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In view of iridium's scarceness and high cost, Ir-containing catalysts for water electrolyzers should have low loadings and maximal utilization. Here, we studied low-Ir-content, highly active, double perovskites (Sr2MIrO6, M = Ni, Co, Sc and Fe) for the oxygen evolution reaction (OER) in acid combining electrochemical experiments, DFT, and advanced characterization techniques. The initial OER performance depends on Ir's oxidation state and the geometric features of IrO6 frameworks, which are tuned by the choice of M. Higher oxidation states, particularly Ir6+, enhance the OER activity: Sr2NiIrO6 and Sr2CoIrO6 display potentials of ∼1.53 V at 10 mA cm−2, comparable to the best Ir-based catalysts in the literature. However, because of their less symmetric structures, perovskites with Ir6+ are less stable, prone to surface reconstruction and their cations leach under OER conditions. These results show that improved iridium-based OER electrocatalysts in acid can be designed by balancing their activity and stability.

Direct bandgap semiconductors of the hybrid-perovskite family CH3NH3PbX3 (X = I, Br, Cl) exhibit outstanding light absorption properties and are the materials of choice for solar energy applications. As an alternative to poisonous Pb, tin-containing perovskites would show a lower effective mass thus exhibiting a higher charge carrier mobility. An auspicious candidate is CH3NH3SnBr3, with an estimated band gap of 1.902 eV, anticipating applications in photovoltaic devices for the visible to ultra-violet wavelength region. We describe that this perovskite can be prepared by ball milling in a straightforward way, yielding specimens with a superior crystallinity. A structural investigation from synchrotron X-ray powder diffraction (SXRD) data was essential to revisit the successive phase transitions this compound experiences down to 120 K, guided by specific heat capacity and DSC measurements. From the cubic structure identified at RT and 270 K, there is a gradual evolution of the patterns, analysed as a phase admixture between the cubic and the low-symmetry phase present at 160 K. This corresponds to an orthorhombic Pmc21 superstructure; this acentric space group enables polarization along the c-axis where there is a twofold screw axis, evidenced in the distribution of Sn–Br distances. Furthermore, there are two conspicuous changes in the orthorhombic framework, yet keeping the Pmc21 space group, which agree with the main calorimetric events (observed at 224 and 147 K). We interpret these changes as an interplay between the tilting of the SnBr6 octahedra of the inorganic framework and the breaking and reconstruction of H-bond interactions with the organic CH3NH+3 unit. The stereochemical effect of the lone electron pair of the Sn2+ ion is clear in the SnBr6 octahedral distortion. Diffuse reflectance UV/Vis spectroscopy yields an optical gap of ∼2.1 eV, in agreement with ab- initio calculations. A Seebeck coefficient of ∼2000 μV K−1 is determined near RT, which is one order of magnitude higher than those reported for other halide perovskites.

The double perovskite Ba2CoOsO6, synthesized using solid-state methods at ambient pressure, is shown as a rare example of an oxide adopting the 6L-trigonal (S.G.: P3m1) perovskite structure. The s...

Proton exchange membrane water electrolysis is a promising technology to produce green hydrogen from renewables, as it can efficiently achieve high current densities. Lowering iridium amount in oxygen evolution reaction electrocatalysts is critical for achieving cost-effective production of green hydrogen. In this work, we develop catalysts from Ir double perovskites. Sr

Hybrid methyl-ammonium (MA:CH3NH3+) lead halide MAPbX3 (X = halogen) perovskites exhibit an attractive optoelectronic performance that can be applied to the next generation of solar cells. To extend the field of interest of these hybrid materials, we describe the synthesis by a solvent-free ball-milling procedure, yielding a well crystallized, pure and moisture stable specimen of the Cd tribromide counterpart, MACdBr3, which contains chains of face-sharing CdBr6 octahedra in a framework defined in the Cmc21 (No 36) space group. The details of the structural arrangement at 295 K have been investigated by high angular resolution synchrotron x-ray diffraction (SXRD), including the orientation of the organic MA units, which are roughly aligned along the c direction, given the acentric nature of the space group. UV-vis spectra unveil a gap of 4.6 eV, which could be useful for ultraviolet detectors.

A theoretical study has recently revealed Bi2PdO4 to be a promising p-type oxide thermoelectric material, with excellent electrical properties and low thermal conductivity, due to its structural peculiarities. Polycrystalline samples of the pristine material and Pb-doped Bi1.9Pb0.1PdO4 have been prepared by a conventional solid-state reaction, and an exhaustive structural characterization has been performed by high-energy synchrotron X-ray diffraction. Thermoelectric transport properties have been measured in the 300–800 K temperature range. The undoped compound displays p-type semiconductor behaviour and extremely high values of thermopower, up to 1458 μV K−1 at 340 K, along with low lattice thermal conductivity, related to large vibrations of Bi and O atoms. In the Pb-doped derivative the electrical resistivity is greatly enhanced, which along with a reduction of the lattice thermal conductivity results in an improved thermoelectric performance by more than one order of magnitude. These features underline the promising further optimization of this material aiming at thermoelectric applications.

