Browsing by Author "Norén, L"
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- ItemA correlated electron diffraction, in situ neutron diffraction and dielectric properties investigation of poled (1-x)Bi0.5Na0.5TiO3-xBaTiO3 ceramics(American Institute of Physics, 2011-10-15) Wang, J; Liu, Y; Withers, RL; Studer, AJ; Li, Q; Norén, L; Guo, YPA correlated electron diffraction, temperature-dependent in situ neutron diffraction, and temperature-dependent dielectric properties investigation of poled (1-x)Bi(0.5)Na(0.5)TiO(3)-xBaTiO(3) (BNTBT100x) (x = 0.04, 0.07, and 0.12) samples has been carried out. The results show that the depolarization temperature, T(d), of the rhombohedral BNTBT 4 sample is associated with the disappearance of G +/- 1/2 [111]*(p) satellite reflections and a(-)a(-)a(-) octahedral tilting while that of the BNTBT 12 sample is associated with a metrically tetragonal to metrically cubic or pseudo-cubic symmetry. In the case of the poled BNTBT 7 sample in the MPB region, the dielectric properties show a quite distinct two stage transition from a room temperature clearly metrically tetragonal phase again to a metrically cubic or pseudo-cubic symmetry above 150 degrees C. There is no apparent change in its average structure in vicinity of T(d) in BNTBT 7. Electron diffraction shows the presence of considerable octahedral tilt twin disorder in all three samples. (C) 2011 American Institute of Physics.
- ItemCoupled Li1+/Nb5+ and O2-/F- ordering on the Na and Cl sites of the average NaCl structure of Li4NbO4F(Elsevier, 2009-05) Norén, L; Withers, RL; Goossens, DJ; Elcombe, MM; Kearley, GJThe average, as well as the cation and anion 'disordered', crystal Structure of Li4NbO4F has been carefully investigated via coupled neutron and X-ray powder diffraction studies as well as via electron diffraction studies. The existence of a spectacular highly structured diffuse intensity distribution in the latter provides strong evidence for coupled Li1+/Nb5+ and O2-/F- ordering on the Na and Cl sites of the average NaCl structure of Li4NbO4F. Bond valence sum calculations have been used to investigate local crystal chemistry as well as to Suggest plausible local crystal chemical constraints while ab initio DFT based theoretical calculations of a 2 x 2 x 2 supercell have been carried Out in order to provide additional insight into the local crystal chemistry of this compound. © 2009, Elsevier Ltd.
- ItemThe crystal and magnetic structures of LaCa2Fe3O8 and NdCa2Fe3O8(Institute of Physics, 2009-03-25) Hudspeth, JM; Goossens, DJ; Studer, AJ; Withers, RL; Norén, LThe 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
- ItemDisordered structures and low temperature dielectric relaxation properties of two misplaced-displacive cubic pyrochlores found in the Bi2O3–MIIO–Nb2O5 (M=Mg, Ni) systems(Elsevier, 2007-09) Nguyen, HB; Norén, L; Liu, Y; Withers, RL; Wei, XY; Elcombe, MMThe disordered structures and low temperature dielectric relaxation properties of Bi1.667Mg0.70Nb1.52O7 (BMN) and Bi1.67Ni0.75Nb1.50O7 (BNN) misplaced-displacive cubic pyrochlores found in the Bi2O3–MIIO–Nb2O5 (M=Mg, Ni) systems are reported. As for other recently reported Bi-pyrochlores, the metal ion vacancies are found to be confined to the pyrochlore A site. The B2O6 octahedral sub-structure is found to be fully occupied and well-ordered. Considerable displacive disorder, however, is found associated with the O' A2 tetrahedral sub-structure in both cases. The A-site ions were displaced from Wyckoff position 16d (1/2, 1/2, 1/2) to 96 h (1/2, 1/2-éA, 1/2 + éA while the O' oxygen was shifted from position 8b (3/8, 3/8, 3/8) to Wyckoff position 32e (3/8 + éọ, 3/8 + éọ, 3/8 + éọ). The refined displacement magnitudes off the 16d and 8b sites for the A and O' sites were 0.408 Å/0.423 Å and 0.350 Å/0.369 Å for BMN/BNN, respectively. © 2007, Elsevier Ltd.
