Naveed Zafar Ali

Max Planck Institute for Solid State Research, Stuttgart, Baden-Württemberg, Germany

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Publications (14)27.19 Total impact

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    ABSTRACT: We have investigated the structural and magnetic transitions in CsCoO$_2$ using calorimetric measurements, neutron powder diffraction (NPD), density functional theory (DFT) calculations and muon-spin relaxation ($\mu$SR) measurements. CsCoO$_2$ exhibits three-dimensional long-range antiferromagnetic (AFM) order at 424~K, resulting in antiferromagnetic alignment of chains of ferromagnetically ordered Co-Co spin dimers. Although there is no change in magnetic structure around a structural transition at $T^{*}=100$~K, the resulting bifurcation of corner-shared Co--O--Co bond angles causes a weakening of the AFM interaction for one set of bonds along the chains. Consequently, the system undergoes a complex freezing out of relaxation processes on cooling.
    Preview · Article · Oct 2014 · Physical Review B
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    ABSTRACT: We present the results of muon-spin relaxation measurements on the triangular lattice Heisenberg antiferromagnet $\alpha$-KCrO$_{2}$. We observe sharp changes in behaviour at an ordering temperature of $T_{\mathrm{c}}=23$ K, with an additional broad feature in the muon-spin relaxation rate evident at T=13 K, both of which correspond to features in the magnetic contribution to the heat capacity. This behaviour is distinct from both the Li- and Na- containing members of the series. These data may be qualitatively described with the established theoretical predictions for the underlying spin system.
    Preview · Article · Jul 2013 · Physical Review B
  • Naveed Zafar Ali · Juergen Nuss · Martin Jansen
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    ABSTRACT: Dark green single crystals of β-KCrO2 are obtained by further annealing of α-KCrO2 (Ag crucible, 773 K, 200 h) which is synthesized by the azide/nitrate route from stoichiometric mixtures of KN3, KNO3, and Cr2O3 (Ag crucible, 853 K, 5 h).
    No preview · Article · Apr 2013 · ChemInform
  • Naveed Zafar Ali · Jürgen Nuss · Martin Jansen
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    ABSTRACT: α-KCrO2 was synthesized by the azide/nitrate route. Stoichiometric mixtures of the precursors (KN3, KNO3, and Cr2O3) were heated in a special regime up to 853 K and annealed at this temperature for 5 h in a specially designed silver container. Dark green single crystals of β-KCrO2 were grown by post annealing the reaction product at 773 K for 200 h in silver crucibles. According to single-crystal X-ray analysis, the new high temperature modification, β-KCrO2, crystallizes in the β-RbScO2 type of structure [P63/mmc; a = 3.0427(4) Å, c = 11.924(2) Å; c/a = 3.918; Z = 2]. The characteristic feature of the structure are CdCl2 analogue slabs of [CrO6/3]– edge shared octahedra stacked along the [001] direction with K+ atoms incorporated between the layers in a trigonal prismatic arrangement, rare for lighter alkali atoms. We have reinvestigated the specific heat (from 2 to 300 K) and the magnetic susceptibility (2 to 350 K) on α-KCrO2, and clearly identified in the specific heat measurements, a λ-type anomaly at 23 K, indicating the onset of long-range AFM ordering. The Curie-Weiss fit of experimental data in the temperature range above 100 K, yields the negative Curie-Weiss temperature of –220 K, indicating predominant antiferromagnetic spin-exchange interaction, with an effective magnetic moment (μeff) of the order of 3.91 μB, corresponding to a g-factor [μeff /μB = g[S(S+1)]1/2] of 2.019 well in accordance for Cr3+ (d3 system) with spin S = 3/2.
    No preview · Article · Feb 2013 · Zeitschrift für anorganische und allgemeine Chemie
  • Naveed Zafar Ali · Jürgen Nuss · Reinhard K Kremer · Martin Jansen
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    ABSTRACT: CsCoO(2) has been prepared along the azide/nitrate route as a reddish black microcrystalline powder at 833 K. According to single crystal X-ray analysis, the title compound crystallizes as a structure type novel for oxides (Cmca,Z = 8, a = 5.9841(1) Å, b = 12.2458(2) Å, c = 8.2650(1) Å). The prominent features of the structure are pairs of edge-linked CoO(4) tetrahedra to form Co(2)O(6) dimers, which are condensed by vertex sharing of the four remaining unshared oxygen atoms to form a two-dimensional architecture of a puckered polyoxyanion spreading along (010). Upon cooling, CsCoO(2) undergoes a virtually second order phase transition at ∼100 K leading to a systematic dovetail twin (C2/c). The magnetic susceptibilities show the dominance of antiferromagnetic interactions with a remarkably high Néel temperature of 430 K indicating a very strong antiferromagnetic superexchange between the Co(3+) ions. The spin-exchange for CsCoO(2) can be addressed as a set of strongly antiferromagnetically coupled quasi-one-dimensional chains, which are weakly coupled to neighboring chains by ferromagnetic interaction.