Synchrotron X-ray powder diffraction methods have been used to obtain accurate structures of the lanthanoid orthoniobates, LnNbO4 at room temperature. The observed decrease in the unit cell volume in going from Ln = La to Lu correlates with a decrease in the volume of the LnO8 polyhedra due to the changing size of the Ln3+ cation. In all cases the Nb is in a highly distorted six-coordinate geometry with two long, ~2.5 A, and four short ~1.9 A Nb–O distances. The importance of the long Nb–O bonds is evident from Bond Valence Sum Calculations. Variable temperature structural studies of three examples, Ln = La, Sm and Gd, showed each transformed to a high temperature tetragonal scheelite type structure in space group I41/a. In the high temperature structure the Nb are tetrahedrally coordinated and it appears that monoclinic – tetragonal transition is best described as a reconstructive transition in which the long Nb–O(1) bond is broken upon heating.

Hybrid methyl-ammonium (MA:CH3NH3+) lead halide MAPbX3 (X = halogen) perovskites exhibit an attractive optoelectronic performance that can be applied to the next generation of solar cells. To extend the field of interest of these hybrid materials, we describe the synthesis by a solvent-free ball-milling procedure, yielding a well crystallized, pure and moisture stable specimen of the Cd tribromide counterpart, MACdBr3, which contains chains of face-sharing CdBr6 octahedra in a framework defined in the Cmc21 (No 36) space group. The details of the structural arrangement at 295 K have been investigated by high angular resolution synchrotron x-ray diffraction (SXRD), including the orientation of the organic MA units, which are roughly aligned along the c direction, given the acentric nature of the space group. UV-vis spectra unveil a gap of 4.6 eV, which could be useful for ultraviolet detectors.

Double perovskites containing Ir4+ were synthesised by a citrate technique, followed by an annealing treatment in air at 1100 °C. The crystal structure of the three compounds, with formula R2NiIrO6 (R= La, Pr and Nd), were determined using a combined refinement against neutron powder diffraction (NPD) and synchrotron x-ray powder diffraction (SXRPD) data sets. At room temperature, all the samples were indexed in the space group P21/n and the monoclinic symmetry remains in the 300 to 1273 K temperature range. Magnetization measurements suggest competitive antiferromagnetic and ferromagnetic interactions, with an unexpected increment of the ordering temperature (TN) along the series. The magnetic structures of all the samples were defined with the propagation vector k = 0; the Ni2+ and Ir4+ moment arrangement, different for each compound, shows a strong dependence on the nature of the rare-earth ion. We thank the financial support of the Spanish Ministry of Economy and Competitiveness to the project MAT2017-84496-R and of EPSRC. We are grateful to the Institut Laue-Langevin and ALBA cells synchrotron (proposal ID 201702206) for making all facilities available.

We report the temperature-dependent THz to mid-infrared response of hexagonal-${\mathrm{ErMnO}}_{3}$ using absorption, reflectivity, and emissivity techniques from 2 to 1700 K. At low temperatures, lowest frequency vibrational modes, which extend up the lock-in ferroelectric temperature, coexist with paramagnon excitations and are associated with well-defined crystal-field rare-earth pure magnetic replicas in an intriguing phonon-magnetic convergence enhancing the multiferroic character of $h\text{\ensuremath{-}}{\mathrm{ErMnO}}_{3}$. Increasing the temperature, a number of vibrational bands close to the space group predicted undergo profile broadening and softening. In particular, a distinctive set of bands in the $288--329\text{\ensuremath{-}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ (300 K) range has a component whose profile is carrier screened becoming nearly fully blurred in the intermediate phase between \ensuremath{\sim}830 and \ensuremath{\sim}1500 K. Below ${T}_{C}\ensuremath{\sim}830$ K this asymmetric band, having one component still partially screened, further splits as spin phonon interaction and the tripling of the unit cell take place revealing at ${T}_{\mathrm{N}}\ensuremath{\sim}79$ K a delicate balance of long- and short-range interactions. Ambient Raman scattering brings up evidence of a Fr\"ohlich resonance due to Coulomb interactions between carriers and the macroscopic field linked to the corresponding longitudinal optical-phonon mode. We found it is dynamically correlated to the hexagonal $c$-axis negative thermal expansion. Quantitative analyses of the mid-infrared (MIR) optical conductivity show that it also plays a role in small polarons and mediates in high-temperature bipolarones. Bipolaron profiles at high temperatures change as the sample opacity increases when at \ensuremath{\sim}900 K straight stripes turn curly toward complex vortex-antivortex domain patterns in the paraelectric phase. At still higher temperatures a low-frequency Drude contribution is triggered by electron hopping signaling an insulator-metal phase transition at \ensuremath{\sim}1600 K while the MIR response suggests coexistence between single small polarons and bipolarons. On closing, we draw a parallel with improper ferroelectrics sustaining a lattice incommensurate intermediate phase and unit-cell tripling. We argue that in the $h\text{\ensuremath{-}}R{\mathrm{MnO}}_{3}$ $(R=\text{rare earth}, \mathrm{Y})$ family of compounds the intermediate phase be considered incommensurate with onset at ${T}_{\mathrm{INC}}\ensuremath{\sim}1500$ K and ferroelectric lock-in at ${T}_{C}\ensuremath{\sim}830$ K delimiting this regime in $h\text{\ensuremath{-}}{\mathrm{ErMnO}}_{3}$.

