Browsing by Author "Fuess, H"
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- ItemDefect structure and property consequence when small Li+ ions meet BaTiO3(American Physical Society, 2020-08-31) Narayanan, N; Lou, Q; Rawal, A; Lu, T; Liu, Z; Chen, J; Langley, J; Chen, H; Hester, JR; Cox, N; Fuess, H; McIntyre, GJ; Li, G; Yu, DH; Liu, YIn the present work the longstanding issue of the structure and dynamics of smaller ions in oxides and its impact on the properties was investigated on 7% Li-doped BaTiO3. The investigation combined several techniques, notably neutron powder diffraction (NPD), nuclear magnetic resonance (7Li-NMR), electron paramagnetic resonance (EPR), electron microprobe, electric polarization (EP) measurement, and electronic structure calculations based on density-functional theory (DFT). Electron microprobe confirmed multiple phases, one containing incorporated Li in the BaTiO3 host lattice and another glassy phase which breaks the host lattice due to excessive Li accumulation. While the average structure of Li in BaTiO3 could not be determined by NPD, 7Li-NMR revealed one broad “disordered” and multiple “ordered” peaks. Local structure models with different defect types involving Li+ were modeled and the corresponding chemical shifts (δ) were compared with experimental values. It is found that the closest defect model describing the ordered peaks, is with Ti4+ being replaced by four Li+ ions. The biexponential behavior of the spin-lattice relaxation of the ordered peaks each with a short and a long relaxation discloses the existence of paramagnetic ions. Finally, EPR revealed the existence of the paramagnetic ion Ti3+ as a charge-transfer defect. DFT calculations disclosed local antipolar displacements of Ti ions around both types of defect sites upon insertion of Li+. This is in accordance with the experimental observation of pinching effects of the EP in Li-doped BaTiO3. These studies demonstrate the huge impact of the local structure of the doped smaller/lighter ions on the functional properties of oxides. ©2020 American Physical Society
- ItemDefect structure-property correlations in Li doped BaTiO3(Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Narayanan, N; Lou, Q; Rawal, A; Lu, T; Liu, Z; Chen, J; Langley, J; Chen, H; Hester, JR; Cox, N; Fuess, H; McIntyre, GJ; Li, G; Yu, DH; Liu, Y; Li, GIn the present work we investigate the important issue of the structure and dynamics of smaller ions in oxides and the resulting impact on its functional properties. For this purpose, we selected a 7% Li-doped BaTiO3. Li is a vital ingredient in novel energy storage technologies such as Li-ion batteries. The smaller Li-ion can influence the structural stability, homogeneity, local environment, and dynamic behavior of the host lattice, affecting and optimizing the dielectric and multiferroic properties of novel polar functional materials [1-2]. However, the Li-ion positions and dynamics in functional materials are not completely understood, controversially discussed and are the subject of extensive ongoing research [3]. Furthermore, sample inhomogeneity due to Li migration to the grain boundary and/or development of multiple phases complicates the elucidation of the structure-property correlations that may lead to incorrect interpretations [4]. The selection of BaTiO3 as the host lattice is due to materials based on this being considered as the alternative to the piezoelectric lead zirconate titanate, citing environmental issues [5]. BaTiO3 also crystallizes in a simple perovskite structure and Li ions can be effectively doped into it at lower doping levels. Very recently, field-dependent electric polarization measurements on BaTiO3 exhibited a polarization–electric field double hysteresis loop upon Li doping [4]. These drastic changes to the electric polarization, related to the doping poses a good test case for the investigation of the Li induced defect structure model and its influence on the functional properties. To elucidate the above structure-property correlations, we combined several techniques, such as neutron powder diffraction electron microprobe associated with the wavelength-dispersive spectroscopy, 7Li nuclear magnetic resonance spectroscopy (NMR), electron paramagnetic resonance (EPR), electric polarization measurement, and theoretical calculations based on density functional theory [6].
- ItemLattice dynamics of hydrogenated austenitic steels(Australian Institute of Physics, 2005-01-31) Danilkin, SA; Hoelzel, M; Udovic, TJ; Rameriz-Cuesta, T; Parker, SF; Wipf, H; Fuess, HWe investigated hydrogen vibrations in of Fe 18Cr-10Ni and Fe-25Cr-20Ni austenitic steels doped in H gas atmosphere at pressures up to 7 GPa. Measurements were performed with neutron spectrometers FANS at NIST and TOSCA at ISIS. Experiments show that vibrational energy of H atoms in studied steels decreases from 132 meV at H/Me=0.0033 to 111 meV at H/Me=0.9 due to lattice dilatation. The hydrogen peaks are broadened. At H contents from 0.003 to 0.4-where the single broad peak is observed-the broadening is most probably connected with the Me-H force constant disorder. At H/Me>0.4-0.5-where H-peak has the two-component structure-the H-H interaction becomes important resulting in the dispersion of the optical phonon branches.
