Browsing by Author "Allison, MC"

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    Crystal structure and monoclinic distortion of glaserite-type Ba3MnSi2O8
    (Elsevier, 2018-10-01) Avdeev, M; Xia, Q; Sale, M; Allison, MC; Ling, CD
    Crystal structure and magnetic properties of glaserite-type Ba3MnSi2O8 were investigated using variable temperature neutron powder diffraction and magnetometry. At room temperature the composition is hexagonal and the crystal structure is best described by the P-3m1 space group (a~ 5.7 Å, c~ 7.3 Å) with the apical oxygen atom modelled on a split site. On cooling below ~ 250 K the structure undergoes a phase transition into a monoclinic C2/c form (√3ahex, ahex, 2chex, β~ 90°). Analysing diffraction data in terms of symmetry-adapted distortion modes suggests that the transition is primarily driven by increasing in-plane displacements of O1, which in turn results in the coupled tilting of [SiO4] and [MnO6] octahedra and in-plane displacements of Ba1 atoms. Magnetic susceptibility measurements and neutron powder diffraction data show no evidence for long-range magnetic ordering down to 1.6 K, although the development of magnetic diffuse scattering suggests that a magnetic transition may take place at lower temperature. Crown Copyright © 2018 Published by Elsevier Inc.
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    FeMn3Ge2Sn7O16 : a spin-liquid candidate with a perfectly isotropic 2-D kagomé lattice
    (Australian Institute of Physics, 2020-02-05) Allison, MC; Wurmehl, S; Büchner, B; Valla, J; Söhnel, T; Avdeev, M; Schmid, S; Ling, CD
    The compound Fe4Si2Sn7O16 has a hitherto unique crystal structure, consisting of ionic oxide layers based on edge-sharing FeO6 and Sn4+O6 octahedra alternating with layers of intermetallic character based on FeSn2+6 octahedra, separated by covalent SiO4 tetrahedra. [1,2] The ionic layers contain kagomé lattices of magnetic Fe2+ cations (octahedral crystal field, high-spin [HS] d6, S = 2) with perfect trigonal symmetry; while the intermetallic layers are non-magnetic because the Fe2+ is in the low-spin (S = 0) state. The formula is more correctly written as Fe4Si2Sn7O16 to differentiate the one LS-Fe2+ per formula unit in the intermetallic layer from the three HS-Fe2+ per formula unit in the kagomé oxide layer. Fe4Si2Sn7O16 also has a unique magnetic ground state below a Néel ordering temperature TN = 3.5 K, in which the spins on 2/3 of the Fe2+ sites in the kagomé oxide layers order antiferromagnetically, while 1/3 remain disordered and fluctuating down to at least 0.1 K. [3] The nature and origin of this unique “striped” partial spin-liquid state is unclear. The fact that it breaks trigonal symmetry, which the more conventional q = 0 or √3×√3 kagomé states would not, raises the possibility that the anisotropic distribution of the 6 unpaired spins on HS-Fe2+ (t2g4eg2) plays a role. To test this possibility, we have now synthesised an isotropic analogue with a kagomé lattice of HS Mn2+ (t2g3eg2), by co-substituting Ge4+ for Si4+ in the bridging/stannite layers to match the lattice dimensions between layers. We found that FeMn3Ge2Sn7O16 has the same “striped” magnetic ground state as Fe4Si2Sn7O16, in the same temperature range, ruling out this explanation. However, the zero-field striped structure is collinear for FeMn3Ge2Sn7O16 vs. non-collinear for Fe4Si2Sn7O16, which may indeed be a consequence of the change in anisotropy on the magnetic kagomé site, and suggests that FeMn3Ge2Sn7O16 is an even more ideal spin-liquid candidate than Fe4Si2Sn7O16. We also found that an external applied magnetic field lifts the degeneracy on the disordered site, giving rise to another ordered magnetic structure never before observed nor predicted on a kagomé lattice.