The double perovskite Ba

The perovskite Ca0.5Bi0.5FeO3 containing unusually high valent Fe3.5+ undergoes sequentially charge disproportionation (CD) of the Fe centres and intersite charge transfer (CT) between Bi and Fe. From structural, magnetic, and transport property characterization, we found that substitution of Fe with Co occurs isoelectronically to form Ca0.5Bi0.5 3+(Fe1−xCox)3.5+O3 and destabilizes the CD state. This results in materials exhibiting only intermetallic charge transfer behaviour in the region 0.01 < x < 0.67. The CT transitions for these materials only involves Fe3.5+ whereas Co remains in the 3.5+ oxidation state at all temperatures. The doped Co3.5+ ions give Pauli-paramagnetic like conducting behavior. The Co-doping effect is very different from that observed in CaFe1−xCoxO3. The charge transition behavior of Fe3.5+ in the present system are also in contrast to those in Ca0.5Bi0.5FeO3 and Sr0.5Bi0.5FeO3

The series of ruthenate double perovskites Ba 2-x Sr x HoRuO 6 (0 ≤ x ≤ 2) have been synthesized using solid state methods. The crystal structures of the series have been determined by high resolution synchrotron X-ray diffraction and show the sequence of structures to be Fm 3 m (a 0 a 0 a 0 ) (0 ≤ x ≤ 0.6) → I4/m (a 0 a 0 c - ) (x = 0.8) → I2 / m (a 0 b - b - ) (1.0 ≤ x ≤ 1.2) → P2 1 / n (a - a - c + ) (1.4 ≤ x ≤ 2.0). A similar progression of structures is observed in the sample BaSrHoRuO 6 with increasing temperature. Magnetic characterisation of these materials was undertaken utilising variable temperature bulk magnetic susceptibility, isothermal magnetisation and low temperature neutron powder diffraction measurements. All members of the series order antiferromagnetically with the Ru and Ho sublattices ordering at different temperatures between 50 and 16 K. The presence of the magnetic Ho 3+ at the perovskite B site stabilises the antiferromagnetic (AFM) ordering of the Ru sublattice. The addition of Sr for Ba beyond x = 1.2 weakens the AFM superexchange interactions through induced structural distortions, resulting in a canted AFM ground state.

Polycrystalline Nd2MgRuO6 has monoclinic P21/n structure with an ordered arrangement of Mg2+ and Ru4+ ions. Magnetic ordering is observed at 1.5 K where the Ru sublattice exhibits type-I antiferromagnetic order with the magnetic moments parallel to the c-axis. The Nd3+ cations show a ferromagnetic arrangement at this temperature. Nd2MgRuO6 exhibits semiconductor like behaviour. Impedance analysis shows the contribution of grain alone to the conduction process till 231 K, above which both the grain and grain boundary contribute.

The crystal and magnetic structures of double perovskites of the type Ba2-xSrxYOsO6 have been studied by synchrotron X-ray and neutron powder diffraction methods, bulk magnetic susceptibility measurements and X-ray absorption spectroscopy. The structures were refined using a combined neutron and synchrotron data set and are based on ordered array of corner sharing YO6 and OsO6 octahedra, with the Ba/Sr cations being completely disordered. The structure evolves from cubic to monoclinic as the Sr content is increased, due to the introduction of cooperative tilting of the octahedra. Bulk magnetic susceptibility measurements demonstrate the oxides are all antiferromagnets. The decrease in symmetry results in a, non-linear, increase in the Neel temperature. Low temperature neutron diffraction measurements of selected examples show these to be type-I antiferromagnets. X-ray absorption spectra collected at the Os L3- and L2-edges confirm the Os is pentavalent in all cases, and there is no detectable change in the covalency of the Os cation as the A-cation changes. Analysis of the L3:L2 branching ratio shows that the spin-orbit coupling is constant and insignificant across the series. Australian Synchrotron Australian Research Council

The impact of chemical doping of the type Sr2−xAxYRuO6 (A=Ca, Ba) on the low temperature magnetic properties of Sr2YRuO6, probed using variable temperature magnetic susceptibility, neutron diffraction and heat capacity measurements, are described. Specific-heat measurements of un-doped Sr2YRuO6 reveal two features at ∼26 and ∼30 K. Neutron scattering measurements at these temperatures are consistent with a change from a 2D ordered state to the 3D type 1 AFM state. Magnetic and structural studies of a number of doped oxides are described that highlight the unique low temperature behavior of Sr2YRuO6 and demonstrate that doping destabilizes the intermediate 2D ordered state.

