Browsing by Author "Mohamed, Z"
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- ItemComplex 5d magnetism in a novel S= 1/2 trimer system, the 12L hexagonal perovskite ba4biir3o12(American Chemical Society, 2013-10-21) Miiller, W; Dunstan, MT; Huang, Z; Mohamed, Z; Kennedy, BJ; Avdeev, M; Ling, CDThe 12L hexagonal perovskite Ba4BiIr3O12 has been synthesized for the first time and characterized using high-resolution neutron and synchrotron X-ray diffraction as well as physical properties measurements. The structure contains Ir3O12 linear face-sharing octahedral trimer units, bridged by corner-sharing BiO6 octahedra. The average electronic configurations of Ir and Bi are shown to be +4(d5) and +4(s1), respectively, the same as for the S = 1/2 dimer system Ba3BiIr2O9, which undergoes a spin-gap opening with a strong magnetoelastic effect at T* = 74 K. Anomalies in magnetic susceptibility, heat capacity, electrical resistivity, and unit cell parameters indeed reveal an analogous effect at T* ≈ 215 K in Ba4BiIr3O12. However, the transition is not accompanied by the opening of a gap in spin excitation spectrum, because antiferromagnetic coupling among S = 1/2 Ir4+ (d5) cations leads to the formation of a S = 1/2 doublet within the trimers, vs S = 0 singlets within dimers. The change in magnetic state of the trimers at T* leads to a structural distortion, the energy of which is overcompensated for by the formation of S = 1/2 doublets. Extending this insight to the dimer system Ba3BiIr2O9 sheds new light on the more pronounced low-temperature anomalies observed for that compound. © 2013, American Chemical Society.
- ItemCrystal and magnetic structure of Li2MnSiO4 and Li2CoSiO4 characterized by neutron diffraction measurement(Australian Institute of Physics, 2014-02-06) Mohamed, Z; Avdeev, M; Ling, CDLithium orthosilicates compounds Li2MnSiO4 and Li2CoSiO4 were synthesized by solid state reaction and characterized using X-ray powder diffraction, magnetic susceptibility measurement, heat capacity and neutron powder diffraction. The magnetic susceptibility measurement shows that Li2MnSiO4 and Li2CoSiO4 obey Curie Weiss behaviour at high temperature and undergo antiferromagnetic ordering below TN = ~12 K and ~13 K respectively. The magnetic structures of both compound have been solved for the first time using low temperature neutron diffraction data. The results reveal that the magnetic structure of Li2CoSiO4 can be described as antiferromagnetic quasi-layers stacked along the a-axis with the magnetic moment ~2.92 μB aligned parallel to the a-axis. The magnetic structure of Li2MnSiO4 showed quite different behaviour compared to Li2CoSiO4. The origin of this complex magnetic structures will be discussed in terms of super-superexchange interactions among the transition metal ions, mediated by bridging SiO4 tetrahedra.
- ItemDetermination of magnetic structure in LiFe 1-xMn xPO 4 by neutron powder diffraction(Australian Institute of Nuclear Science and Engineering, 2013-12-03) Mohamed, Z; Ling, CD; Avdeev, MLithium orthophosphates LiFe 1-xMn xPO 4 (x=0,0.2,0.4,0.6,0.8,1) were successfully synthesized using a solid state reaction method and characterized by X-ray powder diffraction, neutron powder diffraction and magnetic susceptibility measurement. Structure refinements against powder diffraction data were carried out in the orthorhombic space group Pnma. Magnetic susceptibility measurements showed all samples undergo AFM ordering at low temperature. Mn"2"+ doped in Fe-site resulted the below Néel temperature T_N decreased from 52 K (x=0) to 38 K (x=1). The magnetic structures from neutron powder diffraction data confirmed the AFM ordering with magnetic space group Pnma' for LiFePO"4 and all intermediate compounds, with a propagation vector k=(0 0 0). Only the pure LiMnPO_4 sample behaves differently showing an AFM ordering with magnetic space group Pn'm'a'. A weak ferromagnetic component in the c-direction also observed in LiMnPO 4.
