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1.
Inorg Chem ; 2020 Sep 02.
Artigo em Inglês | MEDLINE | ID: mdl-32876445

RESUMO

Mn11Ta4O21 is presented as the first hexagonal A-site manganite. Based on simple rules, the structure is compatible with a 14H-layer (cchchch)2 stacking sequence that is related to BaVO3 and BaCrO3 high-pressure polymorphs. The A-site overstoichiometry is explained through difference in ionic radii sizes between Ba and Mn. Magnetic properties show two transitions at TN1 = 88 K and TN2 = 56 K. Neutron powder diffraction evidence two magnetic structures with purely antiferromagnetic and ferrimagnetic orders below TN1 and TN2, respectively. A complementary description with 14H-(hhccccc)2 sequence of only Mn octahedra provides a direct comparison with BaMnO3-δ hexagonal perovskites and naturally explains the AFM order. Below TN2 a magneto-elastic coupling along with uniaxial negative thermal expansion are observed.

2.
Inorg Chem ; 2020 Aug 07.
Artigo em Inglês | MEDLINE | ID: mdl-32806009

RESUMO

The potential of the perovskite system Nd1-xSrxCoO3-δ (x = 1/3 and 2/3) as cathode material for solid oxide fuel cells (SOFCs) has been investigated via detailed structural, electrical, and electrochemical characterization. The average structure of x = 1/3 is orthorhombic with a complex microstructure consisting of intergrown, adjacent, perpendicularly oriented domains. This orthorhombic symmetry remains throughout the temperature range 373-1073 K, as observed by neutron powder diffraction. A higher Sr content of x = 2/3 leads to stabilization of the cubic perovskite with a homogeneous microstructure and with a higher oxygen vacancy content and cobalt oxidation state than the orthorhombic phase at SOFC operation temperature. Both materials are p-type electronic conductors with high total conductivities of 690 and 1675 S·cm-1 at 473 K in air for x = 1/3 and 2/3, respectively. Under working conditions, both compounds exhibit similar electronic conductivities, since x = 2/3 loses more oxygen on heating than x = 1/3, associated with a greater loss of p-type charger carriers. However, composite cathodes prepared with Nd1/3Sr2/3CoO3-δ and Ce0.8Gd0.2O2-δ present lower ASR values (0.10 Ω·cm2 at 973 K in air) than composites prepared with Nd2/3Sr1/3CoO3-δ and Ce0.8Gd0.2O2-δ (0.34 Ω·cm2). The high activity for the oxygen electrochemical reaction at intermediate temperatures is likely attributable to a large disordered oxygen-vacancy concentration, resulting in a very promising SOFC cathode for real devices.

3.
Nat Phys ; 16(5): 546-552, 2020 May.
Artigo em Inglês | MEDLINE | ID: mdl-32802143

RESUMO

Spin liquids are highly correlated yet disordered states formed by the entanglement of magnetic dipoles1. Theories define such states using gauge fields and deconfined quasiparticle excitations that emerge from a local constraint governing the ground state of a frustrated magnet. For example, the '2-in-2-out' ice rule for dipole moments on a tetrahedron can lead to a quantum spin ice2-4 in rare-earth pyrochlores. However, f-electron ions often carry multipole degrees of freedom of higher rank than dipoles, leading to intriguing behaviours and 'hidden' orders5-6. Here we show that the correlated ground state of a Ce3+-based pyrochlore, Ce2Sn2O7, is a quantum liquid of magnetic octupoles. Our neutron scattering results are consistent with a fluid-like state where degrees of freedom have a more complex magnetization density than that of magnetic dipoles. The nature and strength of the octupole-octupole couplings, together with the existence of a continuum of excitations attributed to spinons, provides further evidence for a quantum ice of octupoles governed by a '2-plus-2-minus' rule7-8. Our work identifies Ce2Sn2O7 as a unique example of frustrated multipoles forming a 'hidden' topological order, thus generalizing observations on quantum spin liquids to multipolar phases that can support novel types of emergent fields and excitations.

