Browsing by Author "Woodward, PM"

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    Chemical, magnetic and orbital order in the substituted double perovskite Sr(1-x)Ca(x)Mn0.5Ru0.5O3
    (American Physical Society, 2010-03) Ricciardo, RA; Woodward, PM; Cuthbert, HL; Zhou, QD; Kennedy, BJ; Zhang, Z; Avdeev, M; Jang, LY
    The structural and magnetic properties of Sr(1x)Ca(x)Mn0.5Ru0.5O3 have been investigated. SrMn0.5Ru0.5O3 exhibits antiferromagnetic ordering, TN ≈200 K. Neutron powder diffraction of this perovskite indicates the presence of orbital ordering of the occupied Mn+3 dz2 orbitals, stabilizing the spin ordering corresponding to the AFM C-type structure. The substitution of smaller Ca+2 for Sr+2 on the A-site induces a change in the octahedral tilt system, (a0a0c−) to (a−b+a−) forcing a loss of this type of orbital ordering for x ≥ 0.2. This is accompanied by a crossover to a ferromagnetic ground state with 200K ≤ TC ≤ 300K even in the absence of long range chemical order. Magnetic data and neutron powder diffraction of CaMn0.5Ru0.5O3 and Sr0.5Ca0.5Mn0.5Ru0.5O3 reveal reduced magnetic moments and imply a complex magnetic behavior as well as a complete loss of orbital ordering for CaMn0.5Ru0.5O3. X-ray absorption studies show increased electron transfer from Mn to Ru. © 2021 American Physical Society
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    Structural and magnetic properties of RTiNO2 (R=Ce, Pr, Nd) perovskite nitride oxides
    (Elsevier, 2015-03-01) Porter, SH; Huang, ZG; Cheng, ZX; Avdeev, M; Chen, ZX; Dou, SX; Woodward, PM
    Neutron powder diffraction indicates that CeTiNO2 and PrTiNO2 crystallize with orthorhombic Pnma symmetry (Ce: a=5.5580(5), b=7.8369(7), and c=5.5830(4) Å; Pr: a=5.5468(5), b=7.8142(5), and c=5.5514(5) Å) as a result of a–b+a– tilting of the titanium-centered octahedra. Careful examination of the NPD data, confirms the absence of long range anion order in both compounds, while apparent superstructure reflections seen in electron diffraction patterns provide evidence for short range anion order. Inverse magnetic susceptibility plots reveal that the RTiNO2 (R=Ce, Pr, Nd) compounds are paramagnetic with Weiss constants that vary from −28 to −42 K. Effective magnetic moments for RTiNO2 (R=Ce, Pr, Nd) are 2.43 μB, 3.63 μB, and 3.47 μB, respectively, in line with values expected for free rare-earth ions. Deviations from Curie–Weiss behavior that occur below 150 K for CeTiNO2 and below 30 K for NdTiNO2 are driven by magnetic anisotropy, spin–orbit coupling, and crystal field effects. © 2015 Elsevier Inc.
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    Structural, magnetic, and optical properties of A3V4(PO4)6 (A = Mg, Mn, Fe, Co, Ni)
    (American Chemical Society, 2016-05-26) Porter, SH; Xiong, J; Avdeev, M; Merz, D; Woodward, PM; Huang, Z
    Combined synchrotron and neutron powder diffraction indicates that A3V4(PO4)6 (A = Mg, Mn, Fe, Co, Ni) compounds crystallize with triclinic P1̅ symmetry. Lattice parameters expand as expected with successive increases in the ionic radius of the A2+ ion. Cation disorder on the octahedral sites increases as the ionic radii of A2+ ion decreases. Direct-current magnetic susceptibility measurements indicate that all compounds with magnetic A2+ ions order anti-ferromagnetically with transition temperatures ranging from 12 to 15 K. Effective magnetic moments for A3V4(PO4)6 (A = Mg, Mn, Fe, Co, Ni) are 5.16, 11.04, 10.08, 9.76, and 7.96 μB per formula unit, respectively, in line with calculated values for high-spin transition metal ions. With the exception of Co3V4(PO4)6 the ultraviolet–visible spectra are dominated by d–d transitions of the V3+ ions. The striking emerald green color of Co3V4(PO4)6 arises from the combined effects of d–d transitions involving both V3+ and Co2+. © 2016 American Chemical Society
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    Structure and properties of Sr1-xCaxMn0.5Ru0.5O3 perovskites: using chemical pressure to control Mn/Ru mixed valency
    (American Chemical Society, 2010-05-10) Ricciardo, RA; Cuthbert, HL; Woodward, PM; Zhou, QD; Kennedy, BJ; Zhang, Z; Avdeev, M; Jang, LY
    The structure and properties of Sr1−xCaxRu0.5Mn0.5O3 compositions have been investigated. Both bond distances and X-ray absorption measurements reveal Mn3+ + Ru5+ ↔ Mn4+ + Ru4+ mixed valency across the entire series. Despite a complete lack of Ru/Mn chemical order, all samples magnetically order between 220 and 300 K. The characteristics and type of magnetic order are sensitive to the occupation and ordering of the Mn eg orbitals, which can be manipulated by changes in chemical pressure, via the Sr/Ca ratio. Sr-rich samples are tetragonally distorted by a cooperative Jahn−Teller distortion (CJTD) that leads to an elongation of the c-axis as well as antiphase rotations of the octahedra about the c-axis (a0a0c− tilt system). The CJTD results from orbital ordering involving occupied dz2 orbitals on Mn3+, which stabilize C-type antiferromagnetic order. For Sr-rich samples, the various oxidation states contribute in approximately equal proportions (i.e., Ru+4.5 and Mn+3.5). Substituting Ca2+ for Sr2+ triggers additional rotations of the octahedra (a−b+a− tilt system) that result in orthorhombic symmetry for Sr1−xCaxMn0.5Ru0.5O3 samples with x ≥ 0.3. The crossover to orthorhombic symmetry is accompanied by the loss of orbital order and the emergence of an itinerant electron ferrimagnetic state. X-ray absorption near edge structure (XANES) measurements show that as the Ca2+ content increases there is a shift in the valence degeneracy toward Mn4+ + Ru4+. This helps to explain the absence of orbital ordering in Ca-rich samples, as well as the crossover from antiferromagnetism to ferrimagnetism. Neutron diffraction and ac susceptibility measurements show that CaRu0.5Mn0.5O3 undergoes magnetic phase separation into a disordered ferrimagnetic state (TC ≈ 230 K) and a G-type antiferromagnetic state (TN ≈ 95 K). © 2010, American Chemical Society