- ItemLong-range ordered magnetic structures in Li2MnSiO4 and Li2CoSiO4(International Union of Crystallography, 2014-08-12) Mohamed, Z; Avdeev, M; Ling, CDThe lithium orthosilicates Li2MnSiO4 and Li2CoSiO4 have been synthesized by solid state reaction and characterized using X-ray powder diffraction (XRD), magnetic susceptibility measurement, heat capacity and neutron powder diffraction (NPD). The monoclinic Li2MnSiO4 and orthorhombic Li2CoSiO4 compound were found to be antiferromagnetically ordered below Neel temperature = ~12 K and ~13 K respectively. The ordered magnetic structures of both compounds have been solved for the first time using low temperature neutron diffraction data. The magnetic structure of Li2CoSiO4 can be described as antiferromagnetic quasi-layers stacked along the a-axis. The ordered magnetic moments of the Co2+ and Mn2+ are aligned perpendicularly and obliquely to the distorted closed-packed layers of oxygen atoms and the values, 2.9 bohr magneton and 4.6 Bohr magneton, are close to the expected values for d7 Co2+ and d5 Mn2+, respectively. The origin of these complex magnetic structures will be discussed in terms of super-superexchange interactions among the transition metal ions, mediated by bridging SiO4 tetrahedra. Figure 1: Magnetic sublattices in Li2CoSiO4 (left) and Li2MnSiO4 (right) with respect to crystal structure. Blue, yellow, and light and dark green show the M, Si, and Li1 and Li2 sites in Pbn21 Li2CoSiO4 and P21/n Li2MnSiO4. © International Union of Crystallography
- ItemMagnetic structure and properties of centrosymmetric twisted-melilite K2CoP2O7(Royal Society of Chemistry, 2017-04-24) Sale, M; Avdeev, M; Mohamed, Z; Ling, CD; Barpanda, PTwisted-melilite dipotassium cobalt pyrophosphate (K2CoP2O7, P42/mnm, #136), originally reported by Gabelica-Robert (1981), was synthesized in powder form by a standard solid-state reaction route. The magnetic properties of the material were studied by magnetometry and its magnetic structure determined using neutron powder diffraction for the first time. Below TN = 11 K, the material adopts a G-type antiferromagnetic structure with moments aligned in the ab-plane (magnetic space group Pn′nm, #58.3.473). Ab initio calculations were performed to examine the isotropic magnetic spin exchange parameters as well as the preferred direction of magnetic moments due to spin–orbit coupling. The relationship between crystal structure geometry and the strength of the magnetic interactions was examined and compared to those of melilite-type Sr2CoGe2O7. © The Royal Society of Chemistry 2017
- ItemMagnetic structure of some battery materials and why it matters(International Battery Materials Association, 2014-03-02) Avdeev, M; Ling, CD; Mohamed, Z; Barpanda, P; Yamada, A; Ben Yahia, H; Shikano, MOperation of batteries with insertion cathodes is typically based on a redox reaction Mn+/Mn+p (p=1,2) of transition metals. The motivation to increase gravimetric energy density and reduce cost naturally drives research to the light, i.e. 3d, transition metals M=V-Ni, which in turn are also paramagnetic ions and may order magnetically. Using neutron diffraction and magnetometry we explored magnetic structure and properties of some materials recently synthesized in search of better cathode materials: Na2CoP2O7, maricite- and triphylite-NaFePO4, Na2FePO4F, M(OH)xF2-x (M=Co,Fe), Li2MnSiO4, Li2CoSiO4, LiNaCoPO4F, LiNaFePO4F. The detailed magnetic structures will be presented and features of crystal structures affecting the type of magnetic ordering will be discussed. Although polyanionic materials (phosphates and silicates) are magnetically low density systems and thus order at low temperature (< 50 K), the information on magnetic structure is important for accurate DFT calculations. The effect of neglecting magnetic order on the DFT based quantitative predictions will be illustrated.
- ItemMagnetic structures of NaFePO4 maricite and triphylite polymorphs for sodium-ion batteries(American Chemical Society, 2013-08-05) Avdeev, M; Mohamed, Z; Ling, CD; Lu, J; Tamaru, M; Yamada, A; Barpanda, PThe magnetic structure and properties of polycrystalline NaFePO4 polymorphs, maricite and triphylite, both derived from the olivine structure type, have been investigated using magnetic susceptibility, heat capacity, and low-temperature neutron powder diffraction. These NaFePO4 polymorphs assume orthorhombic frameworks (space group No. 62, Pnma), built from FeO6 octahedral and PO4 tetrahedral units having corner-sharing and edge-sharing arrangements. Both polymorphs demonstrate antiferromagnetic ordering below 13 K for maricite and 50 K for triphylite. The magnetic structure and properties are discussed considering super- and supersuperexchange interactions in comparison to those of triphylite-LiFePO4. © 2013, American Chemical Society
- ItemMagnetic structures of βI-Li2CoSiO4 and γ0-Li2MnSiO4: crystal structure type vs. magnetic topology(Elsevier, 2014-08) Avdeev, M; Mohamed, Z; Ling, CDThe magnetic structure and properties of the candidate lithium-ion battery cathode materials Pbn21(≡Pna21) Li2CoSiO4 and P21/n Li2MnSiO4 have been studied experimentally using low-temperature neutron powder diffraction and magnetometry. Both materials undergo long-range antiferromagnetic ordering, at 14 K and 12 K respectively, due to super–super-exchange mediated by bridging silicate groups. Despite having different crystal structures (wurtzite- vs. “dipolar”-type), Li2CoSiO4 and Li2MnSiO4 have the same topology in terms of magnetic interactions, and adopt collinear magnetic structures of the same type with the propagation vectors (0, 1/2, 1/2) and (1/2, 0, 1/2), respectively. The magnetic moments in the two materials are aligned in parallel and obliquely to the distorted closed-packed layers of oxygen atoms. The experimentally observed values of the ordered magnetic moments, 2.9 μB and 4.6 μB, are close to those expected for d7 Co2+ and d5 Mn2+, respectively.© 2014, Elsevier Inc.
- ItemNeutron diffraction study of the li-ion battery cathode Li2FeP2O7(American Chemical Society, 2013-03-18) Barpanda, P; Rousse, G; Ye, T; Ling, CD; Mohamed, Z; Klein, Y; Yamada, AWith a combination of magnetic susceptibility measurements and low-temperature neutron diffraction analyses, the magnetic structure of Li2FeP2O7 cathode has been solved. This pyrophosphate Li2FeP2O7 compound stabilizes into a monoclinic framework (space group P2(1)/c),having a pseudolayered structure with the constituent Li/Fe sites distributed into MO6 and MO5 building units. The magnetic susceptibility follows a Curie Weiss behavior above 50 K. Li2FeP2O7 shows a long-range antiferromagnetic ordering at T-N = 9 K, as characterized by the appearance of distinct additional peaks in the neutron diffraction pattern below TN. Its magnetic reflections can be indexed with a propagation vector k = (0,0,0). The magnetic moments inside the FeO6-FeO5 clusters are ferromagnetic, whereas these clusters are antiferromagnetic along the chains. The adjacent chains are in turn ferromagnetically arranged along the a-axis. The magnetic structure of Li2FeP2O7 cathode material is described focusing on their localized spin spin exchange. The magnetic structure and properties have been generalized for Li2FeP2O7 Li2CoP2O7 binary solid solutions. © 2013, American Chemical Society.