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A nitrogen-containing covalent organic framework obtained from the polymerization of 1,3-dicyanobenzene has been used as a starting material for the synthesis of Fe/N/C catalysts for the oxygen reduction reaction (ORR). In this work we report the effect of the thermal treatments on the nature and catalytic properties of the catalysts obtained after the thermal treatments. After the first thermal treatment, the catalysts obtained contain metallic iron and iron carbide particles, along with a minority fraction of inorganic FeNx sites. After acid leaching and a second thermal treatment, FeNx sites remain in the catalysts, along with a minor fraction of graphite-wrapped Fe3C particles. Both catalysts display high activity for the ORR, with the catalyst subjected to acid leaching and a second thermal treatment, 2HT-1,3DCB, displaying higher ORR activity and a lower production of H2O2. This observation suggests that iron particles, such as Fe3C, display ORR activity but mainly toward the two-electron pathway. On the contrary, FeNx ensembles promote the ORR via the four-electron pathway, that is, via H2O formation.

A copper-chromium mixed oxide with Cu:Cr 1:2 ratio has been prepared by a microemulsion method and employed as support of CeO2 in different amounts from 5 to 80 wt%. The catalysts have been characterized by XRD, HRTEM/XEDS, H2-TPR and XPS. The characterization results have been examined in correlation with their catalytic properties for the CO-PROX process complemented by operando-DRIFTS. The catalysts are formed by composites of CuCr2O4 and CuO on which aggregates of CeO2 nanoparticles are dispersed. Incorporation of CeO2 onto the CuCr mixed oxide appreciably enhances the CO-PROX performance of the catalyst. This is related to a promotion of the generation and stabilization of active partially reduced copper sites during the course of the interaction of the catalysts with the reactant mixture and which are evidenced to be formed on both CuCr oxide and CeO2 components of the catalysts.

This study presents the influence of polymorphism on the magnetic properties of Co5TeO8. This compound with a spinel-like structure [Co2]A[Co3Te]BO8 was synthesized into two polymorphs: one disordered within a cubic Fd3m structure, where Co2+ and Te6+ ions are randomly distributed on the octahedral B sites [the disordered polymorph can also be presented as an inverse spinel of the formula Co(Co1.5Te0.5)O4] and the other ordered with a cubic P4332 structure where Co2+ and Te6+ ions are ordered on the B sites. The macroscopic magnetic measurements showed that both polymorphs present a ferrimagnetic ordering, below ∼40 K, and a second transition is also observed at 27 K for the ordered polymorph. Neutron powder diffraction data between room temperature and 1.7 K showed as well the presence of short-range magnetic ordered clusters, which appears for both polymorphs below 200 K. At lower temperature, these short-range orders are transformed into long-range ferrimagnetic orders. Below TC = 40 K, the colinear ferrimagnetic structure of the disordered polymorph is described with the I41/am'd' space group. The ordered polymorph undergoes an incommensurate ferrimagnetic spiral spin ordering below TC1 = 45 K, followed by a second magnetic phase transition at TC2 = 27 K. This last transition is associated with the emergence of an additional ferrimagnetic component and an abrupt change in the magnitude of the magnetic propagation vector k = [0, 0, γ] from γ = 0.086 at T = 30 K to γ ≈ 0.14 in the range between 27 and 1.7 K. The magnetic symmetry of the ordered polymorph is described with the P43(00γ)0 magnetic superspace group. We evidenced that the ordering of Co2+/Te6+ on the B sites changes all of the Co-Co and Co-O distances and thus all JAB, JAA, and JBB exchange interactions, between the A and B sites, which are able to stabilize the incommensurate spin modulation in the ordered polymorph.

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.

Understanding the electric dipole switching in multiferroic materials requires deep insight of the atomic-scale local structure evolution to reveal the ferroelectric mechanism, which remains unclear and lacks a solid experimental indicator in high-pressure prepared LiNbO3-type polar magnets. Here, we report the discovery of Zn-ion splitting in LiNbO3-type Zn2FeNbO6 established by multiple diffraction techniques. The coexistence of a high-temperature paraelectric-like phase in the polar Zn2FeNbO6 lattice motivated us to revisit other high-pressure prepared LiNbO3-type A2BB'O6 compounds. The A-site atomic splitting (∼1.0-1.2 A between the split-atom pair) in B/B'-mixed Zn2FeTaO6 and O/N-mixed ZnTaO2N is verified by both powder X-ray diffraction structural refinements and high angle annular dark field scanning transmission electron microscopy images, but is absent in single-B-site ZnSnO3. Theoretical calculations are in good agreement with experimental results and suggest that this kind of A-site splitting also exists in the B-site mixed Mn-analogues, Mn2FeMO6 (M = Nb, Ta) and anion-mixed MnTaO2N, where the smaller A-site splitting (∼0.2 A atomic displacement) is attributed to magnetic interactions and bonding between A and B cations. These findings reveal universal A-site splitting in LiNbO3-type structures with mixed multivalent B/B', or anionic sites, and the splitting-atomic displacement can be strongly suppressed by magnetic interactions and/or hybridization of valence bands between d electrons of the A- and B-site cations.

The electrooxidation of ethanol with Pt/C, PtRu/C and Pt3Sn in alkaline electrolyte has been studied using electrochemical coupled infrared spectroscopy. In order to identify the evolution of the species formed during the electrooxidation of ethanol infrared spectra have been collected in KOH/H2O and KOD/D2O electrolytes. Whereas the electrochemical studies show different catalytic performance for the EOR (Ethanol Oxidation Reaction) over the three catalysts in terms of onset potential and actual current density, especially at low potentials, the IR spectra show that carbonates and acetates are the main species formed during the EOR with all catalysts. The infrared spectra reveal that the scission of the C–C bond of ethanol takes place at the surface of Pt/C, PtRu/C and Pt3Sn catalysts at potentials as low as 20 mV vs RHE yielding carbonates. At higher potentials, C2 species resulting from the partial oxidation of ethanol to acetates are the main species detected with all catalysts. Upon the progress of the ethanol oxidation, the electrolyte becomes acidified and acetaldehyde and acetic acid (C2 species typically formed during the EOR at acid electrolyte) are observed.