4.
Artigo em Inglês | MEDLINE | ID: mdl-32649790

RESUMO

Non-centrosymmetric polar compounds have important technological properties. Reported perovskite oxynitrides show centrosymmetric structures, and for some of them high permittivities have been observed and ascribed to local dipoles induced by partial order of nitride and oxide. Reported here is the first hexagonal perovskite oxynitride BaWON2 , which shows a polar 6H polytype. Synchrotron X-ray and neutron powder diffraction, and annular bright-field in scanning transmission electron microscopy indicate that it crystalizes in the non-centrosymmetric space group P63 mc, with a total order of nitride and oxide at two distinct coordination environments in cubic and hexagonal packed BaX3 layers. A synergetic second-order Jahn-Teller effect, supported by first principle calculations, anion order, and electrostatic repulsions between W6+ cations, induce large distortions at two inequivalent face-sharing octahedra that lead to long-range ordered dipoles and spontaneous polarization along the c axis. The new oxynitride is a semiconductor with a band gap of 1.1 eV and a large permittivity.

5.
Sci Rep ; 10(1): 9813, 2020 Jun 17.
Artigo em Inglês | MEDLINE | ID: mdl-32555354

RESUMO

The GaV4S8-ySey (y = 0 to 8) family of materials have been synthesized in both polycrystalline and single crystal form, and their structural and magnetic properties thoroughly investigated. Each of these materials crystallizes in the F[Formula: see text][Formula: see text]3m space group at ambient temperature. However, in contrast to the end members GaV4S8 and GaV4Se8, that undergo a structural transition to the R3m space group at 42 and 41 K respectively, the solid solutions (y = 1 to 7) retain cubic symmetry down to 1.5 K. In zero applied field the end members of the family order ferromagnetically at 13 K (GaV4S8) and 18 K (GaV4Se8), while the intermediate compounds exhibit a spin-glass-like ground state. We demonstrate that the magnetic structure of GaV4S8 shows localization of spins on the V cations, indicating that a charge ordering mechanism drives the structural phase transition. We conclude that the observation of both structural and ferromagnetic transitions in the end members of the series in zero field is a prerequisite for the stabilization of a skyrmion phase, and discuss how the absence of these transitions in the y = 1 to 7 materials can be explained by their structural properties.

6.
Chemistry ; 2020 Jun 15.
Artigo em Inglês | MEDLINE | ID: mdl-32542938

RESUMO

The unprecedented borate hydride Sr5 (BO3 )3 H and deuteride Sr5 (11 BO3 )3 D crystallizing in an apatite-related structure are reported. Despite the presence of hydride anions, the compound decomposes only slowly in air. Doped with Eu2+ , it shows broad-band orange-red emission under violet excitation owing to the 4f6 5d-4f7 transition of Eu2+ . The observed 1 H NMR chemical shift is in good agreement with previously reported 1 H chemical shifts of ionic metal hydrides as well as with quantum chemical calculations and very different from 1 H chemical shifts usually found for hydroxide ions in similar materials. FTIR and Raman spectroscopy of different samples containing 1 H, 2 H, nat B, and 11 B combined with calculations unambiguously prove the absence of hydroxide ions and the sole incorporation of hydride ions into the borate. The orange-red emission obtained by doping with Eu2+ shows that the new compound class might be a promising host material for optical applications.