Iridates having pentavalent Ir${}^{5+}$ ions have emerged in the last years as an exciting playground to tune the atomic magnetic ground state from nonmagnetic $J=$0 to magnetic $J\ensuremath{\ne}$0 by either distortions of the IrO${}_{6}$ octahedra or increased orbital overlap. Here, the authors analyze the origin of the magnetic response observed in Sr${}_{2}$YIrO${}_{6}$ and reveal that in this case neither volume contraction nor structural distortions are sufficient to cause any change in the magnetic state of the Ir${}^{5+}$ ions.

Temperature dependent high resolution neutron diffraction studies demonstrate that the sequence and temperatures of the crystallographic phase transitions in NaTaO3 are not impacted by doping with 1% K to form Na0.99K0.01TaO3. Rietveld analysis of the neutron diffraction data shows the structural transitions to be: P b n m ↔ 723 K C m c m ↔ 803 K P 4 / n b m ↔ 893 K P m 3 ¯ m . The two orthorhombic polymorphs, Pbnm and Cmcm, differ fundamentally in the distortion and tilting of the octahedra, such that they cannot be obtained from each other via a continuous phase transition resulting in their co-existence between RT and 723 K. Chemical doping, does however, dramatically impact on the amount of the metastable Cmcm phase observed at room temperature.

Having identified a set of conditions that predispose a solid-state ionic compound to a pressure-induced valence transition, we investigated a series of Bi(iii) perovskite oxides. We found such a transition below 10 GPa in every case, including one that we synthesised for the first time (double perovskite-type Ba2BiOsO6).

Having identified a set of conditions that predispose a solid-state ionic compound to a pressure-induced valence transition, we investigated a series of Bi(iii) perovskite oxides. We found such a transition below 10 GPa in every case, including one that we synthesised for the first time (double perovskite-type Ba2BiOsO6).

We report on the high-temperature evolution of far- and mid- infrared reflectivity and emissivity spectra of ambient orthorhombic ErMnO3 from 12 K to sample decomposition above 1800 K. At low temperatures the number of phonons agrees with the predictions for orthorhombic space group D2h16-Pbnm (Z=4) and coexists with a paramagnon spin resonance and rare earth crystal field transitions. Increasing the temperature, a number of vibrational bands undergo profile broadening and softening approaching the orbital disordered phase where the orthorhombic O' lower temperature cooperative phase coexists with cubic-orthorhombic O. O-ErMnO3 undergoes a first order order-disorder transition into the perovskite cubic phase at Tcubic ~1329 K where the three triple degenerate phonons allowed by the space group Pm-3m(Z=1) are identified. At about 800 K, a quantitative small polaron analysis of the orthorhombic mid-infrared real part optical conductivity shows that antisymmetric and symmetric breathing modes sustain the strongest electron-phonon interactions. Above Tcubic the bipolaron fingerprint profile is the mid-infrared dominant and only feature. Its appearance correlates with the localized screening of the highest vibrational mode reststrahlen band. We propose that the longitudinal optical mode macroscopic field screening is consequence of dynamically sharing {\delta} disproportioned eg electrons hovering over the JT distorted octahedral dimer [Mn (QJT)3+{\delta} (Mn(QJT)3-{\delta}))O6/2]2. A thermal driven insulator-metal transition is detected with onset ~1600 K. We also address the occurrence of an inhomogeneity induced THz band result of heating the samples in dry air, triggering Mn3+-Mn4+ double exchange, under the presence of Mn4+ smaller ions stabilizing the orthorhombic lattice.

The importance of sample history on the Pbnm-Cmcm phase coexistence in NaTaO 3 has been investigated using high resolution neutron and synchrotron X-ray diffraction methods. These results reveal that the phase coexistence, associated with the first order Pbnm-Cmcm phase transition, is not significantly affected by the annealing conditions used to prepare the sample. Rather it is found that compositional inhomogeneities or defects play a significant role in stabilising the high temperature Cmcm phase at room temperature.

The crystal structures of the series of ordered double perovskites Ba(2-x)Sr(x)YIrO6 (0 ≤ x ≤ 2) were refined using a combination of high-resolution synchrotron X-ray and high-intensity neutron diffraction data. The materials displayed a sequence of structures Fm3m(a(0)a(0)a(0)) (x = 0.6)--> I4/m(a(0)a(0)c(-)) (x = 1.0)--> I2/m(a(-)a(-)c(0)) (x = 1.4)--> P2(1)/n(a(-)a(-)c(+)) associated with increased tilting of the corner-sharing octahedra induced by increasing amount of the smaller Sr cation present. A similar sequence of transitions was induced by heating selected samples. Magnetic susceptibility measurements between 2 and 300 K showed no evidence for long-range magnetic ordering, an observation that was supported by neutron diffraction measurements, and rather strong spin-orbit coupling results in a Jeff = 0 ground state.