[EN] Perovskites (ABO) with transition metals in active B sites are considered alternative catalysts for the water oxidation to oxygen through the oxygen evolution reaction (OER) and for the oxygen reduction through the oxygen reduction reaction (ORR) back to water. We have synthesized a double perovskite (ABB′O) with different cations in A, B, and B′ sites, namely, (LaSr)(NiMn)(NiRu)O (LSNMR), which displays an outstanding OER/ORR bifunctional performance. The composition and structure of the oxide has been determined by powder X-ray diffraction, powder neutron diffraction, and transmission electron microscopy to be monoclinic with the space group P2/n and with cationic ordering between the ions in the B and B′ sites. X-ray absorption near-edge spectroscopy suggests that LSNMR presents a configuration of ∼Ni, ∼Mn, and ∼Ru. This bifunctional catalyst is endowed with high ORR and OER activities in alkaline media, with a remarkable bifunctional index value of ∼0.83 V (the difference between the potentials measured at -1 mA cm for the ORR and +10 mA cm for the OER). The ORR onset potential (E) of 0.94 V is among the best reported to date in alkaline media for ORR-active perovskites. The ORR mass activity of LSNMR is 1.1 A g at 0.9 V and 7.3 A g at 0.8 V. Furthermore, LSNMR is stable in a wide potential window down to 0.05 V. The OER potential to achieve a current density of 10 mA cm is 1.66 V. Density functional theory calculations demonstrate that the high ORR/OER activity of LSNMR is related to the presence of active Mn sites for the ORR- and Ru-active sites for the OER by virtue of the high symmetry of the respective reaction steps on those sites. In addition, the material is stable to ORR cycling and also considerably stable to OER cycling., This work was supported by the ENE2016-77055-C3-3-R project from the Spanish Ministry of Economy and Competitiveness (MINECO) and PIE 201480E122 from CSIC. M.R. thanks MINECO’s Juan de la Cierva program for a grant (FPDI-2013-17582). F.C.-V. thanks the Spanish MEC for a Ramón y Cajal research contract (RYC-2015-18996). M.G. acknowledges the support from NSF-DMR-1507252 grant, NJ, USA.

A series of modified TS-1 catalysts was prepared by post-synthesis hydrothermal treatment of a parent TS-1 zeolite with solutions of different concentrations of tetrapropylammonium hydroxide (TPAOH). The treatment results in an advantageous improvement of the catalytic activity for the liquid oxidation of furfural with H2O2 to produce maleic acid. The highest yield of maleic acid (83%) was obtained when using 0.025 M TPAOH solution; while the maximum yield of the untreated TS-1 was 70%. The catalysts were thoroughly characterised by XRD, UV–Vis, ICP–OES, XPS, TEM, N2-adsorption, DRIFT of chemisorbed deuterated acetonitrile and TEM, in order to elucidate the origin of the catalytic improvement. The characterisation studies allowed us to conclude that, besides the well-known creation of mesopores within the zeolite primary particles, the TPAOH treatment also results in the increase of the hydrophobicity balance of the channels and cavities of the zeolites (via silanols removal). Both properties have a relevant effect on the improvement of the catalytic properties.

Pd-nanosheets with a lateral size of ca. 5 nm have been prepared by a simple chemical method. The Pd-nanosheets obtained exhibited a 2D morphology in which Pd atoms are preferentially exposed at the surface of the particles. In addition, a preferential exposure of Pd atoms in 111 planes is observed. As a result, the Pd-nanosheets exhibit a very high catalytic activity for the electrooxidation of ethanol in alkaline electrolyte, superior to that of commercial nanosized Pd particles. In situ infrared studies conducted during the electrooxidation of ethanol reveal that the scission of the C C bond of ethanol takes place at the surface of Pd-nanosheets at low potentials (as low as 30 mV). However, at higher potentials, the formation of partially oxidized species, typically acetates, is the preferred reaction pathway.

The electrooxidation of ethanol (EOR) in neutral media is attracting a great deal of attention, especially in the area of micro devices and microbial fuel cells. In this work we study the EOR with Pt/C in neutral electrolyte and followed the evolution of the species formed with the potential by in situ FTIR. The EOR in neutral electrolyte displays higher overpotentials than in acid electrolyte but follows a similar reaction pathway, producing mainly C1 species (CO ad , CO 2 ) and C2 species (acetaldehyde, and acetic acid). The scission of the C-C bond of ethanol takes place at 200 mV, a potential higher than in acid electrolyte. According to the FTIR experiments, the production of acetaldehyde during the EOR is higher than that of acetic acid. We have also studied the electrooxidation of acetaldehyde (AOR) in a neutral medium with FTIR. We observed the production of CO ad at 300 mV which is oxidized to CO 2 from E ≥ 700 mV.

In this work, we study the direct synthesis of DME using CO2-rich syngas, with a CO2/CO ratio similar to that obtained from the gasification of biomass, i.e., 1.9. We used catalytic beds consisting of physical mixtures of the benchmark catalysts used for the synthesis of methanol from syngas and for methanol dehydration to DME, namely Cu/ZnO/Al2O3 and γ-Al2O3, respectively. Our results show that the ratio between each catalytic phase determines the productivity and selectivity to DME, as well CO and CO2 conversions. Thus, higher total carbon conversions were obtained with the catalytic bed with the highest content of the Cu/Zn/Al2O3 phase. The presence of γ-Al2O3 allows to exceed the equilibrium conversion of CO for the syngas to methanol synthesis. The highest DME productivity is obtained with the catalytic bed containing equal amounts of both catalytic phases. In addition, we also show that other reaction variables such as temperature, pressure, and contact time also play an important role in terms of DME productivity. The presence of a high fraction of CO2 in the syngas results in a high production of H2O, which after long times on stream result in the deactivation of the Cu/ZnO/Al2O3 catalytic phase due to the sintering of the copper particles. The in situ removal of H2O via the addition of an H2O sorbent, zeolite 3A, into the catalytic bed, results in a significant enhancement of both carbon conversion and DME productivity.

[EN] In this work, we report the synthesis of Fe/N/C electrocatalysts using triazine based porous organic polymers as precursors. Iron-doped triazine porous organic polymers were obtained by in situ polymerization of iron pre- cursor and 1,2- or 1,4- dicyanobenzene (DCB). In order to obtain the actual catalyst, the polymer obtained was subjected to thermal treatment under NH3. The catalysts obtained exhibit activity and durability for the oxygen reduction reaction in acid electrolyte. Thorough characterization of the catalysts reveal the formation of several types of iron species, including metallic iron, iron carbides and Fe-Nx moieties. The latter species is the main responsible for the high activity measured for the oxygen reduction reaction in acid electrolyte. 1,2-DCB results in more active catalysts than 1,4-DCB due to the higher fraction of FeNx ensembles in the former, probably because vicinal positions of N-bearing groups are more prone to coordinate Fe atoms., This work was supported by PEGASUS project, funded by the European Union’s Horizon 2020 Programme (call JTI-FCH-2017-1) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement n° 779550, and by project ENE2016-77055-C3-3-R from the Spanish Ministry of Economy and Competitiveness (MINECO). We acknowledge the Spanish CRG BM25-SpLine, MINECO, The Spanish National Research Council (CSIC) and The European Synchrotron (The ESRF) for provision of synchrotron radiation facilities.