7.
Inorg Chem ; 2020 May 20.
Artigo em Inglês | MEDLINE | ID: mdl-32432871

RESUMO

Several different mechanisms of magnetoresistance (MR) have been observed in 1111 LnMnAsO1-xFx oxypnictides (Ln = lanthanide) as a result of magnetic coupling between the Mn and Ln. Such phases also exhibit interesting magnetic phase transitions upon cooling. Sr2Mn2CrAs2O2 has been synthesized to investigate if it is possible to observe MR and/or magnetic phase transitions as a result of magnetic coupling between the Mn and Cr. Sr2Mn2CrAs2O2 crystallizes in the tetragonal space group I4/mmm containing alternating MO22- and M'2As22- layers, and neutron diffraction results demonstrate that the actual stoichiometry is Sr2Mn2.23Cr0.77As2O2. Cation order is present between Mn and Cr, with Cr predominantly occupying the square planar MO22- site. Below 410 K, the magnetic moments of the Mn/Cr ions in the M'2As22- sublattice exhibit G-type antiferromagnetic order. The Mn/Cr moments within the MO22- layer order below 167 K with a K2NiF4-type antiferromagnetic structure that simultaneously induces a spin flip of the magnetic moments in the M'2As22- layers from a G-type to a C-type antiferromagnetic arrangement. The results demonstrate that the superexchange interactions are finely balanced in Sr2Mn2.23Cr0.77As2O2. Sr2Mn2.23Cr0.77As2O2 is semiconducting, and there is no evidence of MR.

8.
Nat Mater ; 2020 Mar 02.
Artigo em Inglês | MEDLINE | ID: mdl-32123332

RESUMO

Oxide ion and proton conductors, which exhibit high conductivity at intermediate temperature, are necessary to improve the performance of ceramic fuel cells. The crystal structure plays a pivotal role in defining the ionic conduction properties, and the discovery of new materials is a challenging research focus. Here, we show that the undoped hexagonal perovskite Ba7Nb4MoO20 supports pure ionic conduction with high proton and oxide ion conductivity at 510 °C (the bulk conductivity is 4.0 mS cm-1), and hence is an exceptional candidate for application as a dual-ion solid electrolyte in a ceramic fuel cell that will combine the advantages of both oxide ion and proton-conducting electrolytes. Ba7Nb4MoO20 also showcases excellent chemical and electrical stability. Hexagonal perovskites form an important new family of materials for obtaining novel ionic conductors with potential applications in a range of energy-related technologies.

9.
Chem Commun (Camb) ; 55(96): 14470-14473, 2019 Nov 28.
Artigo em Inglês | MEDLINE | ID: mdl-31728460

RESUMO

A new double perovskite Mn2MnTeO6 has been obtained by high pressure phase transformation of a corundum-related precursor. It is antiferromagnetic below 36 K and develops a magnetic structure with magnetic moments of 4.8 µB and 3.8 µB for Mn2+ at the A and B sites respectively. This new polymorph accounts for a recently reported decrease in the bandgap of Mn3TeO6 under pressure that may lead to useful light-harvesting properties.

10.
Inorg Chem ; 58(19): 12809-12814, 2019 Oct 07.
Artigo em Inglês | MEDLINE | ID: mdl-31496236

RESUMO

Ozone oxidation has allowed the stabilization of a very high iron oxidation state in the FeSr2YCu2O7.85 cuprate, in which a long-range magnetic ordering of the high valent iron cations coexists with the superconducting interactions (magnetic ordering temperature TN = 110 K > superconducting critical temperature Tc = 70 K). The somewhat unexpected A-type AFM structure, with a µ(Fe) ∼ 2 µB magnetic saturation moment associated with the hypervalent iron sublattice, suggests an unusual low spin state for the iron cations, while the low dimensionality of the magnetic structure results in a soft switching toward ferromagnetism under small external magnetic fields. The role of the crystal structure and of the high charge concentration in the stabilization of this unusual electronic configuration for the iron cations is discussed.

11.
Dalton Trans ; 48(36): 13651-13661, 2019 Sep 17.
Artigo em Inglês | MEDLINE | ID: mdl-31465070

RESUMO

The partial substitution of up to 5% Nd+3 by Ca+2 results in the oxide Nd1.90Ca0.10MgTiO5.94 that presents some remarkable structural features with a noticeable influence on its properties. In this oxide with a monoclinic perovskite-like structure and an octahedral tilting scheme (a-a-b+), both A- and B-ions are arranged in a rock-salt like manner, representing therefore the first example of a type of perovskite theoretically predicted. Besides this unprecedented arrangement of A- and B-ions, the oxygen vacancies created through doping with acceptor ions are trapped by association with the acceptor defects and hence the mobility of these vacancies is strongly limited. The oxygen conductivity of the substituted material is lower and the activation energy for oxygen motion is higher than those of the parent oxide, in which the concentration of anion vacancies is only due to intrinsic defects.

