Browsing by Author "Świerczek, K"

Now showing 1 - 2 of 2
  • No Thumbnail Available
    Item
    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.
  • No Thumbnail Available
    Item
    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.