The activity of oxides towards the oxygen evolution reaction (OER) is usually tuned by changing the transition metal components and/or the surface facets. An important yet less studied feature is the repercussion of the lattice oxygen content of the oxide in the active site's coordination and catalytic performance. This is illustrated here for the Ba-Ni-O system. We synthesized two oxides with Ni in different coordination and oxidation states, namely BaNiO2 with Ni2+ in square-planar positions, and BaNiO2.78(2) with Ni3+ and Ni4+ in octahedral positions. We show that the square-planar configuration of Ni endows BaNiO2 with high intrinsic OER activity, comparable to the best catalysts in the literature. DFT indicates that progressively lowering the lattice oxygen content from BaNiO3 to BaNiO2 increases the Ni sites' affinity for the reaction intermediates, thereby lowering the OER overpotential. Thus, oxygen content is an important parameter in oxide catalysts, as it modulates the coordination, orbital splitting, oxidation number, and catalytic activity of the active sites.

Employing a combination of infrared, Raman and time-domain THz spectroscopies, we studied spin and lattice excitations in Ni 3 TeO 6 and its isostructural compounds obtained by substitution of Ni with Mn or Co. Low-temperature THz spectra revealed THz-range excitations sensitive to magnetic field; some of them can be assigned to electromagnons.

The nature and evolution of FeNxCy moieties in Fe/C/N catalysts has been studied by analysing Fe and N environments. TEM and Fe-XAS reveal the presence of FeNx moieties and Fe3C particles in the fresh catalyst. NEXAFS reveals the presence of two groups of (Fe)NxCy ensembles, namely (Fe)Nx-pyridinic and (Fe)Nx-pyrrolic. The architecture of the FeNxCy ensembles and their evolution during the ORR has been analysed by XAS, NEXAFS, and identical locations TEM. NxCy, FeNxCy and Fe3C species are partially removed during the ORR, resulting in the formation of Fe2O3 and Fe3O4 particles with different morphologies. The process is more severe in acid electrolyte than in alkaline one. (Fe)Nx-pyrrolic moieties are the main ones in the fresh catalysts, but (Fe)Nx-pyridinic groups are more stable after the ORR. The correlation between the evolution of the ORR activity and that of the FeNxCy ensembles indicates that FeNx-pyridinic ensembles are responsible for the ORR activity.

Fe/N/C catalysts are very active for the oxygen reduction reaction (ORR); however, the nature of the active site(s) is not fully understood. In this work, we study the performance of different types of N/C and Fe/N/C catalysts for the ORR, and the effect of the addition of NaCN. Phthalocyanine and graphene have been studied as model metal-free catalysts for the ORR. Fe-phthalocyanine (FePhcy), Fe-phthalocyanine dispersed in graphene (FePhcy/G) and Fe/N/G, have been used as model Fe-containing catalysts. FePhcy and FePhcy/G only contain Fe atoms coordinated to 4 nitrogen atoms. On the other hand, different species such as Fe-Nx and Fe3C coexist in Fe/N/G. In addition, C C ensembles are present in the graphene present in FePhcy/G and Fe/N/G. The ORR activity is characteristic of each catalyst, being the highest for the catalysts containing FeN4 ensembles. The addition of CN− results in the selective poisoning of the Fe-containing sites but it does not suppress the ORR activity of the Graphene containing samples. In-situ infrared spectroscopy studies during the ORR reveal that CN− poisoning of the Fe sites is reversible, desorbing at potentials less positive than ca. 600 mV. As a consequence, the ORR activity of the Fe-containing sites is recovered gradually.

Cationic rearrangement is a compelling strategy for producing desirable physical properties by atomic-scale manipulation. However, activating ionic diffusion typically requires high temperature, and in some cases also high pressure in bulk oxide materials. Herein, we present the cationic rearrangement in bulk Mn2 FeMoO6 at unparalleled low temperatures of 150-300 (o) C. The irreversible ionic motion at ambient pressure, as evidenced by real-time powder synchrotron X-ray and neutron diffraction, and second harmonic generation, leads to a transition from a Ni3 TeO6 -type to an ordered-ilmenite structure, and dramatic changes of the electrical and magnetic properties. This work demonstrates a remarkable cationic rearrangement, with corresponding large changes in the physical properties in a bulk oxide at unprecedented low temperatures.

Total control over the electronic spin relaxation in molecular nanomagnets is the ultimate goal in the design of new molecules with evermore realizable applications in spin-based devices. For single-ion lanthanide systems, with strong spin-orbit coupling, the potential applications are linked to the energetic structure of the crystal field levels and quantum tunneling within the ground state. Structural engineering of the timescale of these tunneling events via appropriate design of crystal fields represents a fundamental challenge for the synthetic chemist, since tunnel splittings are expected to be suppressed by crystal field environments with sufficiently high-order symmetry. Here, we report the long missing study of the effect of a non-linear (C4) to pseudo-linear (D4d) change in crystal field symmetry in an otherwise chemically unaltered dysprosium complex. From a purely experimental study of crystal field levels and electronic spin dynamics at milliKelvin temperatures, we demonstrate the ensuing threefold reduction of the tunnel splitting., M.A.S., U.B.H., K.L., and J.Be. acknowledge financial support from the Danish Research Councils for Independent Research (12-125226). M.A.S., U.B.H., M.R, and K.L. acknowledge DANSCATT for financial support for the neutron scattering experiments. For the experiments conducted at FRM II, this project has received funding from the European Union’s 7th Framework Programme for research, technological development and demonstration under the NMI3-II Grant number 283883. M.A.S. thanks the Oticon Foundation (16-2669), and the Augustinus Foundation (16-2917) for financial support in relation to a research stay at Institut für Physikalische Chemie, Universität Stuttgart, Germany. M.P. and J.v.S. thank the DFG for funding (SL104/5-1). K.S.P. thanks the Danish Research Council for Independent Research for a DFF-Sapere Aude Research Talent grant (4090-00201). E.B., A.A, and, J.Ba. acknowledge the financial support of Spanish MINECO project MAT2017-83468-R. M.J. acknowledges the Swiss National Science Foundation.

