Browsing by Author "Qasim, I"

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    Crystallographic studies on Ru and Ir-based SrB1-xMxO3-type Perovskites
    (Australian Institute of Nuclear Science and Engineering, 2012-11-12) Qasim, I; Kennedy, BJ; Avdeev, M
    Strontium ruthenate and iridate perovskites are of great interest due to their potential technological importance related to their exotic physical properties. Despite the chemical simplicity of the perovskite structure there are a number of examples where the precise structure is unknown or where different researchers have proposed different structures for the sample material. Understanding the relationship between the structure and physical properties is a significant barrier to the development of these types of materials. Two series of oxides of the type SrRu1-xBx03 and Srlr1-xBxO3- (Β = transition metals) have been synthesized using solid state methods, and selected members of these have been structurally characterized using combination of synchrotron X-ray and neutron diffraction methods and their physical properties investigated. Neutron data were critical to establish precise and accurate structures of a number of these materials including Sr2FeIr06 SrRu0.8Ni0.2O3 and Srlr0.8Ni0.203.
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    Impact of Cu doping on the structure and electronic properties of LaCr1–yCuyO3
    (ACS Publications, 2014-01-31) Qasim, I; Blanchard, PER; Liu, S; Kennedy, BJ; Avdeev, M
    Oxides of the type LaCr1–yCuyO3 have been prepared using solid-state methods and their crystal structures refined using synchrotron X-ray powder diffraction. The solubility limit of Cu was found to be around y = 0.2, and such oxides are orthorhombic in space group Pbnm. X-ray absorption spectroscopy measurements at the Cr and Cu L-edges demonstrated that the Cr remains trivalent upon Cu doping, with the Cu being present as Cu(III). The oxides are found to be antiferromagnets, and the Néel temperature, TN, decreases as the Cu content is increased. The crystal and magnetic structures of one example La(Cr0.85Cu0.15)O3 have been investigated between 3 and 350 K by neutron powder diffraction. The samples are semiconductors. © 2014, American Chemical Society.
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    Multi-scale structural analysis of the A-site and oxygen deficient perovskite Sr11Mo4O23
    (Royal Society of Chemistry, 2017-08-28) King, G; Avdeev, M; Qasim, I; Zhou, Q; Kennedy, BJ
    The long range average crystal structure, as well as the short and medium range structural features, of the A-site deficient and oxygen deficient perovskite Sr11Mo4O23 have been determined. Rietveld refinement of synchrotron X-ray and neutron powder diffraction data show that this compound is cubic with space group Fd[3 with combining macron]m and a lattice parameter of a = 16.4108 Å. These findings contradict earlier reports of a tetragonal crystal structure. Sr11Mo4O23 appears to be isostructural with Ba11W4O23, except that the disordered coordination environment around one of the Mo sites seems to be a mixture of octahedral and square pyramidal instead of octahedral and tetrahedral. The short and medium range structural features have been inspected using the neutron pair distribution function (PDF). Short range correlations between the oxygen polyhedra surrounding the Mo(2) atom exist to avoid short O–O contacts. A model has been constructed which contains such correlations and is verified by reverse Monte Carlo (RMC) modeling of the PDF. The RMC refinements also give the distribution of inter-atomic distances in this compound which reveals how the various atomic positions are correlated and over what length scales. These results are important for understanding the ionic conduction pathways.© Royal Society of Chemistry 2017
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    Ordered vs. disordered perovskites; structural studies of Fe-doped SrIrO3 and SrRuO3
    (Academic Press Inc Elsevier Science, 2013-10-01) Qasim, I; Blanchard, PER; Liu, S; Tang, CG; Kennedy, BJ; Avdeev, M; Kimpton, JA
    The structures of the two Fe containing perovskites Sr2IrFeO6 and SrRu0.5Fe0.5O3 have been established using a combination of synchrotron and neutron diffraction methods. Sr2IrFeO6 and SrRu0.5Fe0.5O3 are shown to be monoclinic I2/m and tetragonal I4/mcm respectively The former exhibits a rock-salt like ordering of the Fe and Ir cations and displays a sequence of phase transitions associated with the loss of the octahedral tilts upon heating; 12 / m → I 4 / m → F m 3 ¯ m . The Fe and Ru cations are disordered in SrRu0.5Fe0.5O3 and this shows a single structural phase transition upon heating due to the loss of the in-phase tilts, viz. I 4 / m c m → P m 3 ¯ m . In both cases XANES measurements show partial oxidation of the Fe3+ to Fe4+. The difference in the structures between the two is remarkable given the similar size of Ir5+ and Ru5+, and this is reflected in their magnetic properties. © 2013, Elsevier Ltd.
