with a superstructure unit cell given by a =3 a p + c p , b =3 b p , c = 1 2 (− a p +3 c p ) , when expressed with respect to the underlying ideal elpasolite-type parent structure. This superstructure, while related, is not isomorphous to that recently reported for K3MoO3F3. The existence of a shared subset of strong G p ±J/5[204] p * type satellite reflections suggests the existence of a common intermediate superstructure. A highly structured, three-dimensional continuous diffuse intensity distribution is observed in Tl3MoO3F3 and Rb2KMoO3F3. This suggests that a particular pattern of local O/F ordering and associated Mo ion shifts, recently shown to be responsible for the existence of this diffuse distribution in the case of K3MoO3F3, may be common to the entire family of elpasolite-related A2BMVIO3F3 compounds.

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).

An electron diffraction, XRD and lattice dynamical investigation of the average structure and rigid unit mode (RUM) modes of distortion of microporous AlPO4-5

Abstract Electron diffraction is used to investigate the unit cell and space group symmetry of the average structure of AlPO4-5 as well as the zero frequency rigid unit mode (RUM) modes of distortion thereof. Despite recent reports to the contrary, absolutely no evidence could be found to justify the lowering of the average structure space group symmetry of AlPO4-5 from P6cc. Direct experimental evidence (in the form of a highly structured, characteristic diffuse intensity distribution) is found, however, for the (presumably dynamic) excitation of numerous zero frequency RUM modes of distortion of the ideal AlPO4-5 tetrahedral framework structure. The lattice dynamic program CRUSH is used to confirm the existence of these zero frequency RUM modes of distortion as well as to show that they are an intrinsic, predictable property of the AlPO4-5 tetrahedral framework structure. Six zero frequency RUM modes of distortion are predicted to exist for modulation wave-vectors of the form [hk0] ∗ , 4 for wave-vectors of the form [00l] ∗ and a single RUM for wave-vectors falling on a curved (hyperbolic) distribution. These RUM modes of distortion involve tetrahedral rotation about in-plane rotation axes. Their simultaneous (presumably dynamic) excitation is necessary to understand the apparent linear AlOP angles in average structure refinements.

New B81-B82 phases in the Ni-In system

Abstract The archetypal B8 2 phase, Ni 2 In, has been shown to be considerably more complicated than previously realized. Electron diffraction patterns of Ni 2 In samples quenched from 1073 K contain bands of highly structured diffuse intensity in addition to the strong Bragg reflections characteristic of the B8 2 type average structure, indicating that short-range ordering plays an essential role in the stability of the high temperature phase. Prolonged annealing of Ni 2 In at 733 K shows that the ideal, fully occupied Ni 2 In structure type does not exist as a stable room temperature phase either, but rather completely dissociates into the previously unreported line phases Ni 7 In 3 and Ni 5 In 3 . The compound Ni 7 In 3 appears to be isostructural with Cu 7 In 3 , a B8 superstructure phase based on the high temperature cell but with an ordered distribution of indium vacancies. The phase with the formal composition Ni 5 In 3 appears to be isostructural with the Ni 5 Ge 3 structure, which in turn is a nickel deficient superstructure relative to Ni 2 In. Annealing experiments suggest that the solid solution which is reported to extend from 37.5 to 41 mole% In does not exist. Rather there exist two line phases at either extreme of this composition range i.e. Ni 5 In 3 as described above and Ni 13 In 9 , which appears to be of Pt 13 In 9 structure type.

A neutron, X-ray and electron diffraction study of the structures of Pb3O2X2 (X = Cl, Br)

Abstract The crystal structures of the compounds Pb 3 O 2 Br 2 and Pb 3 O 2 Cl 2 have been re-refined from neutron powder diffraction data. They are found to be more closely isostructural than previous reports suggested. The major difference between the present refinements and previous refinements is in the oxygen position of the bromide compound which has moved significantly along b from y =0.024(5) to 0.9933(4), rather more in keeping with the previously reported structure of the chloride compound. Electron diffraction was used, prior to refinement, to determine the appropriate space group symmetry, given some previous ambiguity in the literature. The space group symmetry was thereby shown to be at least Pn 2 1 a , but most probably Pnma . Evidence for some displacive disorder along b was also found in the form of weak diffuse streaking perpendicular to b ∗ .

Electric-field-induced AFE-FE transitions and associated strain/preferred orientation in antiferroelectric PLZST.

Electric-field-induced, antiferroelectric-ferroelectric (AFE-FE) phase transitions are common for AFE materials. To date, the strain and preferred orientation evolution as well as the role of the intermediate FE state during the successive AFE-FE-AFE phase transitions has not been clear. To this end, we have herein studied a typical AFE Pb0.97La0.02(Zr0.56Sn0.33Ti0.11)O3 (PLZST) material using in-situ neutron diffraction. It is striking that the AFE-FE phase transition is not fully reversible: in the electric-field-induced FE state, the induced strain exhibits an elliptical distribution, which in turn leads to significant preferred orientation in the final AFE state after withdrawal of the applied electric-field. The ω-dependent neutron diffraction patterns show clear evidence of the induced strain distribution and associated preferred orientation arising from the AFE-FE phase transition. The current work also provides an explanation for several temperature and electric-field dependent dielectric anomalies as well as unrecovered strain change which appear in AFE materials after exposure to sufficiently high electric fields.

Temperature-dependent electrical, elastic and magnetic properties of sol?gel synthesized Bi0.9Ln0.1FeO3 (Ln?=?Nd,?Sm)

This report details correlated electrical, mechanical and magnetic behaviour in BiFeO(3) ceramics doped with 10% Ln (Ln = Sm, Nd) ions on the Bi, or perovskite A, site and synthesized by a sol-gel method. The ceramics exhibit bulk piezoelectric and ferroelectric properties and clear ferroelectric domain patterns through piezoresponse force microscopy. Resonant ultrasound spectroscopy, dielectric spectroscopy and magnetometry studies show correlated magnetoelectromechanical behaviour and the existence of weak ferromagnetism for both compositions. An anomaly with simultaneous mechanical and magnetic signatures is discovered in both materials near room temperature, while previously reported transitions and anomalies are found to exhibit electro- and/or magnetomechanical coupling. Magnetism is significantly enhanced in the Sm doped sample, which is a promising multiferroic material.