Browsing by Author "Kolesnik, S"

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    Enhancement of the Curie temperature in NdBaCo2O5.5 by a-site Ca substitution
    (Americal Physical Society, 2012-08-24) Kolesnik, S; Dabrowski, B; Chmaissem, O; Avci, S; Hodges, JP; Avdeev, M; Świerczek, K
    Layered cobaltites RBaCo2O5.5 (R = rare earth and Y) undergo a sequence of magnetic and electronic transitions between antiferromagnetic/ferrimagnetic/paramagnetic and insulating/paramagnetic metallic states with respective transition temperatures T-N (similar to 230-260 K) < T-C (similar to 250-290 K) < T-MI (similar to 360 K). We have synthesized a Nd1-xCaxBaCo2O5.5 series (0 <= x <= 0.2) of cation-[(Nd,Ca)/Ba]-ordered and oxygen vacancy-ordered materials and investigated them by neutron diffraction, magnetization, and electronic and thermal transport. We observe that upon Ca doping, T-N is decreasing to 0 for x = 0.1 and T-C is increasing and coincides with T-MI for x > 0.12, which weakly changes with Ca substitution from similar to 360 to similar to 340 K. This is the largest enhancement of T-C observed for these cobaltites. Unlike hole doping by adding oxygen, Ca doping does not disrupt the cation and oxygen vacancy orderings up to x = 0.20. © 2012, American Physical Society.
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    Structural and magnetic properties of Nd1-xCaxBaCo2O5.5
    (American Physical Society, 2012-03-02) Chmaissem, O; Kolesnik, S; Dabrowski, B; Avci, S; Avdeev, M; Hodges, JP
    R1-xAxBaCo2O5.5 (R = rare earth, A = alkaline metal) is a relatively new class of complex oxide materials that exhibit a wide range of magnetic attributes in addition to metal/insulator switching properties, structural transitions and superstructure order parameters. In many ways, this family exhibits behaviors similar to those of the famous colossal magnetoresistive manganites; however, more complex properties have also been identified owing to the fact that the oxidation state of the cobalt ions often behave in unpredictable ways depending on the chemical composition of the investigated material and the corresponding Co local environment. Thus, Co^3+ and Co^4+ ions with high, intermediate and low spin states may be produced offering an additional degree of freedom to be accounted for when designing new materials with tunable magnetic properties. In this talk, I will discuss the effects of calcium substitution at the Nd sites and the various structural and magnetic models as determined by neutron powder diffraction and complementary magnetic measurements. © 2021 American Physical Society
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    Structural and physical properties of Re substituted B-site ordered and disordered SrCo1−xRexO3−δ (x=0.1, 0.25, 0.5)
    (Elsevier, 2012-02) Baszczuk, A; Dabrowski, B; Kolesnik, S; Chmaissem, O; Avdeev, M
    Synthesis conditions, structural, magnetic and transport properties have been studied for SrCo1−xRexO3−δ samples with x=0.1, 0.25, 0.5. SrCo0.9Re0.1O3−δ forms in air and remains stable on cooling indicating that small amounts of Re7+ stabilize a B-site disordered Pm-3m phase inhibiting formation of a hexagonal phase observed for SrCoO3−δ. After oxygenation SrCo0.9Re0.1O2.94 becomes ferromagnetic below 125 K and shows metallic-like conductivity with moderate magnetoresistance at low temperatures. Fully oxygenated double perovskite Sr2CoReO6 (x=0.5) forms under reducing conditions and is tetragonal at room temperature. A hysteretic transition to the antiferromagnetic state at low temperatures (∼50–60 K) is coupled with a drastic change of electronic and thermal properties. Contrary to previous reports [A. Nag et al., Chem. Mater. 20(13) (2008) 4420–4424] SrCo0.75Re0.25O3 is not a partially cation ordered Sr4Co3ReO12 phase, but a mixture of two structural and magnetic phases with disordered SrCo1−xdRexdO3 and ordered SrCo1−xoRexoO3 compositions where xd>0.1 and xo<0.5. © 2011 Elsevier Inc.
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    Structural, magnetic and electronic properties of LaNi0.5Fe0.5O3 in the temperature range 5-1000 K
    (Elsevier, 2008-08) Gateshki, M; Suescun, L; Kolesnik, S; Świerczek, K; Short, SM; Dabrowski, B; Mais, J
    The structure, magnetism, transport and thermal expansion of the perovskite oxide LaNi0.5Fe0.5O3 were studied over a wide range of temperatures. Neutron time-of-flight data have shown that this compound undergoes a first-order phase transition between similar to 275 and similar to 310K. The structure transforms from orthorhombic (Pbnm) at low temperatures to rhombohedral (R (3) over barc) above room temperature. This phase transition is the cause for the previously observed co-existence of phases at room temperature. The main structural modification associated with the phase transition is the change of tilting pattern of the octahedra from a(+)b(-)b(-) at low temperatures to a(-)a(-)a(-) at higher. Magnetic data strongly suggests that a spin-glass magnetic state exists in the sample below 83 K consistent with the absence of magnetic ordering peaks in the neutron data collected at 30K. At high temperatures the sample behaves as a small polaron electronic conductor with two regions of slightly different activation energies of 0.07 and 0.05 eV above and below 553 K, respectively. The dilatometric data show an average thermal expansion coefficient of 14.7 x 10(-6) K-1 which makes this material compatible with frequently used electrolytes in solid oxide fuel cells. © 2008, Elsevier Ltd.
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    Structural, magnetic, and oxygen storage properties of hexagonal Dy1−xYxMnO3+δ
    (Elsevier, 2014-09) Abughayada, C; Dabrowski, B; Avdeev, M; Kolesnik, S; Remsen, S; Chmaissem, O
    Single-phase polycrystalline samples of hexagonal Dy1−xYxMnO3+δ were synthesized over the full solubility range under reducing oxygen partial pressure conditions. Rietveld refinements using neutron powder diffraction data (NPD) confirmed the persistence of the parent hexagonal P63cm (Space group #185) structure for all the stoichiometric (δ=0) samples. The (Mn–O) bond lengths derived from NPD data were found to be consistent with non-degenerate Mn+3 high-spin state e“2 e′2 in trigonal–bipyramidal coordination. Detailed thermogravimetric measurements show large reversible oxygen storage/release capabilities (within the range of oxygen content 3.0–3.3) at unusually low temperatures of 200–400 °C in air or oxygen atmospheres. A phase transition to a new hexagonal phase (Hex1) was observed to take place around δ~0.29 for the oxygen-loaded Dy-rich samples (x<0.5). Analysis of x-ray and neutron diffraction data using Rietveld refinements demonstrate that the superstructure is formed by tripling the c-axis of the original P63cm unit cell. Modeling of the Hex1 superstructure converged to a structural symmetry consistent with the noncentrosymmetric space group R3c (#161). In magnetization measurements, a ferrimagnetic order for DyMnO3.0 was observed below 8 K. The dilution of Dy magnetism with nonmagnetic yttrium results in reducing both the transition temperature and the effective paramagnetic moment. © 2014 Elsevier Inc.