The first transition-metal-only double perovskite compound, Mn2+ Fe-2(3+) Re5+ O-6, with 17 unpaired d electrons displays ferrimagnetic ordering up to 520K and a giant positive magnetoresistance of up to 220% at 5K and 8 T. These properties result from the ferrimagnetically coupled Fe and Re sublattice and are affected by a two-to-one magnetic-structure transition of the Mn sublattice when a magnetic field is applied. Theoretical calculations indicate that the half-metallic state can be mainly attributed to the spin polarization of the Fe and Re sites.

Transition metal oxides containing cubic B4O4 subcores are noted for their catalytic activity in water oxidation (OER). We synthesized a series of ternary spinel oxides, AB2O4, derived from LiMn2O4 by either replacement at the tetrahedral A site or Co substitution at the octahedral B site and measured their electrocatalytic OER activity. Atomic emission and powder X-ray diffraction confirmed spinel structure type and purity. Weak activation of the OER occurs upon A-site substitution: Zn2+ > Mg2+ > A-vacancy > Li+ = 0. Zn and Mg substitution is accompanied by (1) B-site conversion of Mn(IV) to Mn(III), resulting in expansion and higher symmetry of the [Mn4O4]4+ core relative to LiMn2O4 (inducing greater flexibility of the core and lower reorganization barrier to distortions), and (2) the electrochemical oxidation potential for Mn(III)/IV) increases by 0.15–0.2 V, producing a stronger driving force for water oxidation. Progressive replacement of Mn(III/IV) by Co(III) at the B site (LiMn2–xCoxO4, 0 ≤ x ≤ 1.5...

Producing hydrogen (H2) by splitting water with fossil-free electricity is considered a grand challenge for developing sustainable energy systems and a carbon dioxide free source of renewable H2. Renewable H2 may be produced from water by electrolysis with either low efficiency alkaline electrolyzers that suffer 50–65% losses, or by more efficient acidic electrolyzers with rare platinum group metal catalysts (Pt). Consequently, research has focused on developing alternative, cheap, and robust catalysts made from earth-abundant elements. Here, we show that crystalline Ni5P4 evolves H2 with geometric electrical to chemical conversion efficiency on par with Pt in strong acid (33 mV dec−1 Tafel slope and −62 mV overpotential at −100 mA cm−2 in 1 M H2SO4). The conductivity of Ni5P4 microparticles is sufficient to allow fabrication of electrodes without conducting binders by pressing pellets. Significantly, no catalyst degradation is seen in short term studies at current densities of −10 mA cm−2, equivalent to ∼10% solar photoelectrical conversion efficiency. The realization of a noble metal-free catalyst performing on par with Pt in both strong acid and base offers a key step towards industrially relevant electrolyzers competing with conventional H2 sources.

Polar oxides are technically of great interest but difficult to prepare. Our recent discoveries predicted that polar oxides can be synthesized in the corundum-derivative A2BB'O6 family with unusually small cations at the A-site and a d(0) electron configuration ion at B'-site. When magnetic transition-metal ions are incorporated more interesting polar magnetic oxides can form. In this work we experimentally verified this prediction and prepared LiNbO3 (LN)-type polar magnetic Zn2FeTaO6 via high pressure and temperature synthesis. The crystal structure analysis indicates highly distorted ZnO6 and (Fe/Ta)O6 octahedra, and an estimated spontaneous polarization (PS) of ∼50 μC/cm(2) along the c-axis was obtained from point charge model calculations. Zn2Fe(3+)Ta(5+)O6 has a lower magnetic transition temperature (TN ∼ 22 K) than the Mn2FeTaO6 analogue but is less conductive. The dielectric and polarization measurements indicate a potentially switchable component.

We have expanded the double perovskite family of materials with the unusual combination of layered order in the A sublattice and rock salt order over the B sublattice to compounds NaLaFeWO6 and NaNdFeWO6. The materials have been synthesized and studied by powder X-ray diffraction, neutron diffraction, electron diffraction, magnetic measurements, X-ray absorption spectroscopy, dielectric measurements, and second harmonic generation. At room temperature, the crystal structures of both compounds can be defined in the noncentrosymmetric monoclinic P2(1) space group resulting from the combination of ordering both in the A and B sublattices, the distortion of the cell due to tilting of the octahedra, and the displacement of certain cations. The magnetic studies show that both compounds are ordered antiferromagnetically below T-N approximate to 25 K for NaLaFeWO6 and at similar to 21 K for NaNdFeWO6. The magnetic structure of NaNdFeWO6 has been solved with a propagation vector k = (1/2 0 1/2) as an antiferromagnetic arrangement of Fe and Nd moments. Although the samples are potential multiferroics, the dielectric measurements do not show a ferroelectric response.

Polar oxides are of much interest in materials science and engineering. Their symmetry-dependent properties such as ferroelectricity/multiferroics, piezoelectricity, pyroelectricity, and second-order harmonic generation (SHG) effect are important for technological applications. [1] However, polar crystal design and synthesis is challenging, because multiple effects, such as steric or dipole-dipole interactions, typically combine to form non-polar structures; so the number of known polar materials, especially polar magnetoelectric materials, is still severely restricted. [2] Therefore, it is necessary for the material science community to develop new strategies to create these materials.

Four oxyfluorides of the title series (x = 0.00, 0.25, 0.50, 0.75) have been stabilized by topotactic treatment of perovskite precursors Sr1−xBaxFeO3−δ prepared by soft-chemistry procedures, yielding reactive materials that can easily incorporate a substantial amount of F atoms at moderate temperatures, thus avoiding the stabilization of competitive SrF2 and BaF2 parasitic phases. XRD and Neutron Powder Diffraction (NPD) measurements assess the phase purity and yield distinct features concerning the unit cell parameters’ variation, the Sr and Ba distribution, the stoichiometry of the anionic sublattice and the anisotropic displacement factors for O and F atoms. The four oxyfluorides are confirmed to be cubic in all of the compositional range, the unit cell parameters displaying Vergard’s law. All of the samples are magnetically ordered above room temperature; the magnetic structure is always G-type antiferromagnetic, as shown from NPD data. The ordered magnetic moments are substantially high, around 3.5 μB, even at room temperature (RT). Temperature-dependent Mössbauer data allow identifying Fe3+ in all of the samples, thus confirming the Sr1−xBaxFeO2F stoichiometry. The fit of the magnetic hyperfine field vs. temperature curve yields magnetic ordering TN temperatures between 740 K (x = 0.00) and 683 K (x = 0.75). These temperatures are substantially higher than those reported before for some of the samples, assessing for stronger Fe-Fe superexchange interactions for these specimens prepared by fluorination of citrate precursors in mild conditions., We thank the financial support of the Spanish Ministry of Science and Innovation to the project MAT2013-41099-R. María Retuerto acknowledges the Juan de la Cierva program of the Spanish Ministry of Economy and Competitiveness for the grant FPDI-2013-17582. We thank the PSI (Switzerland) for providing neutron beam time. We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI).

