Naveed Zafar Ali

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

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Publications (10)14.8 Total impact

  • 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.
    Zeitschrift für anorganische und allgemeine Chemie 11/2012; 639(2):241-245. · 1.16 Impact Factor
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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.
    Inorganic Chemistry 10/2012; · 4.59 Impact Factor
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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.
    Physical Review B 07/2011; 84:035113. · 3.66 Impact Factor
  • 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.
    ChemInform 04/2011; 42(16).
  • NZ Ali, J Nuss, M Jansen
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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.
    Zeitschrift für anorganische und allgemeine Chemie 01/2011; 637(2):183-185. · 1.16 Impact Factor
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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.
    ChemInform 06/2010; 41(24).
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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.
    Journal of Solid State Chemistry 03/2010; 183(3). · 2.04 Impact Factor
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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.
    Acta Crystallographica Section B Structural Science 11/2007; 63(Pt 5):719-25. · 2.18 Impact Factor
  • Jürgen Nuss, Naveed Zafar Ali, Martin Jansen
    Acta Crystallographica Section B-structural Science - ACTA CRYSTALLOGR B-STRUCT SCI. 01/2007; 63(5):719-725.
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    Naveed Zafar Ali