Browsing by Author "Zhou, Q"
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- ItemChemInform abstract: structural studies of the disorder and phase transitions in the double perovskite Sr2YTaO6(Wiley, 2010-10-07) Zhou, Q; Kennedy, BJ; Avdeev, MThe evolution of the crystal structure of the title compound is monitored from room temperature to 1250 °C by powder neutron and synchrotron XRD. At 50 °C Sr2YTaO6 crystallizes with a monoclinic superstructure in the space group P21/n, at 1000 °C in the tetragonal space group I4/m, and at 1200 °C in the cubic space group Fm equation image m. The tilting of the octahedra which exists in the room temperature structure is progressively lost upon heating. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
- ItemCorrigendum to “Synthesis, structures, and phase transitions of barium bismuth iridium oxide perovskites Ba2BiIrO6 and Ba3BiIr2O9” [J. Solid State Chem. 183 (2010) 727–735](Elsevier, 2010-11-01) Ling, CD; Kennedy, BJ; Zhou, Q; Spencer, JR; Avdeev, MThe authors wish to draw the reader’s attention to an unfortunate series of typographical errors in this article. The space group of Ba2BiIrO6 at room temperature was incorrectly referred to throughout the text as R3 , The correct space group is R3c . which was used for all the Rietveld-refinements and to generate all the relevant tables and figures. © 2010 Elsevier Inc.
- ItemCrystal structures and phase transitions in Sr2InTaO6 perovskite(Springer, 2012-10-07) Zhou, Q; Kennedy, BJ; Avdeev, MThe preparation and crystal structure of the double perovskite oxide Sr2InTaO6 are reported. This oxide has a monoclinic structure in space group P21/n at room temperature, where In and Ta display a rock-salt type ordering with a = 5.73356(10), b = 5.74052(10), c = 8.10905(14) Å and β = 90.022(6)°. Variable temperature neutron diffraction measurements demonstrate this displays the sequence of phase transitions P21/n⟶605∘CI2/m⟶705∘CI4/m⟶930∘CFm3ˉm as a consequence of the sequential loss of tilting of the corner shared octahedra upon heating. The evolution of Sr2InTaO6 crystal structure upon heating is analysed and described in terms of symmetry-adapted distortion modes. The GM4+ and X3+, that are responsible for anti-phase and in-phase tilting, respectively, are highly temperature dependent, with the GM4+ mode having the largest amplitude at room temperature. © 2013, Springer.
- ItemDiffraction and spectroscopic study of pyrochlores Bi2−xFe1+xSbO7(Elsevier, 2014-03-15) Zhou, Q; Blanchard, PER; Kennedy, BJ; Ling, CD; Liu, S; Avdeev, M; Aitken, JB; Tadich, A; Brand, HEAThe structural and electronic properties of the series Bi2−xFe1+xSbO7 (0 ⩽ x ⩽ 0.6) were investigated using a combination of diffraction and spectroscopy. Synchrotron and neutron diffraction analysis show that Fe3+ cations substitute for Bi3+ onto the A site with increasing x, which was further confirmed by analysis of the Fe K/L-edge X-ray absorption near-edge spectra. The diffraction analysis indicated the presence of displacive disorder along the A2O chains, likely the result of the Bi3+ 6s2 lone pair, as well as non-Vegard-like behaviour of the lattice parameters in the Fe-poor region. Fe K-edge extended X-ray absorption fine-structure analysis of Bi2FeSbO7 confirmed the displacive disorder of the Bi3+ cations as well as Sb5+ and Fe3+ disorder on the B site. © 2013 Elsevier B.V.
- ItemThe ferroelectric phase of CdTiO3: a powder neutron diffraction study(Elsevier, 2011-11-01) Kennedy, BJ; Zhou, Q; Avdeev, MThe synthesis of bulk samples of polycrystalline CdTiO(3) in both the rhombohedral ilmenite and orthorhombic perovskite forms is described and the structures of these have been refined using powder neutron diffraction data. This involved the preparation of samples enriched in (114)Cd. Cooling perovskite-type CdTiO(3) to 4 K induces a ferroelectric phase transition, with the neutron data suggesting the low temperature structure is in Pna2(1). Mode analysis shows the polar mode to be dominant at low temperatures. The ilmenite-structure of CdTiO(3) is compared with that of ZnTiO(3). The refined scattering length of the (114)Cd is estimated to be 5.56 fm. Attempts to dope CdTiO(3) with Ca and Sr are described. (C) 2011 Elsevier Inc.