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 title perovskites (obtained from citrate precursors at 900 °C for 12 h) are characterized by XRD, neutron powder diffraction, and magnetic measurements.

Pb2MnTeO6, a new double perovskite, was synthesized. Its crystal structure was determined by synchrotron X-ray and powder neutron diffraction. Pb2MnTeO6 is monoclinic (I2/m) at room temperature with a regular arrangement of all the cations in their polyhedra. However, when the temperature is lowered to similar to 120 K it undergoes a phase transition from I2/m to C2/c structure. This transition is accompanied by a displacement of the Pb atoms from the center of their polyhedra due to the 6s2 lone-pair electrons, together with a surprising off-centering of Mn2+ (d5) magnetic cations. This strong first-order phase transition is also evidenced by specific heat, dielectric, Raman, and infrared spectroscopy measurements. The magnetic characterizations indicate an anti-ferromagnetic (AFM) order below TN approximate to 20 K; analysis of powder neutron diffraction data confirms the magnetic structure with propagation vector k = (0 1 0) and collinear AFM spins. The observed jump in dielectric permittivity near similar to 150 K implies possible anti-ferroelectric behavior; however, the absence of switching suggests that Pb2MnTeO6 can only be antipolar. First-principle calculations confirmed that the crystal and magnetic structures determined are locally stable and that anti-ferroelectric switching is unlikely to be observed in Pb2MnTeO6.

A series of perovskites Sr 4− x La x Fe 3 ReO 12 ( x =0.0, 1.0, 2.0) has been prepared by wet chemistry methods. The structure analyses by powder X-ray and neutron diffraction and electron microscopy show that these compounds adopt simple perovskite structures without cation ordering over the B sites: tetragonal ( I 4 /mcm ) for x =0.0 and 1.0 and orthorhombic ( Pbmn ) for x =2.0. The oxidation states of the cations in the compound with x =0.0 appear to be Fe 3+/4+ and Re 7+ and decrease for both with La substitution as evidenced by X-ray absorption spectroscopy. All the compounds are antiferromagnetically ordered above room temperature, as demonstrated by Mossbauer spectroscopy and the magnetic structures, which were determined by powder neutron diffraction. The substitution of Sr by La strongly affects the magnetic properties with an increase of T N up to ∼750 K.

Magnesium base alloys are very attractive for hydrogen storage due to their large hydrogen capacity, small weight and low-cost. We have designed a new synthesis method for the ternary metal hydride perovskite system Na1−xLixMgH3, based on the direct reaction of simple hydrides under high-pressure and moderate-temperature conditions. Well-crystallized samples were obtained in a piston-cylinder hydrostatic press at moderate pressures of 2 GPa and temperatures around 750 °C from mixtures of MgH2, NaH and LiH enclosed in gold capsules. X-ray and neutron powder diffraction analysis were used to identify the purity of the samples and provide an accurate description of the crystal structure features (GdFeO3 type). Na1−xLixMgH3 hydrides series (0 ≤ x ≤ 0.18) show an orthorhombic symmetry with space group Pnma (No. 62). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) have been carried out to determine the hydrogen desorption temperatures.

We describe a simplified synthesis method for the metal hydride Mg2NiH4, based on the direct reaction of MgH2 hydride and Ni metal (2MgH2 + Ni → Mg2NiH4) by ball-milling under nitrogen atmosphere at room temperature. Different milling times and different MgH2:Ni atomic stoichiometries have been evaluated, in order to optimize the synthesis conditions. X-ray diffraction (XRD) analysis was used to identify the purity of the samples and quantify the present phases, through Rietveld analyses of the crystal structure, which can be defined in the cubic space group F m 3 ¯ m . Thermogravimetric analysis has been carried out to determine not only the hydrogen desorption temperature but also the hydrogen contents. Scanning electron microscopy images show an average particle size of 1–2 μm for Mg2NiH4; the microstructure presents a sponge-like aspect adequate to favor the absorption and desorption processes.

Many transition-metal oxides in elevated valence states [e.g. Mn(V), Co(IV), Ni(III), Cu(III) ] present a metastable character and, given the difficulty of their synthesis, have been relatively little studied. However, they are very interesting materials presenting strong electronic correlations that are bound to exotic properties such as superconductivity, metal behavior, metal–insulator transitions or colossal magnetoresistance. The metastability of these compounds requires special synthesis conditions such as the application of high pressure. In the last years, we have prepared and investigated a good number of materials belonging to several families such as RNiO3 (R = rare earths), Ba3Mn2O8 , (Ba,Sr)CoO3 , La2(Ni,Co)O4+δ , etc. In the study and correct characterization of these oxides it has been decisive the use of elastic neutron diffraction, most of the times in powder samples. This technique has allowed us to access the structural details typically related to the octahedral tilting in perovskite structures, the oxygen stoichiometry and order–disorder of the oxygen sublattice, the distinction between close elements in the Periodic Table, the resolution of magnetic structures and, in general, the establishment of a correlation between the structure and the properties of interest. This letter is organized around the binomial "high-pressure synthesis" and "characterization by neutron diffraction" and illustrated with some selected examples among the metastable materials above mentioned.

