Browsing by Author "Zhou, QD"
Now showing 1 - 18 of 18
Results Per Page
Sort Options
- ItemChemical, 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, LYThe 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
- ItemComposition- and temperature-dependent phase transitions in 1:3 ordered perovskites Ba4-xSrxNaSb3O12(Elsevier, 2007-11) Zhou, QD; Kennedy, BJ; Elcombe, MM; Withers, RLA series of 25 members of the 1:3 ordered perovskite family of the type Ba4-xSrxNaSb3O12 has been synthesized and their structures determined using synchrotron X-ray and neutron powder diffraction techniques. At room temperature the sample Ba4NaSb3O12 has a cubic structure in space group Im (3) over barm with a = 8.2821(1) angstrom, where the Na and Sb cations are ordered in the octahedral sites but there is no tilting of the (Na/Sb)O-6 octahedra. As the average size of the A-site cation decreases, through the progressive replacement of Ba by Sr, tilting of the octahedra is introduced firstly lowering the symmetry to tetragonal in P4/mnc then to orthorhombic in Cmca and ultimately a monoclinic structure in P2(1)/n as seen for Sr4NaSb3O12 with a = 8.0960(2) angstrom, b = 8.0926(2) angstrom, c = 8.1003(1) angstrom and β = 90.016(2)degrees. The powder neutron diffraction studies show that the orthorhombic and tetragonal phases in Cmca and P4/mnc co-exist at room temperature for samples with x between 1.5 and 2. © 2007, Elsevier Ltd.
- ItemCrystal structure of Ln(1/3)NbO(3) (Ln = Nd, Pr) and phase transition in Nd1/3NbO3(Elsevier, 2007-06) Zhang, Z; Howard, CJ; Kennedy, BJ; Knight, KS; Zhou, QDThe crystal structure of the A-site deficient perovskite Ln1/3NbO3 (Ln=Nd, Pr) at room temperature has been determined, for the first time, as orthorhombic in space group Cmmm using high-resolution neutron powder diffraction. Pertinent features are the alternation of unoccupied layers of A-sites and layers partly occupied by Ln cations, as well as out-of-phase tilting of the NbO6 octahedra around an axis perpendicular to the direction of the cation/vacancy ordering. The phase transition behaviour of Nd1/3NbO3 has also been studied in situ. This compound undergoes a continuous phase transition at around 650°C to a tetragonal structure in space group P4/mmm due to the disappearance of the octahedral tilting. The analysis of spontaneous strains shows that this phase transition is tricritical in nature. © 2007, Elsevier Ltd.
- ItemCrystal structures and phase transitions in a-site deficient perovskites Ln1/3TaO3(American Chemical Society, 2008-11-11) Zhou, QD; Saines, PJ; Sharma, N; Ting, J; Kennedy, BJ; Zhang, Z; Withers, RL; Wallwork, KSThe synthesis and structures of the perovskites Ln1/3TaO3 are described. As the size of the Ln cation is reduced, the compounds display a sequence of structure: P4/mmm/La → Cmmm/Ce−Gd → Pmma/Tb, Dy → Pmc21/Ho, Er. Although apparently tetragonal in P4/mmm, electron diffraction patterns of Tm1/3TaO3 reveal this has a complex incommensurate structure. Likewise Gd1/3TaO3 appears metrically tetragonal, but electron diffraction and synchrotron X-ray powder diffraction demonstrate this is actually orthorhombic. The suppression of the spontaneous orthorhombic strain in Gd1/3TaO3 is thought to be due to the proximity to the first-order Cmmm−Pmma transition. Variable temperature studies show both Tb1/3TaO3 and Dy1/3TaO3 undergo a first-order Cmmm−Pmma transition upon heating. © 2008, American Chemical Society
- ItemCrystal structures and phase transitions in Sr doped Ba2InTaO6 perovskites(Academic Press INC Elsevier Science, 2013-10-01) Zhou, QD; Tan, TY; Kennedy, BJ; Hester, JRThe crystal structures of the double perovskites of the type Ba2-xSrxInTaO6 have been studied by synchrotron X-ray and neutron powder diffraction methods. The structures are based on ordered array of the tilted InO6 and TaO6 octahedra, with the Ba/Sr cations being completely disordered. The structure evolves from cubic to monoclinic Fm (3) over barm(x similar to 0) (6)-> I4/m(x similar to 1.2)-> I2/m(x similar to 1.6)-> P2(1)/n. The changes in the symmetry induced by replacing the Ba with Sr have been identified by a symmetry-mode analysis. Variable temperature S-XRD studies of selected examples have shown that heating the samples induces the same sequence of transitions observed in the composition dependent studies. © 2013, Elsevier Ltd.
