Browsing by Author "Blanchard, PER"
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- ItemA (3 + 3)-dimensional “hypercubic” oxide-ionic conductor: type ii bi2o3–nb2o5(ACS Publications, 2013-04-09) Ling, CD; Schmid, S; Blanchard, PER; Petříček, V; McIntyre, GJ; Sharma, N; Maljuk, A; Yaremchenko, AA; Kharton, VV; Gutmann, MJ; Withers, RLThe high-temperature cubic form of bismuth oxide, δ-Bi2O3, is the best intermediate-temperature oxide-ionic conductor known. The most elegant way of stabilizing δ-Bi2O3 to room temperature, while preserving a large part of its conductivity, is by doping with higher valent transition metals to create wide solid-solutions fields with exceedingly rare and complex (3 + 3)-dimensional incommensurately modulated ?hypercubic? structures. These materials remain poorly understood because no such structure has ever been quantitatively solved and refined, due to both the complexity of the problem and a lack of adequate experimental data. We have addressed this by growing a large (centimeter scale) crystal using a novel refluxing floating-zone method, collecting high-quality single-crystal neutron diffraction data, and treating its structure together with X-ray diffraction data within the superspace symmetry formalism. The structure can be understood as an ?inflated? pyrochlore, in which corner-connected NbO6 octahedral chains move smoothly apart to accommodate the solid solution. While some oxide vacancies are ordered into these chains, the rest are distributed throughout a continuous three-dimensional network of wide δ-Bi2O3-like channels, explaining the high oxide-ionic conductivity compared to commensurately modulated phases in the same pseudobinary system. © 2013, American Chemical Society.
- ItemAnion Disorder in Lanthanoid Zirconates Gd2-xTbxZr2O7(American Chemical Society., 2013-08-05) Reynolds, EM; Blanchard, PER; Kennedy, BJ; Ling, CD; Liu, S; Avdeev, M; Zhang, Z; Cuello, GJ; Tadich, A; Jang, LYThe pyrochlore?defect fluorite order?disorder transition has been studied for a series of oxides of the type Gd2?xTbxZr2O7 by a combination of diffraction and spectroscopy techniques. Synchrotron X-ray diffraction data suggest an abrupt transition from the coexistence of pyrochlore and defect fluorite phases to a single defect fluorite phase with increasing Tb content. However neutron diffraction data, obtained at ? ≈ 0.497 Å for all Gd-containing samples to minimize absorption, not only provide evidence for independent ordering of the anion and cation sublattices but also suggest that the disorder transition across the pyrochlore?defect fluorite boundary of Ln2Zr2O7 is rather gradual. Such disorder was also evident in X-ray absorption measurements at the Zr L3-edge, which showed a gradual increase in the effective coordination number of the Zr from near 6-coordinate in the pyrochlore rich samples to near 7-coordinate in the Tb rich defect fluorites. These results indicate the presence of ordered domains throughout the defect fluorite region, and demonstrate the gradual nature of the order?disorder transition across the Gd2?xTbxZr2O7 series. © 2013, American Chemical Society.
- ItemAnomalous thermal expansion in orthorhombic perovskite SrIrO3: Interplay between spin-orbit coupling and the crystal lattice(Americal Physical Society, 2014-06-16) Blanchard, PER; Reynolds, EM; Kennedy, BJ; Kimpton, JA; Avdeev, M; Belik, AAThe structure of the orthorhombic (Pbnm) polytype of SrIrO3 has been investigated between 3 and 1100 K using a combination of synchrotron and neutron diffraction methods. The orthorhombic structure persists to 1100 K, the highest temperature available in this work. This is a consequence of the larger than expected octahedral tilting estimated from the neutron diffraction studies. We postulate that the strong spin-orbit coupling of the Ir4+ cation, which splits the t2g band, introduces additional strain on the lattice. This introduces unusual thermal expansion of the cell. SrIrO3 was characterized by resistivity, magnetization, and specific heat measurements. Metallic conductivity was observed between 2 and 300 K without indication of the previously reported metal-insulator transition. The Sommerfeld constant γ was 3.12(2) mJ mol−1 K−2, and a Fermi-liquid behavior was observed between 2 and 30 K with positive magnetoresistence of up to 2% (at 70 kOe and between 2 and 50 K).© 2014, American Physical Society.