12.
ChemSusChem ; 12(17): 4029-4037, 2019 Sep 06.
Artigo em Inglês | MEDLINE | ID: mdl-31282611

RESUMO

Materials with the formula Sr2 CoNb1-x Tix O6-δ (x=1.00, 0.70; δ=number of oxygen vacancies) present a cubic perovskite-like structure. They are easily and reversibly reduced in N2 or Ar and re-oxidized in air upon heating. Oxidation by water (wet N2 ), involving splitting of water at a temperature as low as 700 °C, produces hydrogen. Both compounds displayed outstanding H2 production in the first thermochemical cycle, the Sr2 CoNb0.30 Ti0.70 O6-δ material retaining its outstanding performance upon cycling, whereas the hydrogen yield of the x=1 oxide showed a continuous decay. The retention of the materials' ability to promote water splitting correlated with their structural, chemical, and redox reversibility upon cycling. On reduction/oxidation, Co ions reversibly changed their oxidation state to compensate the release/recovery of oxygen in both compounds. However, in Sr2 CoTiO6-δ , two phases with different oxygen contents segregated, whereas in Sr2 CoNb0.30 Ti0.70 O6-δ this effect was not evident. Therefore, this latter material displayed a hydrogen production as high as 410 µmol H 2 g-1 perovskite after eight thermochemical cycles at 700 °C, which is among the highest ever reported, making this perovskite a promising candidate for thermosolar water splitting in real devices.

13.
Chem Commun (Camb) ; 55(21): 3105-3108, 2019 Mar 07.
Artigo em Inglês | MEDLINE | ID: mdl-30789159

RESUMO

The topotactic nitridation of cation ordered, tetragonal Sr2FeMoO6 in NH3 at moderate temperatures leads to cubic, Fm3[combining macron]m double perovskite oxynitride Sr2FeMoO4.9N1.1 where double-exchange interactions determine ferromagnetic order with TC ≈ 100 K. Substitution of oxide by nitride induces bond asymmetries and local electronically driven structural distortions, which combined with Fermi level lowering restricts charge itinerancy to confined regions and preclude spontaneous long-range magnetic order. Under a magnetic field, ferromagnetic correlations expand, favoring charge delocalization and a negative magnetoresistance is observed.

14.
Inorg Chem ; 57(23): 15023-15033, 2018 Dec 03.
Artigo em Inglês | MEDLINE | ID: mdl-30444118

RESUMO

The BaZr0.7Ce0.2Y0.1O3-δ-BaPrO3-δ perovskite system, of interest for high-temperature electrochemical applications involving mixed protonic-electronic conductivity, forms a solid-solution with a wide interval of Ba substoichiometry in the range Ba(Ce0.2Zr0.7)1- xPr xY0.1O3-δ, 0 ≤ x ≤ 1. Structural phase transitions mapped as a function of temperature and composition by high-resolution neutron powder diffraction and synchrotron X-ray diffraction reveal higher symmetry for lower Pr content and higher temperatures, with the largest stability field observed for rhombohedral symmetry (space group, R3̅ c). Rietveld refinement, supported by magnetic-susceptibility measurements, indicates that partitioning of the B-site cations over the A and B perovskite sites compensates Ba substoichiometry in preference to A-site vacancy formation and that multiple cations are distributed over both sites. Electron-hole transport dominates electrical conductivity in both wet and dry oxidizing conditions, with total conductivity reaching a value of ∼0.5 S cm-1 for the x = 1 end-member in dry air at 1173 K. Higher electrical conductivity and the displacement of oxygen loss to higher temperatures with increasing Pr content both reflect the role of Pr in promoting hole formation at the expense of oxygen vacancies. In more reducing conditions (N2) and at low Pr contents, conductivity is higher in humidified atmospheres (∼0.023 atm pH2O) indicating a protonic contribution to transport, whereas the greater electron-hole conductivity with increasing Pr content results in lower conductivity in humidified N2 due to the creation of protonic defects and the consumption of holes.