Micro-tubular SOFCs have shown an astonishing thermal shock resistance, many orders of magnitude larger than planar SOFCs, opening the possibility of being used in portable applications. However, only few studies have been devoted to study the degradation of large-area micro-tubular SOFCs. This work presents microstructural, electrochemical and long term degradation studies of single micro-tubular cells fabricated by high shear extrusion, operating in the intermediate range of temperatures (T∼700 °C). A maximum power of 7 W per cell has been measured in a wide range of fuel utilizations between 10% and 60% at 700 °C. A degradation rate of 360 mW/1000 h (8%) has been observed for cells operated over more than 1500 h under fuel utilizations of 40%. Higher fuel utilizations lead to strong degradations associated to nickel oxidation/reduction processes. Quick thermal cycling with heating ramp rates of 30 °C /min yielded degradation rates of 440 mW/100 cycles (9%). These reasonable values of degradation under continuous and thermal cycling operation approach the requirements for many portable applications including auxiliary power units or consumer electronics opening this typically forbidden market to the SOFC technology.

A simple crossover from two-dimensional to three-dimensional correlated disorder of O and N atoms on a cubic lattice has been discovered within the Ba1xSrxTaO2N series of perovskite oxynitrides. The crossover is driven by lattice expansion as x decreases, and provides a rapid increase in entropy due to a change from subextensive to extensive configurational entropy regimes., We thank STFC, UK for support for H. J. and provision of ISIS beamtime, and EPSRC for additional support. This work was also supported by the Ministerio de Economia y Competitividad (MINECO), Spain through Project MAT2017-86616-R, the Severo Ochoa Program SEV-2015-0496, and Fellowship support to AB (MAT2011-24757). We thank the ALBA synchrotron for the provision of beamtime and Prof. Rosa Palacin, Dr Carlos Frontera (ICMAB-CSIC) and Dr F. Fauth (ALBA) for assistance with data collection.

Hybrid organic-inorganic perovskites, MAPbX3 (X= halogen), containing methylammonium (MA: CH3-NH3+) in the large voids conformed by the PbX6 octahedral network, are the active absorption materials in the new generation of solar cells. CH3NH3PbBr3 is a promising alternative with a large band-gap that gives rise to a high open circuit voltage. A deep knowledge of the crystal structure and, in particular, the MA conformation inside the perovskite cage across the phase transitions undergone below room temperature, seems essential to establish structure-property correlations that may drive to further improvements. The presence of protons requires the use of neutrons, combined with synchrotron XRD data that help to depict subtle symmetry changes undergone upon cooling. We present a consistent picture of the structural features of this fascinating material, in complement with photocurrent measurements from a photodetector device, demonstrating the potential of MAPbBr3 in optoelectronics., Comment: Authors' version of the peer-reviewed manuscript. 19 pages, 7 figures

Hybrid organic-inorganic perovskites, MAPbX

We have established the stability and range of the cubic type II phase of Bi1 − xWxO1.5 + 1.5x using a combination of X-ray diffraction, neutron diffraction and X-ray absorption spectroscopy. Type II is a high temperature modification that can be obtained by quenching/rapid cooling of samples with compositions between x = 0.148 to x = 0.185. Slower cooling rates yield the stable low temperature polymorph, the tetragonal type Ib phase (Bi rich samples), and mixtures of type Ib and Aurivillius phase (W-rich samples). Throughout the entire solid solution range, type II exhibits a (3 + 3) dimensional incommensurate modulation with modulation vectors slightly smaller than 1/3 based on a cubic fluorite type subcell (δ-Bi2O3). The main structural motifs are well-defined tetrahedra of WO6 octahedra in a δ-Bi2O3-matrix, with additional W being incorporated on corners and face centers of the approximate commensurate 3 × 3 × 3 supercell in octahedral coordination, confirmed by XANES analysis of the W L3-edge. Impedance measurements reveal oxide ionic conductivities comparable to those of yttria-stabilised zirconia even after a decrease in ionic conductivity of about half an order of magnitude on thermal cycling due to transition to the tetragonal type Ib phase.