- ItemMagnetic structure and spin correlations in magnetoelectric honeycomb Mn4Ta2O9(American Physical Society, 2018-10-22) Narayanan, N; Senyshyn, A; Mikhailova, D; Faske, T; Lu, T; Liu, Z; Weise, B; Ehrenberg, H; Mole, RA; Hutchison, WD; Fuess, H; McIntyre, GJ; Liu, Y; Yu, DHWe elucidate the magnetic interactions and the role of spin (electron) correlation in determining the ground state of the honeycomb compound Mn4Ta2O9, by neutron powder diffraction, inelastic neutron scattering (INS), specific-heat (CP) measurements, and electronic-structure calculations. The antiferromagnetic long-range order with moments along c occurs at 102 K with strong exchange striction and small anisotropy. It is escribed by the three-dimensional Ising model. Diffuse magnetic scattering has been observed above TN, which is attributed to the two-dimensional spin correlations within the Mn2+ honeycombs. This is confirmed by the calculated exchange constants. INS experiments and spin-wave simulations together with CP measurements reveal two gapped modes on the ab plane, originating from the rotation of the spins away from the easy axis c. The magnetic anisotropy is mainly determined by an electron-correlation-assisted dipole-dipole interaction. This work provides insight into the competing origins of the magnetic anisotropy, which leads to different magnetic ground states in the family of honeycomb compounds. ©2018 American Physical Society
- ItemMagnetic transitions and site-disordered induced weak ferromagnetism in (1-x)BiFeO3-xBaTiO3(American Physical Society, 2014-01-31) Singh, A; Senyshyn, A; Fuess, H; Kennedy, SJ; Pandey, DWe present evidence for weak ferromagnetism in both the rhombohedral and cubic compositions of BF-x BT solid solutions for x < 0.55. Rietveld refinement of nuclear and magnetic structures reveals that the G -type antiferromagnetic ordering of the Fe 3+ magnetic sublattice survives up to x ∼ 0.50. We address the issue of weak ferromagnetism due to spin canting, which is allowed by the symmetry in the R 3c space group but not in the cubic Pm3 ¯ m space group. It is shown that the local symmetry of the average cubic compositions of BF-x BT for 0.35 < x < 0.55 is broken due to off-centering of Bi 3+ in the (1-10) plane and O 2− along the ⟨110⟩ direction from their special Wyckoff positions at (0,0,0) and (1/2,1/2,0), respectively. The local O 2− disorder is shown to be equivalent to local antiferrodistortive rotation, leading to deviation of the Fe 3+ -O 2− -Fe 3+ bond angle from 180° that allows spin canting due to Dzyaloshinskii-Moriya interaction D ⃗ i,j ⋅(S ⃗ i ×S ⃗ j ) , which is otherwise irreconcilable with the ideal cubic symmetry. The magnetization and neutron powder diffraction measurements confirm the absence of magnetic ordering at room temperature for x ≳ 0.55. ©2014 American Physical Society
- ItemSymmetry analysis of the ferroic transitions in the coupled honeycomb system (Fe, Co, Mn)4Ta2O9(Australian Institute of Physics, 2020-02-04) Narayanan, N; Faske, T; Lu, T; Liu, Z; Brennan, M; Hester, JR; Avdeev, M; Senyshyn, A; Mikhailova, D; Ehrenberg, H; Hutchison, WD; Mole, RA; Fuess, H; McIntyre, GJ; Liu, Y; Yu, DHExotic phenomena such as spin liquid, spin-orbital entities, magnetic order induced multiferroicity (type ii) or quantum criticality have recently triggered extensive research on the ground state properties of frustrated magnetic systems. The ground states of these compounds are determined by the coupling of the spin to the orbital, charge and lattice degrees of freedom. One of the extensively investigated lattices is the honeycomb lattice due to the development of the Kitaev model for quantum spin liquids [1-2]. In this work, we are interested in the coupled honeycomb system M4A2O9 (M=Fe, Co and Mn and A=Nb, Ta). All members have two crystallographically distinct M sites, which are in the distorted octahedral oxygen cages. These cages form edge-shared coplanar and corner-shared buckled honeycombs respectively which are interconnected in the perpendicular direction leading to competing exchange paths. The M=Co and Mn members were magnetoelectrics, whereas Fe2Ta2O9 was reported to exhibit both magnetoelectric and (type ii) multiferroic phases depending on the temperature [3-4]. Magnetoelectrics and multiferroics are technically highly relevant with a variety of applications such as MRAMs and field sensors. However, the coupling mechanism is very complicated [5]. Furthermore, due to the group properties of the symmetry analysis methods such as representation analysis and magnetic space groups, the magnetic structure of the Nb counterpart Co4Nb2O9 is controversially discussed. It is therefore apparent that the above discussed diversities of the properties are determined by the magnetic structure and the closely related electronic structure. These can be elucidated by investigating the structure and dynamics of these compounds, which will help to understand the emergence of different ground states and the diverse phase transitions in this family of materials In this work, we systematically investigate the magnetic and electronic structure of the (Fe, Co, Mn)4Ta2O9 system. We combined several different techniques of neutron powder diffraction, inelastic neutron scattering, heat capacity, electronic band structure calculations and spin wave modeling based on linear spin wave theory.