15.
Chem Commun (Camb) ; 54(88): 12523-12526, 2018 Nov 14.
Artigo em Inglês | MEDLINE | ID: mdl-30345452

RESUMO

The new phase Co2ScSbO6 and Ni2-xCoxScSbO6 solid solutions adopt the polar Ni3TeO6-type structure and order magnetically below 60 K. A series of long-period lock-in [0 0 1/3n] spin structures with n = 5, 6, 8 and 10 is discovered, coexisting with a ferrimagnetic [0 0 0] phase at high Co-contents. The presence of electrical polarisation and spontaneous magnetisations offers possibilities for multiferroic properties.

16.
Inorg Chem ; 57(19): 11942-11947, 2018 Oct 01.
Artigo em Inglês | MEDLINE | ID: mdl-30207462

RESUMO

The oxide ionic conductor Ba3W1.2Nb0.8O8.6 has been synthesized as part of an investigation into the new class of Ba3M'M''O8.5 (M' = W, Mo; M'' = Nb) oxide-ion conducting hexagonal perovskite derivatives. The substitution of W6+ for Nb5+ in Ba3W1+ xNb1- xO8.5+ x/2 leads to an increase in the oxygen content, which enhances the low-temperature ionic conductivity. However, at 400 °C, the ionic conductivity of Ba3W1.2Nb0.8O8.6 is still significantly lower than the molybdenum compound Ba3MoNbO8.5. Remarkably, at 600 °C the bulk oxide ionic conductivities of Ba3MoNbO8.5, Ba3WNbO8.5, and Ba3W1.2Nb0.8O8.6 are very similar (σb = 0.0022, 0.0017, and 0.0016 S cm-1, respectively). The variable-temperature neutron diffraction results reported here demonstrate that Ba3W1.2Nb0.8O8.6 undergoes a similar structural rearrangement to Ba3MoNbO8.5 above 300 °C, but the ratio of (W/Nb)O4 tetrahedra to (W/Nb)O6 octahedra rises at a faster rate upon heating between 300 and 600 °C. There is a clear relationship between the ionic conductivity of Ba3M'1+ xM''1- xO8.5+ x/2 (M' = W, Mo; M'' = Nb) phases and the number of tetrahedrally coordinated M' and M ″ cations present within the crystal structure.

17.
Nat Commun ; 9(1): 3536, 2018 08 28.
Artigo em Inglês | MEDLINE | ID: mdl-30154477

RESUMO

The original version of this Article contained an error in the third sentence of the legend of Fig. 2, which incorrectly read 'The phase fractions of the charge ordered (CO) phase, obtained from synchrotron (X) and neutron (N) diffraction data are shown in the right-hand panel.' The correct version states 'charge averaged (CA)' in place of 'charge ordered (CO)'. This has been corrected in both the PDF and HTML versions of the Article.

18.
Nat Commun ; 9(1): 2975, 2018 07 30.
Artigo em Inglês | MEDLINE | ID: mdl-30061576

RESUMO

Incomplete transformations from ferromagnetic to charge ordered states in manganite perovskites lead to phase-separated microstructures showing colossal magnetoresistances. However, it is unclear whether electronic matter can show spontaneous separation into multiple phases distinct from the high temperature state. Here we show that paramagnetic CaFe3O5 undergoes separation into two phases with different electronic and spin orders below their joint magnetic transition at 302 K. One phase is charge, orbital and trimeron ordered similar to the ground state of magnetite, Fe3O4, while the other has Fe2+/Fe3+charge averaging. Lattice symmetry is unchanged but differing strains from the electronic orders probably drive the phase separation. Complex low symmetry materials like CaFe3O5 where charge can be redistributed between distinct cation sites offer possibilities for the generation and control of electronic phase separated nanostructures.