We describe the preparation and characterization of a new ferrimagnetic oxide of composition TbFeMnO 5 . It has been prepared by a soft-chemistry procedure, involving the thermal treatment at moderate temperatures (600 °C) of citrate precursors, subsequently annealed at 900 °C under high oxygen pressure (200 bar). The crystallographic and magnetic structures have been studied by neutron powder diffraction (NPD) experiments, in complement with magnetization and susceptibility measurements. TbFeMnO 5 is an orthorhombic, Pbam space group; the crystal structure contains infinite chains of Mn 4+ O 6 edge-sharing octahedra running along the c axis and interconnected by Fe 3+ O 5 pyramids and TbO 8 units. Some anti-site disorder is observed from the NPD refinements: about 9% of the 4 f octahedral sites are occupied by Fe 3+ cations and about 18% of the pyramidal 4 h positions contain Mn 4+ . These structural features are confirmed by Mossbauer spectroscopy: two sites for Fe 3+ are identified; one of them corresponds to almost regular octahedra (at 4 f positions), characterized by nearly equal Mn/Fe–O distances of 1.906A at RT (from NPD data), giving a quadrupole doublet in the Mossbauer spectra at RT; the second contributes to a more intensive doublet corresponding to a distorted square pyramid Fe 3+ O 5 (at 4 h sites), for which NPD data demonstrates an axial distortion with three sets of Mn/Fe–O distances at 1.981(4), 1.851(5) and 1.944(3) A. Both doublets are broadened by the Fe/Mn disorder over these sites. TbFeMnO 5 undergoes long-range magnetic ordering below T C ≈ 175 K , as shown from magnetization, NPD and Mossbauer spectroscopy data. The propagation vector k = 0 defines a ferrimagnetic structure that initially concerns the Mn 4+ and Fe 3+ cations, in a magnetic arrangement defined by the basis vectors ( 0 , 0 , F z ) and ( 0 , 0 , F z ′ ) respectively. The Tb 3+ sublattice is ordered at 2 K with a spin arrangement given by ( 0 , 0 , F z ″ ) . The refined magnetic moments at this temperature are 2.66 ( 12 ) μ B for the Mn 4+ , − 0.8 ( 2 ) μ B for the Fe 3+ and − 2.0 ( 3 ) μ B for the Tb 3+ cations.

We have designed a new synthesis method for the ternary metal hydride Mg2FeH6 based on the direct reaction of simple hydrides under high-pressure conditions. Well-crystallized samples were prepared in a piston-cylinder hydrostatic press at 2 GPa and temperatures around 750 °C from mixtures of MgH2 and Fe enclosed in gold or platinum capsules. Seven different samples have been prepared under different conditions. X-ray powder diffraction analysis was used to identify and assess the purity of the samples, through Rietveld analyses of the crystal structure (K2PtCl6-type). Mg2FeH6 shows a cubic symmetry with space group Fm-3m. SEM images show an average particle size of 1–2 μm for Mg2FeH6; the microcrystals present well-grown faces and display a high homogeneity of shapes and sizes. Thermogravimetric analysis has been carried out to determine not only the hydrogen desorption temperature but also the hydrogen contents.

The crystal structure of CaH2 has been studied from neutron powder diffraction (NPD) at 295 K in a non-deuterated sample; a good quality NPD pattern was obtained in spite of the hydrogen incoherent scattering. The structure was refined by the Rietveld method in the Pnma space group (No. 62), Z = 4, with unit-cell parameters a = 5.9600(1), b = 3.6006(7) and c = 6.8167(1) Å. The two kinds of crystallographically independent H atoms, H1 and H2, are located in tetrahedral and square-pyramidal cavities, respectively, while Ca ions are nine-fold coordinated to hydrogen atoms. The average 〈Ca—H1〉 and 〈Ca—H2〉 bond lengths are 2.279 and 2.544 Å, respectively. Bond valence calculations show that Ca—H1 bonds are under compressive stress, whereas Ca—H2 bonds undergo tensile stress in a structure with a relatively high global instability index. It is also remarkable that the displacement factors for H2 are significantly larger than for H1, suggesting an increased lability for the Ca—H2 bonds. We provide with an analysis of the isotope effect, by comparing the present results on CaH2 with literature data on CaD2; we indeed observe a higher distortion of the H1 and H2 coordination polyhedra with respect to the deuteride, as observed in other isostructural dihydrides.

Four members of a new series of oxides of nominal stoichiometry NdMn 2-x Ru x O 5 (x = 0.1, 0.2, 0.4, 0.5) have been prepared in polycrystalline form by wet-chemistry procedures followed by thermal treatment at 1000°C. NdMn 2-x Ru x O 5 materials are isostructural with NdMn 2 O 5 . Their crystal structures have been refined from XRD data and, for the x = 0.5 oxide, from NPD data in the space group Pbam. The crystal contains infinite chains of (Mn,Ru) 4+ O 6 octahedra sharing edges, linked together by dimeric (Mn) 3 +O 5 square pyramidal units and NdO 8 polyhedra. There is a slight anti-site disordering effect, implying that some Ru cations are introduced at the pyramidal positions. For x = 0.5, the crystallographic formula is Nd 3+ (Mn 2+ 0.572(4) Ru 4+ 0.428(4) ) oct (Mn 3+ 0.964(8) -Ru 3+ 0.036(8) ) pyr O 4.92(4) . The abrupt contraction of the c axis for x = 0.5, together with the short (Mn,Ru)—(Mn,Ru) distances (2.60 A) observed within the chains of octahedra suggest the presence of Ru-Ru metal bonding for this composition. The magnetic properties are characteristic of spin-glasses, as confirmed by ac susceptibility measurements.

The synthesis of the Ca2Fe1+xW1–xO6 perovskite solid solution in the range x = 0.13–0.33 by a soft-chemistry procedure followed by annealing at different temperatures under different reducing atmospheres, and its characterization by X-ray and neutron diffraction together with magnetic measurements, is reported. The average Fe oxidation state in this series varies smoothly from Fe2.46+ in Ca2Fe1.13W0.87O6 to Fe3+ in Ca2Fe1.33W0.67O6. A detailed description of the crystal structure of four selected members of this series (x = 0.2, 0.23, 0.30, 0.33) is provided. All these compounds crystallize in the space group P21/n, with a ≈ b ≈ √2a0 and c ≈ 2a0, although they show different levels of long-range ordering between the two different cations placed at the B positions of the A2B′B″O6 perovskite structure. The driving force for this B′/B″ ordering is the charge difference between both kinds of cations, whereby the earlier members of the series, which contain a large proportion of Fe2+, are fully ordered whereas a strong antisite disorder effect is observed for the later members. The evolution of this antisite disordering has a dramatic influence on the magnetic properties across the series. Thus, for x = 0.20, the ferrimagnetic Curie temperature is above 400 K, although the saturation magnetization is very low, since there is a poor coherence between the antiferromagnetic patches formed by Fe–O–Fe interactions between the B′ and B″ positions of the perovskite, whereas the x = 0.33 compound, which has a lower TC of 310 K, shows a considerable saturation magnetization and a surprisingly strong magnetic neutron scattering for such a structurally disordered sample. The observed scattering can be ascribed to the good coherence obtained via Fe–O–Fe superexchange interactions among the Fe-rich, antiferromagnetically ordered patches that occur naturally thorughout the crystal. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