- ItemGiant magnetoelastic effect at the opening of a spin-gap in Ba3BiIr2O9(American Chemical Society, 2012-01-26) Miiller, W; Avdeev, M; Zhou, Q; Kennedy, BJ; Sharma, N; Kutteh, R; Kearley, GJ; Schmid, S; Knight, KS; Blanchard, PER; Ling, CDAs compared to 3d (first-row) transition metals, the 4d and 5d transition metals have much more diffuse valence orbitals. Quantum cooperative phenomena that arise due to changes in the way these orbitals overlap and interact, such as magnetoelasticity, are correspondingly rare in 4d and 5d compounds. Here, we show that the 6H-perovskite Ba3BiIr2O9, which contains 5d Ir4+ (S = 1/2) dimerized into isolated face-sharing Ir2O9 bioctahedra, exhibits a giant magnetoelastic effect, the largest of any known 5d compound, associated with the opening of a spin-gap at T* = 74 K. The resulting first-order transition is characterized by a remarkable 4% increase in Ir–Ir distance and 1% negative thermal volume expansion. The transition is driven by a dramatic change in the interactions among Ir 5d orbitals, and represents a crossover between two very different, competing, ground states: one that optimizes direct Ir–Ir bonding (at high temperature), and one that optimizes Ir–O–Ir magnetic superexchange (at low temperature). © 2012 American Chemical 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, Z; Kennedy, BJ; Liu, S; Miiller, W; Reynolds, EM; Zhou, Q; Avdeev, M; Zhang, Z; 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.
- ItemMulti-scale structural analysis of the A-site and oxygen deficient perovskite Sr11Mo4O23(Royal Society of Chemistry, 2017-08-28) King, G; Avdeev, M; Qasim, I; Zhou, Q; Kennedy, BJThe long range average crystal structure, as well as the short and medium range structural features, of the A-site deficient and oxygen deficient perovskite Sr11Mo4O23 have been determined. Rietveld refinement of synchrotron X-ray and neutron powder diffraction data show that this compound is cubic with space group Fd[3 with combining macron]m and a lattice parameter of a = 16.4108 Å. These findings contradict earlier reports of a tetragonal crystal structure. Sr11Mo4O23 appears to be isostructural with Ba11W4O23, except that the disordered coordination environment around one of the Mo sites seems to be a mixture of octahedral and square pyramidal instead of octahedral and tetrahedral. The short and medium range structural features have been inspected using the neutron pair distribution function (PDF). Short range correlations between the oxygen polyhedra surrounding the Mo(2) atom exist to avoid short O–O contacts. A model has been constructed which contains such correlations and is verified by reverse Monte Carlo (RMC) modeling of the PDF. The RMC refinements also give the distribution of inter-atomic distances in this compound which reveals how the various atomic positions are correlated and over what length scales. These results are important for understanding the ionic conduction pathways.© Royal Society of Chemistry 2017
- ItemNeutron diffraction studies of the ferroelectric phase of CdTiO3(International Union of Crystallography, 2011-08-30) Zhou, Q; Kennedy, BJ; Avdeev, MCadmium titanate (CdTiO3) is relatively poorly studied due to the toxicity of cadmium and difficulties in obtaining pure CdTiO3 since it has only a moderate stability with respect to the oxides. CdTiO3 can be synthesised with either an ilmenite or perovskite type structure. The ilmenite-like phase of CdTiO3 is unstable at high temperatures and undergoes an irreversible reconstructive phase transition to the perovskite phase near 900 °C. The perovskite phase decomposes, through the loss of Cd, if heated above 1000 °C. In recent years, there has been growing interest developing thin films of cadmium for a variety of uses including as a photocatalyst. The precise structure of the perovskite phase of CdTiO3 is uncertain. This is a consequence of the combination of its ferroelectric properties and the subtleties in the various octahedral tilting schemes observed for perovskites. A ferroelectric structure for CdTiO3 at room temperature in Pc21n, and a non-polar in Pbnm have been reported. Studies showed that CdTiO3 undergoes a displacive ferroelectric phase transition at about 80 K, with X-ray analysis suggesting the low temperature phase is in Pn21a or P21ma while the room temperature paraelectric phase is in Pbnm. In the present work we have used high resolution neutron diffraction