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    Low pressure synchrotron x-ray powder diffraction of Cu5-xMxSbO6 (M=Cr, Mn, W)
    (Australian Institute of Physics, 2016-02-04) Wilson, DJ; Söhnel, T; Smith, KL; Brand, HEA; Ulrich, C; Graham, PJ; Chang, FF; Allison, MC; Vyborna, NH
    The large crystallographic and chemical diversity of copper-based metal oxides is one of their highlighting features and cause for pursuit into copper based material research. An interesting feature seen in copper based metal oxides is the coexistence of different copper oxidation states, in different crystallographic positions, within the same compound. This can lead to a mixture of magnetically active Cu2+ and magnetically inactive Cu1+ within the same compound, with different structural motifs. One interesting compound that demonstrates this coexistence of mixed copper oxidation states is Cu5SbO6, which crystallises in a modified delafossite structure type (CuFeO2). Here, the magnetically active brucite-like CuO2 layer was diluted in an ordered fashion with non-magnetic Sb5+. These layers were separated by linearly coordinated, magnetically inactive Cu1+. Rietveld refinements on a range of preparation temperatures revealed a low-temperature (LT) and high-temperature modification (HT) phase transition. This is related to an ordering (HT)/disordering (LT) effect of the Sb5+/Cu2+ brucite-like layers between the Cu1+ ions. Substituting the Cu2+ or Sb5+ in the layers with other transition metals (Cr, Mn, W) could present interesting changes to the properties of the material, and potentially influence the ordered/disordered stacking of the layers. By using solid-state Raman spectroscopy, we could show that this structure displayed a pressure-induced phase transition at room temperature for the ordered modification, which was not observed for the disordered modification. Lowering the pressure from ambient down to 20 mbar showed phonon modes at about 700 cm-1 and 550 cm-1 disappeared almost completely. Neutron powder diffraction experiments were conducted at atmospheric and low pressure on both ordered and disordered modifications. On analysis of the neutron diffraction patterns, we could show a very small shift in the reflections, and thus changes in the unit cell parameters, for the ordered modification, while these shifts were not observed for the disordered modification. These shifts should also be observed in synchrotron powder diffraction patterns. Therefore, we investigated the nature of this phase transition with variable pressure synchrotron X-ray powder diffraction.
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    Striped magnetic ground state of the ideal kagomé lattice compound Fe4Si2Sn7O16
    (Society of Crystallographers in Australia and New Zealand, 2017-12-03) Ling, CD; Allison, MC; Schmid, S; Avdeev, M; Gardner, JS; Ryan, DH; Soehnel, T
    We have used representational symmetry analysis of neutron powder diffraction data to determine the magnetic ground state of Fe4Si2Sn7O16. We recently reported a long-range antiferromagnetic (AFM) Néel ordering transition in this compound at TN = 3.0 K, based on magnetisation measurements [1]. The only magnetic ions present are layers of high-spin Fe2+ (d6, S = 2) arranged on a perfect kagomé lattice (trigonal space group P-3m1). Below TN = 3.0 K, the spins on 2/3 of these magnetic ions order into canted antiferromagnetic chains, separated by the remaining 1/3 which are geometrically frustrated and show no long-range order down to at least T = 0.1 K [2]. Moessbauer spectroscopy shows that there is no static order on the latter 1/3 of the magnetic ions — i.e., they are in a liquid-like rather than a frozen state – down to at least 1.65 K. A heavily Mn-doped sample Fe1.45Mn2.55Si2Sn7O16 has the same ground state. Although the magnetic propagation vector k = (0, ½, ½) breaks hexagonal symmetry, we see no evidence for magnetostriction in the form of a lattice distortion within the resolution of our data. To the best of our knowledge, this type of magnetic order on a kagomé lattice has no precedent experimentally and has not been explicitly predicted theoretically. We will discuss the relationship between our experimental result and a number of theoretical models that predict symmetry-breaking ground states for perfect kagomé lattices.
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    Striped magnetic ground state of the kagome lattice in Fe4Si2Sn7O16
    (American Physical Society, 2017-11-15) Ling, CD; Allison, MC; Schmid, S; Avdeev, M; Gardner, JS; Wang, CW; Ryan, DH; Zbiri, M; Söhnel, T
    We have experimentally identified a different magnetic ground state for the kagome lattice, in the perfectly hexagonal Fe2+ (3d6,S=2) compound Fe4Si2Sn7O16. A representational symmetry analysis of neutron diffraction data shows that below TN=3.5 K, the spins on 23 of the magnetic ions order into canted antiferromagnetic chains, separated by the remaining 13 which are geometrically frustrated and show no long-range order down to at least T=0.1 K. Mössbauer spectroscopy confirms that there is no static order on the latter 13 of the magnetic ions—i.e., they are in a liquidlike rather than a frozen state—down to at least 1.65 K. A heavily Mn-doped sample Fe1.45Mn2.55Si2Sn7O16 has the same magnetic structure. Although the propagation vector q=(0,12,12) breaks hexagonal symmetry, we see no evidence for magnetostriction in the form of a lattice distortion within the resolution of our data. We discuss the relationship to partially frustrated magnetic order on the pyrochlore lattice of Gd2Ti2O7, and to theoretical models that predict symmetry breaking ground states for perfect kagome lattices. ©2017 American Physical Society