- ItemEffect of electric field and temperature on average structure and domain wall motion in 0.93Bi(0.5)Na(0.5)TiO(3)-0.07BaTiO(3) ceramic(Hindawi Publishing Corporation, 2013-06-24) Wang, J; Liu, Y; Studer, AJ; Norén, L; Withers, RLIn situ neutron powder diffraction patterns and dielectric spectra of 0.93Bi0.5Na0.5TiO3-0.07BaTiO3 ceramic were investigated under different electrical fields and temperatures. An electric-field-induced phase transition from metrically cubic to metrically tetragonal, associated with strong domain wall motion, occurs. Such induced phase and domain wall motion are unchanged until the high-temperature phase transition occurs from metrically tetragonal to metrically cubic. All these changes are irrelevant to the observed depolarization temperature (75°C). The depolarization behaviour is thus suggested to be associated with the local structure caused by the octahedral tilt twinning disorder. © 2013, Jian Wang et al.
- ItemIn-situ neutron diffraction study of Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals under uniaxial mechanical stress(American Institute of Physics, 2012-04-26) Li, Q; Liu, Y; Luzin, V; Studer, AJ; Wan, YH; Li, ZR; Norén, L; Withers, RL; Xu, ZIn this paper, we report the phase transition behavior of ternary relaxor ferroelectric single crystals of 0.25Pb(In1/2Nb1/2)O-3-0.44Pb(Mg1/3Nb2/3)O-3-0.31PbTiO(3) subject to a uniaxial mechanical stress up to 400 MPa. The resultant in situ neutron diffraction data are interpreted in terms of the polarization rotation theory and provide direct structural evidence for the stress-induced polarization rotation pathway deduced from studies of macroscopic physical properties under stress. It is suggested that an intermediate, metastable orthorhombic phase is induced above a critical pressure of similar to 75 MPa. This critical stress level appears to be unaffected by sample poling although the ground states (at zero stress) for the poled and unpoled crystals are different. The critical stress level, however, does decrease with increasing temperature. The elastic behavior of the intermediate phases is also studied based on a calculation of the associated lattice strains. (C) 2012 American Institute of Physics.
- ItemLocal microstructure evolution of local microstructure evolution of bismuth sodium titanate-based lead-free piezoelectric systems across the morphotropic phase boundary region(World Scientific Publishing Co, 2012-09-12) Liu, Y; Withers, RL; Wang, J; Norén, L; Studer, AJ; Li, YXMorphotropic phase boundary (MPB) containing piezoelectric systems generally exhibit enhanced piezoelectric performance at compositions within, or close to, the MPB region. The mechanism/s underlying such enhancement, however, are still contentious due to complex micro/nanostructure and apparently inherent local structural variability associated with octahedral tilt disorder/platelet precipitates in such piezoelectric materials. This paper reviews some recent structural analysis results from Bi0.5Na0.5TiO3 (BNT) and other binary, lead-free, piezoelectric materials systems derived from it via electron diffraction and in situ neutron diffraction. The results suggest that intrinsically existing local microstructure (LMS) in BNT essentially continues across the MPB region. The LMS, originating from inherent octahedral tilt disorder, is strongly temperature-, electric field-, pressure- and chemical composition-dependent, and may help to explain a series of phenomena observed in BNT-based binary materials systems, including the enhanced piezoelectric effect in the region of the MPB. © 2012 World Scientific Publishing Co.
- ItemTemperature-dependent electrical, elastic and magnetic properties of sol-gel synthesized Bi(0.9)Ln(0.1)FeO(3) (Ln = Nd, Sm)(IOP Publishing Ltd, 2012-03-28) Schiemer, J; Withers, RL; Carpenter, MA; Liu, Y; Wang, JL; Norén, L; Li, Q; Hutchison, WDThis report details correlated electrical, mechanical and magnetic behaviour in BiFeO3 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. © 2012, IOP Publishing LTD.