Ruthenium-containing double perovskites A

The crystal and magnetic structures of double perovskites of the type Ba

The A-site ordered perovskite oxide CdCu3Mn4O 12 has been synthesized for the first time in polycrystalline form under high pressure (7 GPa) and high temperature (1000 °C) conditions, required to stabilize the small Cd and Cu cations at the A positions of the perovskite. The crystal structure has been studied by X-ray powder diffraction at room temperature. This oxide crystallizes in the cubic space group Im3̄ (no. 204) with the unit-cell parameter a = 7.2179(5) Å at 300 K. The MnO6 network is extremely tilted, giving rise to a square planar coordination for Cu2+ cations. The magnetic characterization shows that this compound is ferrimagnetic with an ordering temperature TC = 347 K, well above room temperature. A metallic behavior is displayed between 10 and 300 K. Negative magnetoresistance (MR) of 15% is achieved at 10 K for H = 9 T; MR is still significant at room temperature, displaying values above 7% for H = 9 T. Density functional theory calculations carried out on the density of states lead to electronic and magnetic features in good agreement with the obtained experimental results. © 2014 American Chemical Society., We thank the financial support of the Spanish Ministry of Science and Innovation to project MAT2010-16404, of the Comunidad de Madrid to project S2009PPQ-1551 (QUIMAPRES), and of the Spanish Consolider Ingenio 2010 Program (project CDS2007-00045). J.S.-B. acknowledges receipt of a Ramón y Cajal Fellowship (RyC-2010-06276) from the Ministerio de Economía y Competitividad.

The temperature dependences of the structures of three polytypes of BaRuO

The temperature dependence of the structures of three polytypes of BaRuO3 have been investigated between room temperature and 1000 °C using high resolution Synchrotron X-ray diffraction. The structural studies reveal a systematic decrease in the Ru-Ru distance as the pressure required to prepare the polytype increases. The O-O distance across the shared face increases as the Ru-Ru separation decreases. The 9R and 4H polytypes undergo unexceptional changes with increasing temperature. In 6H-BaRuO3 there is an apparent increase in the Ru-Ru interaction around 650 °C and concurrent reduction in the O-O distance indicating an anomalous strengthening of the Ru-Ru interactions upon heating. Australian Synchrotron Australian Research Council

Novel members of the RNiRuO family of double perovskites have been prepared via a nitrate-citrate route, followed by annealing treatments in air in the 1100-1300 °C temperature range. The crystal and magnetic structures were characterized from neutron powder diffraction (NPD) data at 300 K and 2 K. All the samples are monoclinic, P2/n space group, with a √2a√2a2a unit-cell, compared to the simple perovskite a edge. Ni and Ru ions occupy distinct octahedral positions, with a certain antisite disordering. The magnetic structures are defined by the propagation vector k = 0. The magnetic moments of the Ni cations are antiparallel to the spins of the Ru sublattice; the structure can be described as ferromagnetic [011] layers antiferromagnetically (AFM) coupled to each other. For R = Ho and Er perovskites, the rare-earth moments participate in the magnetic structure: Ho cations are ordered along the direction (010) in alternate planes AFM coupled and the magnetic moments of Er are ferromagnetically ordered along the c axis., We thank the financial support of the Spanish Ministry of Economy and Competitiveness to the project MAT2013-41099-R.

Ruthenium containing double perovskites A2ScRuO6 have been synthesized as polycrystalline powders and structurally characterized using a combination of synchrotron X-ray and neutron powder diffraction methods. When A = Ba a hexagonal 6L perovskite structure is obtained if the synthesis is conducted at ambient pressure and a rock-salt ordered cubic structure obtained if the sample is quenched from high pressures. The Sr oxide Sr2ScRuO6 is obtained with a rock-salt ordered corner sharing topology. Heat capacity and bulk magnetic susceptibility measurements show the three oxides are antiferromagnets. Cubic Ba2ScRuO6 undergoes a metal-insulator transition near 270 K and hexagonal Ba2ScRuO6 is a semiconductor with an activation energy of 0.207 eV. The magnetic structures of the two rock-salt ordered double perovskites were established using powder neutron diffraction, and are described by k = (001) and k = (000) for the Ba and Sr oxides respectively, corresponding to type I antiferromagnetic structures, with ferromagnetic layers stacked antiferromagnetically. The ambient pressure hexagonal polymorph of Ba2ScRuO6 has partial Sc-Ru ordering at both the face-sharing B2O9 dimer and corner sharing BO6 sites. The magnetic structure is described by k = (½ 0 0) with the basis vector belonging to the irreducible representation Γ3. Australian Synchrotron Australian Research Council

The structural and magnetic properties of the two Ir(V) perovskites Ba2ScIrO6 and Sr2ScIrO6 have been established. The structures were refined using a combined neutron and Synchrotron data set. At room temperature the former has a cubic structure in space group Fm3 ̅m a = 8.1450(3) Å and the latter is monoclinic in P21/n with a = 5.6606(3) b= 5.6366(3) c = 7.9720(4) Å β = 89.977(5)°. Magnetization measurements show both oxides have magnetic moments close to zero as a consequence of strong spin-orbit coupling that results in a Jeff ~ 0 ground state. The distortion of the IrO6 octahedra in Sr2ScIrO6 is insufficient to generate crystal field splitting strong enough to quench the spin orbit coupling. Australian Synchrotron Australian Research Council