- ItemDesigning new n = 2 Sillen-Aurivillius phases by lattice-matched substitutions in the halide and [Bi2O2]2+ layer(Australian Institute of Physics, 2014-02-05) Liu, S; Blanchard, PER; Kennedy, BJ; Ling, CD; Avdeev, MThe chemical and structural flexibility of the perovskite structure, which makes it so ubiquitous in nature and useful in a range of technological applications, extends to layered variants such as Ruddlesden-Popper, Dion- Jacobson and Aurivillius phases. Multi-layered variants such as the Sillen-Aurivillius phases are related to Aurivillius phases by the insertion of an additional halide layer between every second [Bi2O2]2+ layer. Sillen-Aurivillius phases exist in various combinations of n number of perovskite layers and m halide layers. We have synthesised a new n = 2, m = 1 Sillen-Aurivillius compound Bi3Sr2Nb2O11Br based on Bi3Pb2Nb2O11Cl by simultaneously replacing Pb2+ with Sr2+ and Cl- with Br-. Rietveld refinements against X-ray and neutron powder diffraction data revealed a significant relative compression in the stacking axis, in contrary to the belief of inserting a significantly larger halide layer in the new compound. We could not stabilise other combinations such as Bi3Sr2Nb2O11Cl and Bi3Pb2Nb2O11Br due to inter-layer mismatch. Sr2+ doping reduces the impact of the stereochemically active 6s2 lone pair found on Bi3+/Pb2+ site, resulting in a contraction of the stacking axis by 1.22 % and an expansion of the a-b plane by 0.25 %, improving inter-layer compatibility with Br-. XANES analysis shows that the ferroelectric distortion of the B-site cation is less apparent in Bi3Sr2Nb2O11Br compared to Bi3Pb2Nb2O11Cl. Variable-temperature neutron diffraction data show no evidence for a ferroelectric distortion.
- ItemDesigning new n=2 Sillen-Aurivillius phases by lattice-matched substitutions in the halide and [Bi 2Oc2] 2+ layer(Australian Institute of Nuclear Science and Engineering (AINSE), 2013-12-02) Liu, S; Blanchard, PER; Avdeev, M; Kennedy, BJ; Ling, CDThe chemical and structural flexibility of the perovskite structure, which makes is so ubiquitous in nature and useful in a range of technological applications, extends to layered variants such as Ruddleston-Popper, Dion-Jacobsen and Aurivillius phases. Multi-layered variants such as the Sillen-Aurivillius phases are related to Aurivillius phases by the insertion of an additional halide layer between every second [Bi2 02 2+ layer. Sillen-Aurivillius phases exist in various AnXm combinations, where n is the number of perovskite layers A and m the number of halide layers X. We have synthesised a new n=2 Sillen-Aurivillius compound Bi3 Sr2 Nb2 O11 Br based on Bi3 Pb2 Nb2 O11 Cl by simultaneously replacing Pb2+ with Sr2+ and Cl - with Br -. Rietveld refinements against X-ray and neutron powder diffraction data revealed a significant relative compression in the stacking axis (c axis) in contrary to the belief of inserting a significantly larger halide layer in the new compound. We could not stabilise other combinations such as Bi3 Sr2 Nb2 O11 Cl and Bi3 Pb2 Nb2 O11 Br due to inter-layer mismatch. Sr2+ doping reduces the impact of the stereochemically active 6s2 lone pair found on Pb2+and Bi3+, resulting in a contraction of the c axis by 1.22 % and an expansion of the a-b plane by 0.25 %, improving inter-layer compatibility with Br-. XANES analysis shows that the ferroelectric distortion of the B-site cation is less apparent in Bi3 Sr2 Nb2 O11 Br compared to Bi3 Pb2 Nb2 O11 Cl. Variable-temperature neutron diffraction data show no evidence for a ferroelectric distortion.