    No preview · Article · Oct 2012 · Inorganic Chemistry
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    Naveed Zafar Ali · Jesko Sirker · Jürgen Nuss · Peter Horsch · Martin Jansen
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    ABSTRACT: Na5Cu3O6, a new member of one dimensional charge ordered chain cuprates, was synthesized via the azide/nitrate route by reacting NaN3, NaNO3 and CuO. According to single crystal X-ray analysis, one dimensional CuO2 chains built up from planar, edge-sharing CuO4 squares are a dominant feature of the crystal structure. From the analysis of the Cu-O bond lengths we find that the system forms a Wigner lattice. The commensurate charge order allows to explicitly assign the valence states of either +2 or +3 to each copper atom resulting in a repetition according to Cu(2+)-Cu(3+)-Cu(2+)-Cu(2+)-Cu(3+)-Cu(2+). Following the theoretical analysis of the previously synthesized compounds Na3Cu2O4 and Na8Cu5O10, the magnetic susceptibility was expected to show a large dimer gap. Surprisingly, this is not the case. To resolve this puzzle, we show that the magnetic couplings in this compound are strongly affected by excitations across the Wigner charge gap. By including these contributions, which are distinct from conventional superexchange in Mott-insulators, we obtain a quantitative satisfying theoretical description of the magnetic susceptibility data.
    Full-text · Article · Jul 2011 · Physical Review B
  • Naveed Zafar Ali · Juergen Nuss · Martin Jansen
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    ABSTRACT: Dark yellow single crystals of the title compound are prepared by heating stoichiometric mixtures of CuO, KN3, KNO3, and K2CO3 followed by annealing at 833 K for 50 h.
    No preview · Article · Apr 2011 · ChemInform
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    ABSTRACT: Air and moisture sensitive K5[CuO2][CO3] was prepared via the azide/nitrate route from stoichiometric mixtures of the precursors CuO, KN3, KNO3 and K2CO3. According to the single-crystal X-ray analysis of the crystal structure [P4/nbm, Z = 2, a = 7.4067(5), c = 8.8764(8) Å, R1 = 0.053, 433 independent reflections] K5[CuO2][CO3] represents an ordered superstructure of Na5[NiO2][CO3]. The structure contains isolated [CuO2]3– dumbbells and CO32– anions, with the latter not connected to the transition element. Raman spectroscopic measurements confirm the presence of CO32– in the structure.
    No preview · Article · Feb 2011 · Zeitschrift für anorganische und allgemeine Chemie
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    Naveed Zafar Ali
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    ABSTRACT: The main focus of the research work accomplished was to prepare new ternary alkalioxometallates of the first-row transition-metal elements containing, providing highly defined materials showing interesting structural and physical properties. ‘Azide nitrate Route’ was employed successfully for the complete synthesis, and overall 11 modifications of alkali oxometallates were characterized alongwith in depth study of their structural property relationships. Out of these, nine compounds were unknown. For all the compounds the crystal structure has been determined from single crystal data (07) and rest from powder data using state of the art techniques (High resolution synchrotron, neutron and advance D8-Bruker diffractometers). The "azide/nitrate route" has proven to be an efficient approach in present work for the solid state synthesis of new magnetic oxides covering the whole dimensionality, starting from 3-dimensional oxoferrates, two dimensional oxochromate and oxocobaltate respectively, and also new intrinsically doped one dimensional cuprates. We observed and illustrated, how by changing the dimensionality of a magnetic lattice, influence dramatically the thermodynamic properties by concerted study of the specific heat and magnetic susceptibility behaviour of the magnetic system particularly at low temperature. Later in order to construct a model for the spin interactions in these alkali oxometallates, the intrinsic magnetic properties of total 10 compounds have also been measured. The resulting magnetic properties were later justifiably analysed by employing various theoretical models. Thermal analysis (DSC, low and high temperature specific heat) bulk and electrical conductivity measurements, Moessbauer spectroscopy, High-resolution synchrotron powder diffractometry, neutron diffraction experiments were performed to study the structure property relation in detail and to validate the authenticity of results received.
    Preview · Article · Jan 2011
  • D Sheptyakov · N Z Ali · M Jansen
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    ABSTRACT: In continuation of our recent x-ray study of the structural phase transitions in the AFeO2 (A = K, Rb, Cs) family, we have systematically investigated the respective structural and magnetic phase transitions by neutron powder diffraction. While the temperatures of the first-order structural phase transitions are strongly different for the three compounds (~1003, ~ 737 and ~ 350 K for A = K, Rb, Cs) and systematically decrease with increasing ionic radius of the A-cation, the magnetic transition temperatures in all three compounds have been found to be almost the same—slightly above 1000 K. The magnetic ordering type is similar in all three compounds—antiferromagnetic ordering of magnetic Fe3 + ions within the system of the three-dimensional Fe–O–Fe linkages such that the Fe–Fe exchange between the nearest neighboring ions is always antiferromagnetic. The directions of magnetic Fe moments were found to be parallel to the crystallographic axis c in RbFeO2 and CsFeO2 and parallel to the axis b in KFeO2 in notations of their low-temperature orthorhombic modifications.