- ItemImpact of Jahn-Teller active Mn3+ on strain effects and phase transitions in Sr0.65Pr0.35MnO3(American Physical Society, 2012-03-19) Tan, TY; Kennedy, BJ; Zhou, QD; Ling, CD; Miiller, W; Howard, CJ; Carpenter, MA; Knight, KSThe mixed-valence manganite Sr(0.65)Pr(0.35)MnO(3) has been prepared and its crystal and magnetic structure investigated between 7 and 1200 K using high-resolution powder neutron diffraction. The structural and lattice parameter data have been used to determine the octahedral tilting and spontaneous strains associated with the structural, electronic, and magnetic phase transitions. At room temperature, the structure is tetragonal and is characterized by cooperative out-of-phase tilts of the MnO(6) octahedra about the c axis and a large Jahn-Teller-type distortion due to the presence of Mn(3+). The sample exhibits a reversible phase transition from the cubic Pm3m perovskite to a tetragonal I4/mcm structure at 750 K. The Pm (3) over barm <-> I4/mcm phase transition is continuous, and the tetragonal strain, which is dominated by the Jahn-Teller-type distortion of the MnO(6) octahedra, exhibits an unusual e(0.5)(t)alpha(T(c)-T) temperature dependence. At low temperatures, a C-type antiferromagnetic structure develops with a Neel temperature T(N) of 250 K. TheMnmagnetic moment at 7 K is 2.99(2) mu(B)/Mn. The magnetic ordering introduces additional tetragonal strain, and this strain shows the expected quadratic dependence on the magnetic moment at low temperatures. An increase in the octahedral tilt angle at TN demonstrates an effective coupling between the magnetic ordering process and octahedral tilting. © 2012, American Physical Society
- ItemKey role of Bismuth in the magnetoelastic transitions of Ba3BiIr2O9 and Ba3BiRu2O9 as revealed by chemical doping(America Chemical Society, 2013-12-24) Blanchard, PER; Huang, ZX; Kennedy, BJ; Liu, S; Miiller, W; Reynolds, EM; Zhou, QD; Avdeev, M; Zhang, ZM; Aitken, JB; Cowie, BCC; Jang, LY; Tan, TT; Li, S; Ling, CDThe key role played by bismuth in an average intermediate oxidation state in the magnetoelastic spin-gap compounds Ba3BiRu2O9 and Ba3BiIr2O9 has been confirmed by systematically replacing bismuth with La3+ and Ce4+. Through a combination of powder diffraction (neutron and synchrotron), X-ray absorption spectroscopy, and magnetic properties measurements, we show that Ru/Ir cations in Ba3BiRu2O9 and Ba3BiIr2O9 have oxidation states between +4 and +4.5, suggesting that Bi cations exist in an unusual average oxidation state intermediate between the conventional +3 and +5 states (which is confirmed by the Bi L3-edge spectrum of Ba3BiRu2O9). Precise measurements of lattice parameters from synchrotron diffraction are consistent with the presence of intermediate oxidation state bismuth cations throughout the doping ranges. We find that relatively small amounts of doping (∼10 at%) on the bismuth site suppress and then completely eliminate the sharp structural and magnetic transitions observed in pure Ba3BiRu2O9 and Ba3BiIr2O9, strongly suggesting that the unstable electronic state of bismuth plays a critical role in the behavior of these materials. © 2013 American Chemical Society.
- ItemNeutron diffraction studies of Gd(2)Zr(2)O(7) pyrochlore(Elsevier, 2011-07-01) Kennedy, BJ; Zhou, QD; Avdeev, MThe structure of Gd(2)Zr(2)O(7) pyrochlore over the temperature range 4-300 K has been refined from powder neutron diffraction data. The sample was enriched in (160)Gd to avoid the high neutron absorption of naturally occurring Gd. The diffraction pattern showed well resolved superlattice reflections indicative of the pyrochlore structure and no evidence is found for anion-disorder from the structural refinements. (C) 2011 Elsevier Inc.