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    Stabilising the orthorhombic perovskite structure in SrIrO3 through chemical doping. Synthesis, structure and magnetic properties of SrIr1−xMgxO3(0.20 ≤ x ≤ 0.33)
    (Royal Society of Chemistry, 2013-01-01) Qasim, I; Kennedy, BJ; Avdeev, M
    The synthesis, structure, magnetic and electric conductivity properties of Mg doped SrIrO3 are described. Doping SrIrO3 with Mg favours an orthorhombic structure over the hexagonal face-sharing structure. Polycrystalline samples of the type SrIr1-xMgxO3 with an orthorhombic perovskite-type structure have been formed for 0.2 < x < 0.33. At x = 0.2 the formal oxidation state of the Ir is +4.5 and the material displays metallic conductivity. At x = 0.33 the Ir is in the +5 state and the material is non-metallic. Magnetic susceptibility measurements demonstrate the materials to be spin-glasses in which short range ferromagnetic interactions are evident.© 2013, Royal Society of Chemistry
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    Structural and magnetic properties of Ni doped SrRuO3
    (Society of Crystallographers in Australia and New Zealand, 2011-04-30) Qasim, I; Kennedy, BJ; Avdeev, M
    SrRuO3 has been the subject of numerous studies since it is the only ferromagentic metal among 4d transition metal oxide. It has a Curie Temperature T ~ 165K. It has been established that the Curie Temperature is sensitive to chemical substitution and there are reports that Ru-site substitution with 3d metals including Mn, Co, Ni, Cu, Zn significantly decrease T. Detailed structural studies have been reported for Mn and Cu doped oxides and these show a transition to a tetragonal structure as a consequence of orbital ordering associated with the Jahn-Teller cation. Little is know of the analogous Ni doped materials.
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    Synthesis, structures and properties of transition metal doped SrIrO3
    (Royal Society of Chemistry, 2013-01-10) Qasim, I; Kennedy, BJ; Avdeev, M
    The synthesis of some transition metal SrIr1-xMxO3 type oxides is described. These have an orthorhombic perovskite type-structure and are only found when the formal oxidation state of the Ir is approximately equal to +4.5 and for M = Fe, Co, Ni and Zn. Occupancy of the TM eg type orbitals appears to contribute to the break-down of the dimeric motif seen in the 6H-SrIrO3 structure. Variable temperature resistivity measurements show that SrIr0.8Zn0.2O3 is a metal; SrIr0.8Ni0.2O3 an non-metal and SrIr0.8Co0.2O3 undergoes a temperature dependent metal-insulator transition. © 2013, Royal Society of Chemistry
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    X-ray absorption near edge structure and crystallographic studies of the mixed valence oxide SrRu(0.8)Ni(0.2)O(3)
    (IOP Publishing LTD, 2011-10-13) Qasim, I; Kennedy, BJ; Zhang, Z; Avdeev, M; Jang, LY
    Neutron diffraction methods are used to refine the structure of SrRu(0.8)Ni(0.2)O(3) at room temperature and 4 K. X-ray absorption measurements at the Ru L-edge demonstrate that partial oxidation of the Ru(4+) to Ru(5+) occurs upon introducing Ni into the SrRuO(3) structure to form SrRu(0.8)Ni(0.2)O(3). Whilst the diffraction measurements show that SrRu(0.8)Ni(0.2)O(3) is isostructural with SrRuO(3), the Ni doped compound exhibits an unusual first order orthorhombic-cubic phase transition near 670 K as revealed by x-ray diffraction. The Curie temperature in SrRu(0.8)Ni(0.2)O(3) is lower than that found in SrRuO(3) as a consequence of local distortions reducing the p-d hybridization. (c) 2011 IOP Publishing LTD