    No preview · Article · Oct 2010 · Journal of Physics Condensed Matter
  • Naveed Zafar Ali · Juergen Nuss · Denis Sheptyakov · Martin Jansen
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    ABSTRACT: Structures and phase transitions for the isostructural series of compounds KFeO2, RbFeO2 and CsFeO2 have been systematically studied by synchrotron X-ray high resolution powder diffraction experiments and in case of CsFeO2 also by single crystal diffractometry. At room temperature, all of the three compounds crystallize in the orthorhombic (Pbca) KGaO2 type of structure consisting of a three dimensional network of corner-sharing [FeO4/2]− tetrahedra, which at elevated temperatures shows a reversible phase transformation to a cubic structure (space group Fd3¯m). For KFeO2, RbFeO2 and CsFeO2 this phase transformation takes place at 1003 K, 737 K and 350 K respectively, as confirmed by differential scanning calorimetry and X-ray diffraction. Upon heating through the transitions the major structural changes are driven by the onset or enhancement of librational motion of the FeO4 tetrahedra. Due to this phenomenon the Fe–O–Fe bonds appear to step-wise getting straight, seemingly approaching 180° within the time and space averaged structure.
    No preview · Article · Jun 2010 · ChemInform
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    ABSTRACT: The first order phase transition of CsFeO 2 was investigated using synchrotron powder diffraction data as a function of temperature. Two alternative approaches were used to describe the deviation of the framework crystal structure relative to the high-symmetry parent structure: symmetry (a.k.a. distortion) modes and polyhedral-tilt parameters. In both cases, the relevant parameters were refined as a function of temperature using the method of parametric Rietveld refinement. We demonstrate a semi-automated and generally applicable method for the determination of spontaneous lattice strain variations, order parameters and power-law exponents as derived from Landau theory.
    No preview · Article · May 2010 · Materials Science Forum
  • Naveed Zafar Ali · Juergen Nuss · Denis Sheptyakov · Martin Jansen
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    ABSTRACT: Structures and phase transitions for the isostructural series of compounds KFeO{sub 2}, RbFeO{sub 2} and CsFeO{sub 2} have been systematically studied by synchrotron X-ray high resolution powder diffraction experiments and in case of CsFeO{sub 2} also by single crystal diffractometry. At room temperature, all of the three compounds crystallize in the orthorhombic (Pbca) KGaO{sub 2} type of structure consisting of a three dimensional network of corner-sharing [FeO{sub 4/2}]{sup -} tetrahedra, which at elevated temperatures shows a reversible phase transformation to a cubic structure (space group Fd3-barm). For KFeO{sub 2}, RbFeO{sub 2} and CsFeO{sub 2} this phase transformation takes place at 1003 K, 737 K and 350 K respectively, as confirmed by differential scanning calorimetry and X-ray diffraction. Upon heating through the transitions the major structural changes are driven by the onset or enhancement of librational motion of the FeO{sub 4} tetrahedra. Due to this phenomenon the Fe-O-Fe bonds appear to step-wise getting straight, seemingly approaching 180 deg. within the time and space averaged structure. - Graphical abstract: Reciprocal layer hk0 for the orthorhombic room temperature phase, with pseudo cubic symmetry due to twinning, and the cubic high temperature phase of CsFeO{sub 2}. Upon heating the major structural changes are driven by the enhancement of librational motion of the [FeO{sub 4/2}]{sup -} corner-sharing tetrahedra, a phenomenon related to the high/low phase transition of cristobalite.
    No preview · Article · Mar 2010 · Journal of Solid State Chemistry
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    Jürgen Nuss · Naveed Zafar Ali · Martin Jansen
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    ABSTRACT: RbFeO(2), rubidium oxoferrate, crystallizes as an orthorhombic structure. The crystals under investigation were composed of six individuals representing reticular pseudomerohedral twins with a pseudocubic twin lattice of index 4 because of the approximate equations a(8)(1/2) = b(2)(1/2) = c. The compound is isostructural with KFeO(2) (KGaO(2) type of structure) and is composed of [FeO(4/2)](-) corner-sharing tetrahedra, forming a three-dimensional cristobalite-like network, with Rb(+) ions occupying its interstices.
    Preview · Article · Nov 2007 · Acta Crystallographica Section B Structural Science
  • Jürgen Nuss · Naveed Zafar Ali · Martin Jansen

    No preview · Article · Jan 2007