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    Striped magnetic ground state on an ideal S = 2 Kagomé lattice
    (International Union of Crystallography, 2017) Ling, CD; Allison, MC; Schmid, S; Avdeev, M; Ryan, DH; Soehnel, T
    We have used representational symmetry analysis of neutron powder diffraction data to determine the magnetic ground state of Fe4Si2Sn7O16. We recently reported a long-range antiferromagnetic (AFM) Néel ordering transition in this compound at TN = 3.0 K, based on magnetization measurements. [1] The only magnetic ions present are layers of high-spin Fe2+ (d6, S = 2) arranged on a perfect kagomé lattice (trigonal space group P-3m1). [2] Below TN = 3.0 K, the spins on 2/3 of these magnetic ions order into canted antiferromagnetic chains, separated by the remaining 1/3 which are geometrically frustrated and show no long-range ordered down to at least T = 0.1 K. Moessbauer spectroscopy shows that there is no static order on the latter 1/3 of the magnetic ions – i.e., they are in a liquid-like rather than a frozen state – down to at least 1.65 K. A heavily Mn-doped sample Fe1.45Mn2.55Si2Sn7O16 has the same ground state. Although the magnetic propagation vector k = (0, 1/2, 1/2) breaks hexagonal symmetry, we see no evidence for magnetostriction in the form of a lattice distortion within the resolution of our data. To the best of our knowledge, this type of magnetic order on a kagomé lattice has no precedent experimentally and has not been explicitly predicted theoretically. We will discuss the relationship between our experimental result and a number of theoretical models that predict symmetry breaking ground states for perfect kagomé lattices. © International Union of Crystallography
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    Structure and magnetic properties of the AB3Si2Sn7O16 layered oxides
    (Australian Institute of Physics, 2017-02-03) Allison, MC; Ling, CD; Schmid, S; Avdeev, M; Stuart, G; Söhnel, T
    Layered transition metal compounds with geometrically frustrated architectures are widely studied due to the novel effects that arise in a material where lattice geometry prevents the formation of a stable low temperature magnetic ground state in which all interactions between electron spins are satisfied. The parent compound for this study, Fe4Si2Sn7O16, provides a novel situation in oxide compounds. It can be described as a layered composite of oxygen linked (FeSn6) octahedra (the stannide layer) and (FeO6)/(SnO6) octahedra with a kagomé topology (the oxide layer). These layers are separated by SiO4 tetrahedra and the divalent iron in both layers appear to highly substitutionally liable, this combination of features therefore provides a rare opportunity to study a new series of materials with two discrete magnetically frustrated lattices (triangular and kagomé). To date, we have studied the changes in structure as iron is systematically replaced in the structure with iridium, ruthenium, cobalt and/or manganese. Refinements of the X-ray and neutron powder diffraction data show that each transition metal has strong preferences for either the stannide or oxide layer positions dependent upon ionic size and electronic configuration. In this presentation we will show the current results of our studies on the structure, electronic configuration and magnetic properties.
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    Synthesis, structure and geometrically frustrated magnetism of the layered oxide-stannide compounds Fe(Fe3−xMnx)Si2Sn7O16
    (Royal Society of Chemistry, 2016-05-23) Allison, MC; Avdeev, M; Schmid, S; Liu, S; Söhnel, T; Ling, CD
    Fe4Si2Sn7O16 has a unique crystal structure that contains alternating layers of Fe2+ ions octahedrally coordinated by O (oxide layer) and Sn (stannide layer), bridged by SiO4 tetrahedra. The formula can be written as FeFe3Si2Sn7O16 to emphasise the distinction between the layers. Here, we report the changes in structure and properties as iron is selectively replaced by manganese in the oxide layer. Solid-state synthesis was used to produce polycrystalline samples of Fe(Fe3−xMnx)Si2Sn7O16 for x ≤ 2.55, the structures of which were characterised using high-resolution synchrotron X-ray and neutron powder diffraction. Single-crystal samples were also grown at x = 0.35, 0.95, 2.60 and characterised by single crystal X-ray diffraction. We show that manganese is doped exclusively into the oxide layer, and that this layer contains exclusively magnetically active high-spin M2+ transition metal cations; while the stannide layer only accommodates non-magnetic low-spin Fe2+. All samples show clear evidence of geometrically frustrated magnetism, which we associate with the fact that the topology of the high-spin M2+ ions in the oxide layer describes a perfect kagomé lattice. Despite this frustration, the x = 0 and x = 2.55 samples undergo long-range antiferromagnetic ordering transitions at 3.0 K and 2.5 K, respectively. © The Royal Society of Chemistry 2016 - CC BY 3.0