methods to refine the structure of the hree phases of CdTiO3, namely the paraelectric ilmenite and perovskite phases and the ferroelectric perovskite phase. It is expected that neutron diffraction will provide a more accurate and precise description of these structures compared with X-ray diffraction methods due to the presence of the heavy Cd cations. To circumvent the high neutron absorption cross section of naturally occurring Cd we used samples enriched in 114Cd. Cooling perovskite-type CdTiO3 to 4 K induces a ferroelectric phase transition, with the neutron data suggesting the low temperature structure is in Pna21 (Figure 1). Solid solutions of the type Cd1-xCaxTiO3 could be prepared. Invariably this required the use of relatively high temperatures resulting in the formation of perovskite-type oxides and we did not find any evidence to suggest appreciable amounts of Ca could be incorporated into the ilmenite type CdTiO3 structure. Interestingly we could not prepare solid solutions of the type Cd1-xSrxTiO3 using conventional methods. There are only 5% of Sr and 5% of Ca can be doped in CdTiO3 in the solid solution of CaxSrxCd1-xTiO3. This is somewhat remarkable given the relative ease with which oxides of the type Ca1-xSrxTiO3 can be prepared and suggests the A-O bonding is playing a significant, but poorly understood role in stabilising the oxides. There is ample evidence that altering the A-cation significantly alters the hybridisation between the B-site metal t2g d states and the O p π orbitals © International Union of Crystallography.
- ItemPressure-induced intersite Bi--M (M=Ru, Ir) valence transitions in hexagonal perovskite(Wiley Online Library, 2014-02-24) Huang, Z; Auckett, JE; Blanchard, PER; Kennedy, BJ; Miller, W; Zhou, Q; Avdeev, M; Johnson, MR; Zbiri, M; Garbarino, G; Marshall, WG; Gu, QF; Ling, CDPressure-induced charge transfer from Bi to Ir/Ru is observed in the hexagonal perovskites Ba3+nBiM2+nO9+3n (n=0,1; M=Ir,Ru). These compounds show first-order, circa 1 % volume contractions at room temperature above 5 GPa, which are due to the large reduction in the effective ionic radius of Bi when the 6s shell is emptied on oxidation, compared to the relatively negligible effect of reduction on the radii of Ir or Ru. They are the first such transitions involving 4d and 5d compounds, and they double the total number of cases known. Ab initio calculations suggest that magnetic interactions through very short (ca. 2.6 Å) M[BOND]M bonds contribute to the finely balanced nature of their electronic states. © 2014 Wiley‐VCH.
- ItemSpin-gap opening accompanied by a strong magnetoelastic response in the S=1 magnetic dimer system Ba3BiRu2O9(American Physical Society, 2011-12-15) Miiller, W; Avdeev, M; Zhou, Q; Studer, AJ; Kennedy, BJ; Kearley, GJ; Ling, CDNeutron diffraction, magnetization, resistivity, and heat-capacity measurements on the 6H-perovskite Ba3BiRu2O9 reveal simultaneous magnetic and structural dimerization driven by strong magnetoelastic coupling. An isostructural but strongly displacive first-order transition on cooling through T∗=176 K is associated with a change in the nature of direct Ru–Ru bonds within Ru2O9 face-sharing octahedra. Above T∗, Ba3BiRu2O9 is an S=1 magnetic dimer system with intradimer exchange interactions J0/kB=320 K and interdimer exchange interactions J′/kB=−160 K. Below T∗, a spin-gapped state emerges with Δ≈220 K. Ab initio calculations confirm antiferromagnetic exchange within dimers, but the transition is not accompanied by long-range magnetic order. ©2011 American Physical Society
- ItemStructural and magnetic studies of the electron doped manganites Sr 0.65 Pr 0.35− x Ce x MnO 3 (0.00 ≤ x ≤ 0.35)(IOP Publishing, 2013-07-13) Heyraud, S; Blanchard, PER; Liu, S; Zhou, Q; Kennedy, BJ; Brand, HEA; Tadich, A; Hester, JRThe nuclear and magnetic structures and properties of Sr0.65Pr0.35−xCexMnO3 (0.00 ≤ x ≤ 0.35) were investigated using a combination of synchrotron x-ray and neutron powder diffraction, along with magnetic and x-ray absorption near edge structure measurements. At room temperature, doping with Ce results in a transition from a tetragonal structure in I4/mcm to an orthorhombic one in Imma associated with the loss of long range orbital ordering. At low temperatures, we observe the formation of an orthorhombic Fmmm phase. XANES measurements demonstrate that the Ce exists as a mixture of Ce3+ and Ce4+. © 2013, IOP Publishing Ltd.