19.
Inorg Chem ; 57(17): 11058-11067, 2018 Sep 04.
Artigo em Inglês | MEDLINE | ID: mdl-30113162

RESUMO

Layered perovskite titanium oxyhydrides have been prepared by low-temperature topochemical CaH2 reduction from Ruddlesden-Popper Sr n+1Ti nO3 n+1 phases ( n = 1, 2) and structurally characterized by combined synchrotron X-ray and neutron diffraction data refinements. In the single-layered Sr2TiO3.91(2)D0.14(1) material, hydride anions are statistically disordered with oxides on the apical site only, as opposed to known transition-metal oxyhydrides exhibiting a preferred occupation of the equatorial site. This unprecedented site selectivity of H- has been reproduced by periodic DFT+ U calculations, emphasizing for the hydride defect a difference in formation energy of 0.24 eV between equatorial and apical sites. In terms of electronic structure, the model system Sr2TiO3.875H0.125 is found to be slightly metallic and the released electron remains mostly delocalized over several Ti atoms. On the other hand, hydride anions in the double-layered Sr3Ti2O6.20H0.12 material show a clear preference for the bridging apical site within the perovskite slabs, as confirmed by DFT calculations on the Sr3Ti2O6.875H0.125 model system. Finally, the influence of the B-site chemical nature on the hydride site selectivity for early 3d transition metals is theoretically explored in the single-layered system by substituting vanadium for titanium. The V3+ electronic polaron is suggested to play a role in stabilizing H- on the equatorial site in Sr2VO4- xH x for x = 0.125.

20.
Inorg Chem ; 57(6): 3360-3370, 2018 Mar 19.
Artigo em Inglês | MEDLINE | ID: mdl-29518311

RESUMO

The effect of substituting iron and zinc for cobalt in CaBaCo4O7 was investigated using neutron diffraction and X-ray absorption spectroscopy techniques. The orthorhombic distortion present in the parent compound CaBaCo4O7 decreases with increasing the content of either Fe or Zn. The samples CaBaCo3ZnO7 and CaBaCo4- xFe xO7 with x ≥ 1.5 are metrically hexagonal, but much better refinements in the neutron diffraction patterns are obtained using an orthorhombic unit cell. The two types of substitution have opposite effects on the structural and magnetic properties. Fe atoms preferentially occupy the sites at the triangular layer. Thus, the replacement of Co by Fe suppresses the ferrimagnetic ordering of the parent compound, and CaBaCo4- xFe xO7 (0.5 ≤ x ≤ 2) samples are antiferromagnetically ordered following a new propagation vector k = (1/3,0,0). However, the Zn atoms prefer occupying the Kagome layer, which is very detrimental for the long-range magnetic interactions giving rise to a magnetic glass-like behavior in the CaBaCo3ZnO7 sample. The oxidation states of iron and zinc are found to be 3+ and 2+, respectively, independently of the content, as confirmed by X-ray absorption spectroscopy. Therefore, the average Co oxidation state changes accordingly with the Fe3+ or Zn2+ doping. Also, X-ray absorption spectroscopy data confirm the different preferential occupation for both Fe and Zn cations. The combined information obtained by neutron diffraction and X-ray absorption spectroscopy indicates that cobalt atoms can be either in a fluctuating Co2+/Co3+ valence state or, alternatively, Co2+ and Co3+ ions being randomly distributed in the lattice. These results explain the occurrence of local disorder in the CoO4 tetrahedra obtained by EXAFS. An anomaly in the lattice parameters and an increase in the local disorder are observed only at the ferrimagnetic transition for CaBaCo4O7, revealing the occurrence of local magneto-elastic coupling.

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