- ItemNeutron powder diffraction studies of Ca2−xSrxCoWO6 double perovskites(Elsevier, 2006-11-15) Zhou, QD; Kennedy, BJ; Elcombe, MMNeutron powder diffraction data has been used to determine precise structures for four oxides in the series Ca2−xSrxCoWO6 (x=1, 1.5, 1.7, 2). The studies demonstrated that the symmetry increases with Sr content as a consequence of the gradual reduction, and ultimately removal, of the in-phase tilting of the BO6 octahedra. The sample of Sr2CoWO6 was a mixture of monoclinic (P21/n) and tetragonal (I4/m) phases at room temperature but was purely monoclinic at 20 K. Crown Copyright © 2006 Published by Elsevier B.V.
- ItemNeutron powder diffraction studies of Ca2−xSrxCoWO6 double perovskites(The Bragg Institute, Australian Nuclear Science and Technology Organisation, 2005-11-27) Zhou, QD; Kennedy, BJ; Elcombe, MMA series of double perovskite compounds of A2-xSrxCoWO6 (A = Ca, Ba) were synthesized and the room- and variable-temperature structural phase transitions have been studied by synchrotron and neutron powder diffraction techniques. These studies demonstrated that the symmetry increases as the average size of the A-site cation increases. These transitions are associated with the gradual reduction and ultimately removal of the octahedral tiles of the BO6 octahedra. Temperature dependent structural studies have been undertaken for selected samples. The transition to cubic is continuous in the three Ca doped samples studied as a function of temperature, Ca2-xSrxCoWO6 x = 1.8, 1.7, 1.6, however in each case analysis of the spontaneous strain shows the transition to be tricritical rather than second order in nature. Where observed the temperature induced P21/n to I4/m transition is first order as required by symmetry. © 2005 The Authors
- ItemStructural and electronic phase transitions in Sr1-xCexMnO3 perovskites(Elsevier, 2008-10-01) Kennedy, BJ; Saines, PJ; Zhou, QD; Zhang, Z; Matsuda, M; Miyake, MThe structures of eight members of the series Sr1-xCexMnO3 with 0.075≤x≤0.4 have been established using synchrotron X-ray powder diffraction. Unusual peak broadening due to domainwall scattering is observed near the composition-induced first-order cubic–tetragonal transition. The temperature dependence of the structures for three examples x = 0.1, 0.2 and 0.35 is described. For the x = 0.1 and 0.35 samples evidence for a first-order loss of the orbital ordering associated with a cooperative Jahn–Teller distortion is presented. The onset of the orbitally disordered state is accompanied by a discontinuous reduction in cell volume. © 2008, Elsevier Ltd.
- ItemStructural characterisation of the perovskite series Sr0.9-xCaxCe0.1MnO3: influence of the Jahn-Teller effect(Elsevier, 2009-04) Kennedy, BJ; Ting, J; Zhou, QD; Zhang, Z; Matsuda, M; Miyake, MFifteen perovskite-type compounds Sr0.9−xCaxCe0.1MnO3, x=0–0.9 in steps as fine as 0.05, have been synthesised by solid state methods, and the room temperature structures characterised using X-ray synchrotron powder diffraction. At low Ca contents (x≤0.45) the structures are tetragonal in space group I4/mcm and at high Ca contents (x≥0.55) the compounds are orthorhombic in space group Pbnm. At room temperature these two phases co-exist in the compound with x=0.5. XANES measurements show the Ce to be present as Ce4+ in all the oxides. High temperature structures are reported for selected members. © 2009, Elsevier Ltd.
- ItemStructural studies of the disorder and phase transitions in the double perovskite Sr2YTaO6(Elsevier, 2010-08) Zhou, QD; Kennedy, BJ; Avdeev, MThe evolution of the crystal structure of the double perovskite Sr2YTaO6 from room temperature to 1250°C has been studied using powder neutron and synchrotron X-ray diffraction. At room temperature Sr2YTaO6 crystallises in a monoclinic superstructure with the space group P21In. The tilting of the octahedra evident in the room temperature structure is progressively lost on heating, resulting in a sequence of phase transitions that ultimately yields the cubic structure in space group Fm3m. The high temperature tetragonal and cubic phases are characterised by strongly anisotropic displacements of the anions. The amount of defects in the crystal structure of Sr2YTaO6 is found to be sensitive to the preparative method. © 2010, Elsevier Ltd.