We report the occurrence of a magnetodielectric effect and its correlation with structure and magnetism in the $A$-site ordered chromate spinel oxides $\mathrm{Li}M\mathrm{C}{\mathrm{r}}_{4}{\mathrm{O}}_{8}$ ($M=\mathrm{Ga}$, In). In addition to magnetic and dielectric measurements, temperature dependent synchrotron and neutron diffraction experiments have been carried out for the Ga compound. The results are compared and contrasted with that of a corresponding conventional $B$-site magnetic chromate spinel oxide, $\mathrm{ZnC}{\mathrm{r}}_{2}{\mathrm{O}}_{4}$. Like $\mathrm{ZnC}{\mathrm{r}}_{2}{\mathrm{O}}_{4}$, the $A$-site ordered chromate spinels exhibit a magnetodielectric effect at the magnetic ordering temperature (${T}_{\mathrm{N}}\ensuremath{\sim}13--15\phantom{\rule{0.16em}{0ex}}\mathrm{K}$), resulting from magnetoelastic coupling through a spin Jahn-Teller effect. While the presence of a broad magnetic anomaly, associated with a short-range magnetic ordering (${T}_{\mathrm{SO}}\ensuremath{\sim}45\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) in $\mathrm{ZnC}{\mathrm{r}}_{2}{\mathrm{O}}_{4}$, does not cause any dielectric anomaly, a sharp change in dielectric constant has been observed in $\mathrm{LiInC}{\mathrm{r}}_{4}{\mathrm{O}}_{8}$ at the magnetic anomaly, which is associated with the opening of a spin gap (${T}_{\mathrm{SG}}\ensuremath{\sim}60\phantom{\rule{0.16em}{0ex}}\mathrm{K}$). Contrary to the In compound, a broad dielectric anomaly exists at the onset of short-range antiferromagnetic ordering (${T}_{\mathrm{SO}}\ensuremath{\sim}55\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) in $\mathrm{LiGaC}{\mathrm{r}}_{4}{\mathrm{O}}_{8}$. The differences in dielectric behavior of these compounds have been discussed in terms of breathing distortion of the $\mathrm{C}{\mathrm{r}}_{4}$ tetrahedra.

The ordered double perovskites Ba2-xSrxYIrO6 (x = 0—2) are prepared by calcination of stoichiometric mixtures of BaCO3, SrCO3, Y2O3, and Ir in air (alumina crucible, staged heating at 650—850 °C, 12 h per stage) followed by staged sintering in air at 1050—1350 °C (72 h per stage).

Polycrystalline samples of Sr2BIrO6 (B: In, Sc, Fe, Co, Ti) are prepared from citrate precursors using stoichiometric mixtures of Sr(NO3)2, IrO2, In2O3, Sc2O3, FeC2O4, Co(NO3)2, and TiC10H14O5 in aqueous solutions of citric acid and HNO3 followed by heating of the resulting resins (600 °C, 12 h) and calcination in air (1100—1200 °C, 12 h).

The preparation, crystal structure and magnetic properties of Sr2BIrO6 (B = In, Sc, Fe, Co or Ti) oxides are reported. For B = Sc, Fe, Co or In materials, X-ray and neutron powder diffraction (NPD) studies confirm the presence of 1:1 B-ordered perovskite-like structures crystallizing in the monoclinic I2/m and P21/n space groups, with unit-cell parameters a ≈ √2a0, b ≈ √2a0, c ≈ 2a0 and β ≈ 90°. Sr2TiIrO6 is described as a disordered perovskite with orthorhombic symmetry in the Pbnm space group. For B = Fe and Co, low-temperature NPD data and magnetic measurements indicate the existence of an antiferromagnetic structure resulting from the Fe3+/Co3+ magnetic moment interaction. X-ray absorption spectroscopy corroborates that the oxidation states of B and Ir are 3+ and 5+, respectively. The bond lengths and other structural features are discussed in the framework of other recently reported iridium double perovskites where iridium adopts tetravalent, pentavalent and hexavalent oxidation states, and in the wider context of Sr double perovskites., We thank the Spanish Ministry of Economy and Competitiveness (project MAT2013‐41099‐R) for financial support. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE‐AC02‐ 98CH10886.