We have studied the crystal structure and magnetic properties of Y bFeMnO5 obtained by substituting Fe3+ for Mn3+ in the parent Y bMn2O5 compound, through x-ray (XRD) and neutron (NPD) powder diffraction, magnetometry and Mossbauer spectroscopy. The samples were prepared in polycrystalline form by a soft chemistry route, followed by thermal treatments under high-oxygen pressure. The Rietveld analysis of diffraction data shows that Y bFeMnO5 is isostructural with the oxides of stoichiometry RMn2O5 (R=rare earth, Y or Bi); the crystal structure is orthorhombic, Pbam space group, formed by chains of edge-sharing Mn4+O6 octahedra linked together by dimer groups of square pyramids Fe3+O5 and Y b3+O8 scalenohedra. A low level of disorder was established between the two transition metal positions 4f and 4h, occupied ideally by Mn4+ and by Fe3+: about 6% of Mn cations is replaced by Fe and 16% of Fe by Mn. Mossbauer spectroscopy data confirm the existence of two distinct crystallographic sites for Fe3+. One of them corresponds to almost regular octahedra (at 4 f positions), characterized by nearly equal Mn/Fe–O distances of 1.890 A at R T (from NPD data), giving a quadrupole doublet in the Mossbauer spectra at R T , broadened by the Fe/Mn disorder over this site. The second environment for Fe3+ contributes to a less broadened, but more intensive doublet in the Mossbauer spectra, which corresponds to a distorted square pyramid Fe3+O5 (at 4 h sites), for which NPD data demonstrates an axial distortion with three sets of Fe–O distances at 2.010(2) A, 1.859(5) A and 1.925(3) A. Magnetic studies and the thermal evolution of the NPD patterns show that below a transition temperature T c ∼ 178 K a long-range magnetic order is developed, resolved from NPD data as a ferrimagnetic structure with propagation vector k=0. The spin arrangements for the Mn4+ ions ( 4 f site) and Fe3+ ions ( 4 h site) are given by the basis vectors ( 0 , 0 , F z ) and ( 0 , 0 , F z ′ ), respectively. The Mn and Fe ordered magnetic moments are oriented along the c axis. At lower temperatures, the magnetic moment of the Y b3+ cation also participates in the magnetic structure, adopting a parallel arrangement with the Mn4+ spins; at T=1.8 K the magnitudes of the magnetic moments of the effective scatterers are 2.00(8), −3.04(17) and 0.35(14) μB, over the 4 f , 4 h and 4 g sites respectively.

New electron-doped derivatives of the Sr2FeMoO6 double perovskite have been studied. We report on the synthesis of the Sr2–xNdxFeMoO6 (x = 0, 0.2, 0.4 and 0.6) perovskite series by a wet chemistry procedure and its characterization. The evolution of the crystal and magnetic structure has been analyzed from neutron powder diffraction data. The partial replacement of Sr2+ cations by higher valence Nd3+ cations induces an injection of electrons into the Fe/Mo sublattice. A bond-valence study demonstrates that this electron-injection preferentially affects the Mo orbitals. This substitution leads to a distortion of the structure from tetragonal (I4/m) to monoclinic (P21/n) at room temperature for x ≥ 0.4 and an increase of the disorder between Fe and Mo cations. This effect has severe consequences for the magnetic properties, notably a reduction of the saturation magnetization and a non-monotonic behavior of the Curie temperature. Magnetotransport measurements show a significant increment of the magnetoresistance at low temperature for high values of x. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

The title compound has been first synthesized by a citrate technique followed by thermal treatments under moderate oxygen pressure conditions, and characterized by X-ray and neutron powder diffraction (NPD) and magnetization measurements. The crystal structure of DyCrMnO 5 has been refined from NPD data in the space group Pbam ; a =7.2617(6) A, b =8.5161(6) A, and c =5.7126(5) A at 295 K. This oxide is isostructural with R Mn 2 O 5 oxides ( R =rare earths) and it contains infinite chains of (Cr, Mn) 4+ O 6 octahedra-sharing edges, linked together by (Mn, Cr) 3+ O 5 pyramids and DyO 8 units. The high degree of antisite disordering exhibited by DyCrMnO 5 is noteworthy. The octahedral positions are occupied by roughly 50% of Mn and Cr cations, and the pyramidal groups contain two thirds of Mn and one third of Cr cations. We assume that Mn and Cr cations at the octahedral positions exhibit a tetravalent oxidation state, whereas the metals at the pyramidal positions are trivalent, in order to preserve the electroneutrality of this oxide. The susceptibility vs temperature curve of DyCrMnO 5 does not suggest the establishment of a long-range magnetic structure even at low temperatures; the NPD technique does not provide any signal of magnetic ordering, since the reflections do not show any magnetic contribution.

The title oxide has been obtained by replacing Mn 3+ by Fe 3+ in the parent oxide DyMn 2 O 5 . The crystallographic and magnetic structures have been analysed from neutron powder diffraction (NPD) data, in complement with susceptibility and magnetic measurements. DyFeMnO 5 is orthorhombic, belonging to the Pbam space group as the parent compound. The crystal structure contains infinite chains of edge-sharing Mn 4+ O 6 octahedra, interconnected by dimer units of Fe 3+ O 5 square pyramids. There is a certain antisite disorder in the crystal structure, with 8.0% of the Mn 4+ sites occupied by Fe cations, and 8.2% of the Fe 3+ positions occupied by Mn 3+ cations. The magnetization measurements show that DyFeMnO 5 presents magnetic order below T C ≈178 K; a study of the magnetic structure from the low-temperature NPD patterns indicates an antiferromagnetic coupling of the Mn 4+ and Fe 3+ spins, with the polarization of the Dy 3+ magnetic moments parallel to the those of the Fe sublattice.