- ItemDesigning new n=2 sillen-aurivillius phases by lattice-matched substitutions in the halide and [Bi2O2](2+) layers(Academic Press Inc Elsevier Science, 2013-09-01) Liu, S; Blanchard, PER; Avdeev, M; Kennedy, BJ; Ling, CDAbstract A new n=2 Sillen–Aurivillius compound Bi3Sr2Nb2O11Br has been synthesised based on Bi3Pb2Nb2O11Cl by simultaneously replacing Pb2+ with Sr2+ and Cl− with Br−. Rietveld refinements against X-ray and neutron powder diffraction data revealed a significant relative compression in the stacking axis (c-axis) of the new compound. Sr2+ doping reduces the impact of the stereochemically active 6s2 lone pair found on Pb2+ and Bi3+, resulting in a contraction of the c-axis by 1.22% and an expansion of the ab plane by 0.25%. This improves the inter-layer compatibility with the larger halide Br−. Analysis of X-ray absorption near-edge spectroscopy data show that the ferroelectric distortion of the B-site cation is less apparent in Bi3Sr2Nb2O11Br compared to Bi3Pb2Nb2O11Cl, and variable-temperature neutron diffraction data show no evidence for a ferroelectric distortion.© 2013, Elsevier Inc
- 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.
- ItemDoes local disorder occur in the pyrochlore zirconates?(American Chemical Socity, 2012-12-17) Blanchard, PER; Clements, R; Kennedy, BJ; Ling, CD; Reynolds, EM; Avdeev, M; Stampfl, APJ; Zhang, Z; Jang, LYThe zirconates Ln2Zr2O7 (Ln = lanthanoid) have been studied using a combination of Zr L-edge X-ray absorption near edge structure (XANES) and synchrotron X-ray and neutron powder diffraction methods. These studies demonstrate that as the size of the lanthanoid cation decreases, the local structure evolves smoothly from the ideal pyrochlore toward the defect fluorite rather than undergoing an abrupt transformation. The Zr L-edge spectrum is found to be extremely sensitive to changes in the local coordination environment and demonstrates an increase in local disorder across the pyrochlore oxides. The sensitivity of the XANES measurements enables us to identify the progressive nature of the transition that could not be detected using bulk diffraction techniques. © 2012, American Chemical Society.
- ItemExperimental observation and computational study of the spin-gap excitation in Ba3BiRu2O9(American Physical Society, 2016-11-01) Ling, CD; Huang, Z; Kennedy, BJ; Rols, S; Johnson, MR; Zbiri, M; Kimber, SAJ; Hudspeth, J; Adroja, DT; Rule, KC; Avdeev, M; Blanchard, PERBa3BiRu2O9 is a 6H-type perovskite compound containing face-sharing octahedral M2O9 (M=Ir, Ru) dimers, which are magnetically frustrated at low temperatures. On cooling through T∗=176 K, it undergoes a pronounced magnetostructural transition, which is not accompanied by any change in space group symmetry, long-range magnetic ordering, or charge ordering. Here, we report the first direct evidence from inelastic neutron scattering that this transition is due to an opening of a gap in the excitation spectra of dimers of low-spin Ru4+ (S=1) ions. X-ray absorption spectroscopy reveals a change in Ru-Ru orbital overlap at T∗, linking the emergence of this spin-gap excitation to the magnetostructural transition. Ab initio calculations point to a geometrically frustrated magnetic ground state due to antiferromagnetic interdimer exchange on a triangular Ru2O9 dimer lattice. X-ray total-scattering data rule out long-range magnetic ordering at low temperatures, consistent with this geometrically frustrated model. ©2016 American Physical Society
- 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
- ItemGradual structural evolution from pyrochlore to defect-fluorite in Y2Sn2-xZrxO7: average vs local structure(ACS Publications, 2013-11-25) Zhang, ZM; Middleburgh, SC; de los Reyes, M; Lumpkin, GR; Kennedy, BJ; Blanchard, PER; Reynolds, EM; Jang, LYWe have studied the long-range average and local structures in Y2Sn2−xZrxO7 (x = 0−2.0) using synchrotron X-ray powder diffraction and X-ray absorption spectroscopy, respectively, and by theoretical methods. While the diffraction data indicate a clear phase transition from ordered pyrochlore to disordered defectfluorite at x ∼ 1.0−1.2, X-ray absorption near-edge structure (XANES) results at the Zr L3- and Y L2-edges reveal a gradual structural evolution