Element- and orbital-selective x-ray absorption and magnetic circular dichroism measurements are carried out to probe the electronic structure and magnetism of Ir 5d electronic states in double perovskite Sr2MIrO6 (M=Mg, Ca, Sc, Ti, Ni, Fe, Zn, In) and La2NiIrO6 compounds. All the studied systems present a significant influence of spin-orbit interactions in the electronic ground state. In addition, we find that the Ir 5d local magnetic moment shows different character depending on the oxidation state despite the net magnetization being similar for all the compounds. Ir carries an orbital contribution comparable to the spin contribution for Ir4+ (5d5) and Ir5+ (5d4) oxides, whereas the orbital contribution is quenched for Ir6+ (5d3) samples. Incorporation of a magnetic 3d atom allows getting insight into the magnetic coupling between 5d and 3d transition metals. Together with previous susceptibility and neutron diffraction measurements, the results indicate that Ir carries a significant local magnetic moment even in samples without a 3d metal. The size of the (small) net magnetization of these compounds is a result of predominant antiferromagnetic interactions between local moments coupled with structural details of each perovskite structure., This work was partially supported by the Spanish MINECO Projects No. MAT2014-54425-R and No. MAT2013-41099-R and by the Comunidad de Madrid Project No. S2009/PPQ-1551. M. A. Laguna-Marco acknowledges CSIC and European Social Fund for a JAE-Doc contract. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357.

Polycrystalline Sr2CaIrO6 (I) and Sr2MgIrO6 (II) are prepared from citrate-nitrate precursors using Sr(NO3)2, IrO2, CaCO3, or MgO in aqueous citric acid and HNO3 evaporated at 140 °C and heated to 600 °C to decompose organic residues and NO3 (950 °C, 12 h, O2 pressure of 200 bar (I), 800 °C, 36 h for (II)).

Sr2CaIrO6 and Sr2MgIrO6 double perovskites have been prepared as polycrystalline powders from citrate precursors treated under oxygen pressure conditions. These compounds have been studied by X-ray and neutron powder diffraction (NPD), magnetic measurements, and X-ray absorption spectroscopy. The crystal symmetry of both oxides is monoclinic (space group P21/n), and the unit-cell parameters are related to the ideal simple perovskite cell as a ≈ √2a0, b ≈ √2a0, c ≈ 2a0, β ≈ 90°, where a0 is the edge of the simple cubic ABO3 perovskite. This superstructure is a result of the 1:1 ordering of Ca/Mg and Ir ions over the B site and the distortion of the oxygen sublattice, which leads to a tilted three-dimensional network of corner-sharing BO6 octahedra. Magnetic measurements indicate an antiferromagnetic ordering below TN = 58 K for Sr2CaIrO6 and 74 K for Sr2MgIrO 6. The low-temperature magnetic structure of Sr2CaIrO 6 was determined by NPD and selected from the possible magnetic solutions compatible with the P21/n space group according to the group-theory representation. The propagation vector is k = (1/2 1/2 0). A canted antiferromagnetic structure is observed below TN with an ordered magnetic moment of 1.33(2) μB for the Ir6+ cations. Sr2MgIrO6 does not present magnetic reflections. An X-ray absorption spectroscopy investigation gives an insight into the Ir oxidation states and suggests a hexavalent state for the Ca compound and an admixture of Ir6+ and Ir5+ for the Mg perovskite, in agreement with the NPD data, which indicate a significant oxygen understoichiometry for this material. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim., The authors thank the Spanish Ministry of Science and Technology (project number MAT2010-16404) and the Comunidad de Madrid (QUIMAPRES project number S2009PPQ-1551) for financial support.

Double perovskites containing Ir6+/Ir5+ with formula Sr2MIrO6 (M = Ni, Zn) have been synthesized under high oxygen pressure conditions. Their crystal structures have been studied by X-ray and neutron powder diffraction at room temperature (RT) and 2 K. At RT, these oxides crystallize in the monoclinic space group P21/n with unit-cell parameters a ≈ √2a0, b ≈ √2a0, and c ≈ 2a0, and β ≈ 90°. The thermal evolution of the structure of the Ni-containing compound shows the presence of two phase transition in the 373–673 K interval following the sequence P21/n → I4/m → Fm3m. These materials have also been characterized by magnetic measurements, suggesting the onset of antiferromagnetic interactions at TN = 58 and 46 K, for M = Ni, Zn, respectively. X-ray absorption spectroscopy sheds light on the oxidation states of M and Ir ions within these double perovskites.

Double perovskites containing Ir6+/Ir5+ with formula Sr2MIrO6 (M = Ni, Zn) have been synthesized under high oxygen pressure conditions. Their crystal structures have been studied by X-ray and neutron powder diffraction at room temperature (RT) and 2 K. At RT, these oxides crystallize in the monoclinic space group P21/n with unit-cell parameters a ≈ √2a0, b ≈ √2a0, and c ≈ 2a0, and β ≈ 90. The thermal evolution of the structure of the Ni-containing compound shows the presence of two phase transition in the 373-673 K interval following the sequence P21/n → I4/m → Fm3̄m. These materials have also been characterized by magnetic measurements, suggesting the onset of antiferromagnetic interactions at T N = 58 and 46 K, for M = Ni, Zn, respectively. X-ray absorption spectroscopy sheds light on the oxidation states of M and Ir ions within these double perovskites. © 2013 American Chemical Society., We acknowledge the financial support of the Spanish Ministry of Science and Technology to the project MAT2010-16404 and of the Comunidad de Madrid to the Project S2009PPQ-1551.