- ItemStructure 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, LYThe 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
- ItemSynthesis, structural and magnetic studies of the double perovskites Ba2CeMO6 (M = Ta, Nb)(American Chemical Society, 2012-07-16) Zhou, QD; Blanchard, PER; Kennedy, BJ; Reynolds, EM; Zhang, Z; Miiller, W; Aitken, JB; Avdeev, M; Jang, LY; Kimpton, JATwo Ce(3+) containing double perovskites Ba(2)CeMO(6) (M = Nb and Ta) have been prepared through the use of mildly reducing conditions, and the Ce valence state has been shown to be +3 through Ce L-edge X-ray absorption measurements. Both oxides adopt a monoclinic structure in I2/m at room temperature and undergo two phase transitions upon heating, a discontinuous I2/m -> R (3) over bar and a continuous R (3) over bar -> Fm (3) over barm transition. Analysis of the first order I2/m -> R (3) over bar transitions is impeded by the complex peak shapes and diffuse scattering evident in the synchrotron powder diffraction data because of domain wall effects. © 2012, American Chemical Society.
- ItemSynthesis, structures, and phase transitions of barium bismuth iridium oxide perovskites Ba2BiIrO6 and Ba3BiIr2O9(Elsevier, 2010-03) Ling, CD; Kennedy, BJ; Zhou, QD; Spencer, JR; Avdeev, MThe Ba-Bi-Ir-O system is found to contain two distinct perovskite-type phases a rock-salt ordered double perovskite Ba2BiIrO6, and a 6H-type hexagonal perovskite Ba3BiIr2O9 Ba2BiIrO6 undergoes a series of symmetry-lowering phase transitions on cooling Fm (3) over barm -> R (3) over barc -> 12/m(C2/m)->/(1) over bar (P (1) over bar), all of which are second order except the rhombohedral monoclinic one, which is first order The monoclinic phase is only observed in a 2-phase rhombohedral+monoclinic regime. The transition and 2-phase region lie very close to 300K, making the room-temperature X-ray diffraction patterns extremely complex and potentially explaining why Ba2BiIrO6 had not previously been identified and reported A solid solution Ba2Bi1+xIr1-xO6, analogous to Ba2Bi1+xRu1-xO6, 0 <= x <= 2/3, was not observed The 6H-type phase Ba3BiIr2O9 undergoes a clean second-order phase transition P6(3)/mmc -> C2/c at 750K, unlike 6H-type Ba3LaIr2O9, the P6(3)/mmc structure of which is highly strained below similar to 750K but fails to distort coherently to the monoclinic phase. © 2010, Elsevier Ltd.
- ItemThermal expansion behaviour in the oxygen deficient perovskites Sr2BSbO5.5 B=Ca, Sr, Ba. Competing effects of water and oxygen ordering(Elsevier, 2011-09-01) Zhou, QD; Kennedy, BJ; Avdeev, MNeutron diffractions studies reveal the presence of oxygen disorder in the oxygen deficient perovskites Sr(2)BSbO(5.5) (B=Ca, Sr, Ba). Synchrotron X-ray studies demonstrate that these oxides have a double perovskite:type structure with the cell size increasing as the size of the B cation increases from 8.2114(2) angstrom for B=Ca to 8.4408(1) angstrom for B=Ba. It is postulated that a combination of local clustering of the anions and vacancies together with water-water and water-host hydrogen bonds plays a role in defining the volume of the encapsulated water clusters and that changes in the local structure upon heating result in anomalous thermal expansion observed in variable temperature diffraction measurements. (C) 2011 Elsevier Inc.
- ItemX-ray absorption near edge structure and crystallographic studies of the mixed valence oxides CaRu1-xMnxO3(American Chemical Society, 2009-09-22) Zhou, QD; Kennedy, BJ; Zhang, Z; Jang, LY; Aitken, JBEleven members of the series CaRu1-xMnxO3 With 0 <= x <= 1 have been prepared and characterized using a combination of Mn K-edge and Ru L-3,L-2-edge X-ray absorption near edge structure spectroscopy and high resolution synchrotron X-ray powder diffraction. A complex distribution of four Mn3+/4+ and Ru4+/5+ oxidation states was observed and quantified. When incorporated into CaRuO3 at low doping levels, the Mn is present as Mn3+ and Mn4+ with the nominal Mn oxidation state increasing with x (from 3.6 at x = 0.1 to 4 at x = 1). The Ru exists predominantly as Ru5+ in CaMnO3 with the nominal Ru valency as 4.8 at x = 0.9. The formation of Mn3+/4+-Ru4+/5+ redox pairs accounts for the unusual chemical composition dependence of the cell volume, evident from the diffraction studies. The coexistence of mixed valence redox pairs is also believed to contribute to the phase separation observed at 0.1 <= x <= 0.6. © 2009, American Chemical Society