across the whole compositional range. These findings provide experimental evidence that the local disorder occurs long before the pyrochlore to defect-fluorite phase boundary, as determined by X-ray diffraction, and the extent of disorder continues to develop throughout the defectfluorite region. The Zr and Y L-edge spectra are very sensitive to changes in the local structure; such sensitivity enables us to reveal the progressive nature of the phase transition. Experimental results are supported by ab initio atomic scale simulations, which provide a mechanism for disorder to initiate in the pyrochlore structure. Further, the coordination numbers of the cations in both the defect-fluorite and pyrochlore structures are predicted, and the trends agree well with the experimental XANES results. The calculations predict that the coordination of cations in the Y2Zr2O7 defect-fluorite (normally considered to be 7 for all cations) varies depending on the species with the average coordination of Y and Zr being 7.2 and 6.8, respectively. © 2013, American Chemical Society.
- ItemImpact of Cu doping on the structure and electronic properties of LaCr1–yCuyO3(ACS Publications, 2014-01-31) Qasim, I; Blanchard, PER; Liu, S; Kennedy, BJ; Avdeev, MOxides of the type LaCr1–yCuyO3 have been prepared using solid-state methods and their crystal structures refined using synchrotron X-ray powder diffraction. The solubility limit of Cu was found to be around y = 0.2, and such oxides are orthorhombic in space group Pbnm. X-ray absorption spectroscopy measurements at the Cr and Cu L-edges demonstrated that the Cr remains trivalent upon Cu doping, with the Cu being present as Cu(III). The oxides are found to be antiferromagnets, and the Néel temperature, TN, decreases as the Cu content is increased. The crystal and magnetic structures of one example La(Cr0.85Cu0.15)O3 have been investigated between 3 and 350 K by neutron powder diffraction. The samples are semiconductors. © 2014, American Chemical Society.
- ItemInvestigating the local structure of lanthanoid hafnates Ln2Hf2O7 via diffraction and spectroscopy(American Chemical Society, 2013-02-07) Blanchard, PER; Liu, S; Kennedy, BJ; Ling, CD; Avdeev, M; Aitken, JB; Cowie, BCC; Tadich, AThe lanthanoid hafnates Ln2Hf2O7 (Ln = La, Pr, Nd, Sm?Tm) were studied using a combination of synchrotron X-ray and neutron powder diffraction together with X-ray absorption and Raman spectroscopy. Spectroscopic methods revealed a gradual increase in disorder from the ideal pyrochlore structure to the defect fluorite structure as the size of the Ln cation increases. The line shape of the Hf L3-edge X-ray absorption near edge spectra which is sensitive to the local coordination environment changed with increasing disorder. The general line shape of the O K-edge XANES and Raman spectra also indicated an increase in disorder across the Ln2Hf2O7 series. Differences in the diffraction and spectroscopy analysis reflect the greater sensitivity of the spectroscopy techniques to local ordering. © 2013, American Chemical Society
- ItemInvestigating the order-disorder phase transition in Nd2-xYxZr2O7via diffraction and spectroscopy(Royal Society of Chemistry, 2013-01-01) Blanchard, PER; Liu, S; Kennedy, BJ; Ling, CD; Zhang, Z; Avdeev, M; Cowie, BCC; Thomsen, L; Jang, LYThe pyrochlore-defect fluorite phase transition in the mixed-metal zirconate Nd2-xYxZr2O7 (0 <= x <= 2) solid solution was investigated using synchrotron X-ray and neutron diffraction, as well as X-ray absorption spectroscopy. Diffraction analysis revealed a two-phase region between 1.0 <= x <= 1.2. In the pyrochlore phase, Zr L-3-edge XANES analysis demonstrated a gradual change in the local coordination environment of the B site with increasing Y content that was consistent with an increase in disorder. Although Y L-3-edge XANES analysis suggested that the Y cations remained in an ordered coordination environment in the pyrochlore phase, disorder did gradually increase once the fluorite phase formed. It was found that Y cations prefer an ordered coordination environment near the phase boundary whereas Zr cations prefer a disordered coordination environment. © 2013, Royal Society of Chemistry
- 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.