Ba2MSbO6–δ (M = Fe, Co) double perovskites have been prepared in polycrystalline form by the solid-state reaction in air and characterized by X-ray diffraction (XRD), neutron powder diffraction (NPD), magnetic measurements and Mossbauer spectroscopy (for M = Fe). At room temperature, the crystal structure of both compounds can be defined as a 6-layered (6H) hexagonal perovskite structure (space group P63/mmc), with a = 5.7875(1) A and c = 14.2104(2) A for M = Fe and a = 5.7548(2) A and c = 14.1439(7) A for M = Co. M and Sb cations are randomly distributed over 4f and 2a Wickoff positions. The crystal structure is constituted by dimer units of (M,Sb)4fO6 octahedra sharing a face along the c axis; the dimers, which sharing corners, are connected by a single layer of (M,Sb)2aO6 octahedra. A severe degree of antisite disordering was detected in the Fe compound, which indicates the presence of 52.8 % Fe:47.2 % Sb at the (Fe,Sb)2a positions, whereas for Co, 64.0 % Sb is present at the(Co,Sb)2a sites. Mossbauer spectroscopic data for Ba2FeSbO6 confirm the existence of two distinct crystallographic sites for Fe3+. One site corresponds to very regular FeO6 octahedra (at 2a positions), which gives rise to a singlet in the Mossbauer spectra at room temperature and 77 K that is broadened by the strong Fe/Sb disorder over this site. The second environment for Fe3+ contributes to a broadened doublet in the Mossbauer spectra, which corresponds to a very distorted FeO6 octahedron (at 4f sites), for which neutron powder diffraction data demonstrates an axial distortion with two sets of Fe–O distances. This distortion is due to the repulsion of the highly charged Sb5+ cations within the dimer units of the (Fe,Sb)4fO6 octahedra, and it is much reduced in the Co compound, where the amount of Sb5+ at these sites is smaller. Magnetic measurements suggest the absence of long-range magnetic ordering for both samples, which is confirmed by low-temperature neutron diffraction data. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

RVO 3 perovskites have been prepared in the widest range of R 3+ ionic size, from LaVO 3 to LuVO 3 . Pure polycrystalline samples have been obtained by a citrate technique leading to reactive RVO 4 precursors, followed by thermal treatments in a reducing H 2 /N 2 (15/85%) flow to stabilize V 3+ cations. These oxides have been studied at room temperature by high-resolution neutron powder diffraction to follow the evolution of the crystal structures along the series. The distortion of the orthorhombic perovskite (space group Pbnm), characterized by the tilting angle of the VO 6 octahedra, progressively increases from La to Lu due to simple steric factors. Additionally, all of the perovskites show a subtle distortion of the VO 6 octahedra which significantly increases from La to Tb, and then slightly decreases for the last terms of the series. The stability of the crystal structure is also discussed in light of bond-valence arguments.

RNiMnO (R = Tb, Ho, Er, Tm) perovskites have been prepared by soft-chemistry techniques followed by high oxygen-pressure treatments; they have been investigated by X-ray diffraction, neutron powder diffraction (NPD), and magnetic measurements. In all cases the crystal structure is defined in the monoclinic P2/n space group, with an almost complete order between Ni and Mn cations in the octahedral perovskite sublattice. The low temperature NPD data and the macroscopic magnetic measurements indicate that all the compounds are ferrimagnetic, with a net magnetic moment different from zero and a distinct alignment of Ni and Mn spins depending on the nature of the rare-earth cation. The magnetic structures are different from the one previously reported for LaNiMnO, with a ferromagnetic structure involving Mn and Ni moments. This spin alignment can be rationalized taking into account the Goodenough-Kanamori rules. The magnetic ordering temperature (T) decreases abruptly as the size of the rare earth decreases, since T is mainly influenced by the superexchange interaction between Ni and Mn (Ni-O-Mn angle) and this angle decreases with the rare-earth size. The rare-earth magnetic moments participate in the magnetic structures immediately below T., We acknowledge the financial support of the Spanish Ministry of Economy and Competitiveness through the projects MAT2013-41099-R and CSD2007-00045 and funding of the Universidad Complutense de Madrid and Banco Santander through the project UCM2014-971703. J.S.-B. acknowledges receipt of a Ramón y Cajal Fellowship (RyC-2010-06276) from the Spanish Ministry of Economy and Competitiveness.

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.

Pb2MReO6 (M: Co (I), Cr (II)) perovskites are prepared by hot pressing of stoichiometric amounts of PbO, Cr2O3 (CoO), Re, and ReO3 (Pt capsule in multianvil press, 1423 (I) and 1473 K (II), 8 GPa, 30 min).

CsTlCl(3) and CsTlF(3) perovskites have been theoretically predicted to be superconductors when properly hole-doped. Both compounds have been previously prepared as pure compounds: CsTlCl(3) in a tetragonal (I4/m) and a cubic (Fm3̅m) perovskite polymorph and CsTlF(3) as a cubic perovskite (Fm3̅m). In this work, substitution of Tl in CsTlCl(3) with Hg is reported, in an attempt to hole-dope the system and induce superconductivity. The whole series CsTl(1-x)HgxCl(3) (x = 0.0, 0.1, 0.2, 0.4, 0.6, and 0.8) was prepared. CsTl(0.9)Hg(0.1)Cl(3) is tetragonal as the more stable phase of CsTlCl(3). However, CsTl(0.8)Hg(0.2)Cl(3) is already cubic with the space group Fm3̅m and with two different positions for Tl(+) and Tl(3+). For x = 0.4 and 0.5, solid solutions could not be formed. For x ≥ 0.6, the samples are primitive cubic perovskites with one crystallographic position for Tl(+), Tl(3+), and Hg(2+). All of the samples formed are insulating, and there is no signature of superconductivity. X-ray absorption spectroscopy indicates that all of the samples have a mixed-valence state of Tl(+) and Tl(3+). Raman spectroscopy shows the presence of the active Tl-Cl-Tl stretching mode over the whole series and the intensity of the Tl-Cl-Hg mode increases with increasing Hg content. First-principle calculations confirmed that the phases are insulators in their ground state and that Hg is not a good dopant in the search for superconductivity in this system.

We have studied the crystallographic structure, the oxidation states and the electronic structure of the ternary metal hydride Mg2FeH6 prepared by high-pressure and high-temperature reaction from the simple hydrides. This procedure provides a well-crystallized sample essential to perform Neutron Powder Diffraction (NPD) experiments. By NPD we accessed to subtle crystallographic details of Mg2FeH6, such as the hydrogen occupancy and the metal-hydrogen distances and angles; a full hydrogen composition with six hydrogen atoms per unit formula is determined. No magnetic structure was observed by NPD at 2 K. The oxidation states of the atoms inside the material and the electronic structure of Mg2FeH6 have been evaluated by x-ray absorption near-edge spectroscopy (XANES). Unexpectedly, the Fe K-edge spectrum shows a zero oxidation state of Fe atoms in agreement with Mössbauer data. Our results suggest a metallic nature of the hydride in contrast with the ionic character previously reported for this compound., We thank the financial support of the Spanish Ministry of Economy and Competitivity to the project MAT2013-41099-R, and for beamtime provision.