- ItemStructural studies of the phases in Ba2LaIrO6 - new light on an old problem(Elsevier, 2009-11) Zhou, Q; Kennedy, BJ; Avdeev, M; Giachini, L; Kimpton, JAAccurate and precise structures for the cation-ordered double perovskite Ba2LaIrO6 at a number of temperatures are presented. Careful analysis of the synchrotron X-ray and neutron diffraction profiles of Ba2LaIrO6 suggests that, at room temperature, this is monoclinic in space group I2/m. Heating the sample to 375 K results in a transition to a rhombohedral phase in R3¯ with further heating resulting in the loss of the tilting of the octahedra, the structure being in Fm3¯m. Cooling the sample induces an additional phase transition to a triclinic structure in I1¯. Details of the various structures are presented. © 2009, Elsevier Ltd.
- ItemTuning the giant magnetoelastic transition in Ba3BiIr2O9 and Ba3BiRu2O9(IOP Science, 2014-06-17) Huang, Z; Avdeev, M; Kennedy, BJ; Knight, KS; Zhou, Q; Ling, CDWe have experimentally investigated the effects of pressure on the magnetoelastic transitions associated with the opening of spin-gaps in Ba3BiIr2O9 and Ba3BiRu2O9. For both compounds, reducing the unit cell volume by either external physical and internal chemical pressure was found to reduce the temperature T* of the transition and, to a lesser extent, the magnitude of the associated negative thermal volume expansion. The results yield the latent heat associated with the transitions, −3.34(3) × 102 J mol−1 for Ba3BiIr2O9 and −7.1(5) × 102 J mol−1 for Ba3BiRu2O9. The transition in Ba3BiRu2O9 is significantly more robust than in Ba3BiIr2O9, requiring an order of magnitude higher pressures to achieve the same reduction in T*. The differing responses of the two compounds points to differences between the 4d and 5d metals and hence to the importance of spin-orbit coupling, which is expected to be much stronger in the Ir compound. © 2014, IOP Publishing Ltd.
- ItemValence changes of manganese and structural phase transitions in Sr1-xPrxMnO3 (0.1 <= x <= 0.6)(Elsevier, 2013-05-01) Tan, TY; Martin, N; Zhou, Q; Kennedy, BJ; Gu, QF; Kimpton, JA; Zhang, Z; Jang, LYFifteen perovskite manganites Sr1−xPrxMnO3, x=0.1−0.6 in steps as fine as 0.025, have been synthesized by solid state methods and their crystal structures determined using synchrotron X-ray powder diffraction. At room temperature two first order phase transitions associated with changes in the orbital ordering and the tilting of the corner shared MnO6 octahedra are evident cubic/Pm3¯m⟹x≈0.15tetragonal/(I4/mcm)⟹x≈0.45orthorhombic/Imma, with two phase regions associated with both transitions. Heating the Pr rich oxide Sr0.5Pr0.5MnO3 provides a rare example where chemical pressure and temperature result in a different sequence of transitions Imma –R3¯c–Pm3¯m, reflecting the importance of the Pr 4f electrons. X-ray absorption near-edge structure (XANES) measurements at the Mn K and Pr L3 edges show the valence state of the Mn linearly decreases with increasing Pr content whereas the Pr remains +3 across the entire series. Magnetic susceptibility measurements for selected samples are presented. © 2013, Elsevier Inc.