- ItemLong- and short-range structure studies of KBT-KBZ solid-solutions using synchrotron radiation(Royal Society of Chemistry, 2015-01-14) Liu, S; Blanchard, PER; Zhang, Z; Kennedy, BJ; Ling, CDThe relaxor ferroelectric K0.5Bi0.5TiO3 has been synthesised in a solid-solution series with K0.5Bi0.5ZrO3, as K0.5Bi0.5Ti1−xZrxO3. High-resolution synchrotron X-ray powder diffraction and X-ray absorption near edge structure spectroscopy were used to characterise the long-range average and local structural behaviour. Rietveld refinements against diffraction data show that a pseudocubic tetragonal region exists across the whole solid-solution series, with truly cubic symmetry only observed at x = 1 (pure KBZ). Variable-temperature diffraction data for x = 0 (pure KBT) showed a broad ferroelectric transition from tetragonal to cubic symmetry at approximately 683 K with a coexistence of both phases close to that temperature, accompanied by a marked volume contraction. Ti K-edge data showed that Zr doping has a minimal effect on Ti off-centering, and revealed no evidence for local clustering. Metal L-edges showed that Ti4+ cations remain off-centered with increasing Zr content, while Zr4+ cations approach a higher-symmetry coordination environment, most likely due to the increased size of the Zr atoms. Although there is a minimal effect on actual Ti-offsets, an effective dilution of these environments by Zr doping leads to a reduction in polar domains and a diminished ferroelectric response. © 2015 Royal Society of Chemistry
- ItemOrdered vs. disordered perovskites; structural studies of Fe-doped SrIrO3 and SrRuO3(Academic Press Inc Elsevier Science, 2013-10-01) Qasim, I; Blanchard, PER; Liu, S; Tang, CG; Kennedy, BJ; Avdeev, M; Kimpton, JAThe structures of the two Fe containing perovskites Sr2IrFeO6 and SrRu0.5Fe0.5O3 have been established using a combination of synchrotron and neutron diffraction methods. Sr2IrFeO6 and SrRu0.5Fe0.5O3 are shown to be monoclinic I2/m and tetragonal I4/mcm respectively The former exhibits a rock-salt like ordering of the Fe and Ir cations and displays a sequence of phase transitions associated with the loss of the octahedral tilts upon heating; 12 / m → I 4 / m → F m 3 ¯ m . The Fe and Ru cations are disordered in SrRu0.5Fe0.5O3 and this shows a single structural phase transition upon heating due to the loss of the in-phase tilts, viz. I 4 / m c m → P m 3 ¯ m . In both cases XANES measurements show partial oxidation of the Fe3+ to Fe4+. The difference in the structures between the two is remarkable given the similar size of Ir5+ and Ru5+, and this is reflected in their magnetic properties. © 2013, Elsevier Ltd.
- ItemPerovskites in low dimensional multi-layer structure types(International Union of Crystallography, 2014-08) Liu, S; Miiller, W; Liu, Y; Blanchard, PER; Avdeev, M; Kennedy, BJ; Ling, CDThis study introduces examples of structure property relationships within the multi-layered Sillen-Aurivillius family (shown in Figure) and aims to investigate the effect of chemical doping and lattice matching effects. The first example involves doping 1/3 of the n = 3 ferroelectric perovskite layers with magnetic transition metal cations in Bi 5 PbTi 3 O 1 4 Cl  with charge balancing by removing Pb 2 + for Bi 3 + . A statistical 1:2 distribution of M 3 + and Ti 4 + across all three perovskite layers was found in Bi 6 Ti 2 MO 1 4 Cl, M = Cr 3 + , Mn 3 + , Fe 3 + , resulting in highly strained structures (enhancing the ferroelectricity compared to Bi 5 PbTi 3 O 1 4 Cl) and pronounced spin-glass behavior below T i r r (0) = 4.46 K. Ferroelectric transitions were observed at high temperature for each of the new compounds. Ferroelectric properties were also measured on Bi 6 Ti 2 FeO 1 4 Cl using piezoresponse force microscopy showing hysteretic phase behavior. A new n = 2 Sillen-Aurivillius compound Bi 3 Sr 2 Nb 2 O 1 1 Br, based on Bi 3 Pb 2 Nb 2 O 1 1 Cl , was synthesized by simultaneously replacing Pb 2 + with Sr 2 + and Cl - with Br - . Inter-layer mismatch prevented the formation of Bi 3 Sr 2 Nb 2 O 1 1 Cl and Bi 3 Pb 2 Nb 2 O 1 1 Br. Sr 2 + doping reduces the impact of the stereochemically active 6s 2 lone pair found on Pb 2 + and Bi 3 + , resulting in a stacking contraction in the lattice parameters by 1.22 % and an expansion of the a-b plane by 0.25 %, improving inter-layer compatibility with Br - . X-ray Absorption Near Edge Structure spectra analysis shows that the ferroelectric distortion of the B-site cation is less apparent in Bi 3 Sr 2 Nb 2 O 1 1 Br compared to Bi 3 Pb 2 Nb 2 O 1 1 Cl. Variable-temperature neutron diffraction data show no evidence for a ferroelectric distortion. © 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.
- ItemProbing long- and short-range disorder in Y2Ti2–xHfxO7 by diffraction and spectroscopy techniques(American Chemical Society, 2016-11-01) Zhang, ZM; Avdeev, M; de los Reyes, M; Lumpkin, GR; Kennedy, BJ; Blanchard, PER; Liu, S; Tadich, A; Cowie, BCCWe studied the long-range average and short-range local structures in Y2Ti2–xHfxO7 (x = 0–2.0) using diffraction and spectroscopy techniques, respectively. Both neutron and synchrotron X-ray powder diffraction data show a clear phase transition of the average structure from ordered pyrochlore to disordered defect-fluorite at x ≈ 1.6; the long-range anion disorder appears to develop gradually throughout the entire pyrochlore region in contrast to the rapid loss of cation ordering from x = 1.4 to 1.6. The commonly observed two-phase region around the pyrochlore/defect-fluorite phase boundary is absent in this system, demonstrating high sample quality. X-ray absorption near-edge structure (XANES) results at the Y L2-, Ti K- and L3,2-, Hf L3-, and O K-edges indicate a gradual local structural evolution across the whole compositional range; the Y coordination number (CN) decreases and the CN around Ti and Hf increases with increasing Hf content (x). The spectroscopic results suggest that the local disorder occurs long before the pyrochlore to defect-fluorite phase boundary as determined by diffraction, and this disorder evolves continuously from short- to medium- and eventually to long-range detectable by diffraction. This study highlights the complex disordering process in pyrochlore oxides and the importance of a multitechnique approach to tackle disorder over different length scales and in the anion and cation sublattices, respectively. The results are important in the context of potential applications of these oxides such as ionic conductors and radiation-resistant nuclear waste forms. © 2016 American Chemical Society