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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.

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 LiNbO

[EN] Using single-crystal neutron diffraction we show that the magnetic structure Ni3TeO6 at fields above 8.6 T along the c axis and low temperature changes from a commensurate collinear antiferromagnetic structure with spins along c and ordering vector QC=(001.5) to a conical spiral with propagation vector QIC=(001.5±δ), δ∼0.18, having a significant spin component in the (a,b) plane. We determine the phase diagram of this material in magnetic fields up to 10.5 T along c and show the phase transition between the low field and conical spiral phases is of first order by observing a discontinuous jump of the ordering vector. QIC is found to drift both as a function of magnetic field and temperature. Preliminary inelastic neutron-scattering data reveal that the spin-wave gap in zero field has minima exactly at QIC and a gap of about 1.1 meV consisting with a crossover around 8.6 T. Further, a simple magnetic Hamiltonian accounting in broad terms for these is presented. Our findings confirm the exclusion of the inverse Dzyaloshinskii-Moriya interaction as a cause for the giant magnetoelectric due to symmetry arguments. In its place we advocate for the symmetric exchange striction as the origin of this effect., We are indebted to J. A. Alonso for support and access to his synthesis laboratory. We thank J. White for creating the software for planning experiments in the restricted geometry of the horizontal-field magnet at SINQ. The project was founded by the Danish Agency for Research and Innovation through DanScatt grant 7055-00007B. J.L. was supported by Nordforsk project 81695: NNSP School, and by the Paul Scherrer Institute. We also thank S. Ward for assistance with the SpinW computations. Neutron-scattering experiments were performed at the SINQ neutron source, Paul Scherrer Institute, Villigen, Switzerland and at the BER-II facility, Helmholz Center, Berlin.

We present results of a multimethod investigation of the polar antiferromagnets $\mathrm{N}{\mathrm{i}}_{2}\mathrm{CoTe}{\mathrm{O}}_{6}$ and $\mathrm{NiC}{\mathrm{o}}_{2}\mathrm{Te}{\mathrm{O}}_{6}$, inspired by the colossal magnetoelectric effect present in $\mathrm{N}{\mathrm{i}}_{3}\mathrm{Te}{\mathrm{O}}_{6}$. Both compounds crystalize in the same polar space group $R3$ as $\mathrm{N}{\mathrm{i}}_{3}\mathrm{Te}{\mathrm{O}}_{6}$, preserving the crystal symmetry at least from room temperature down to 2 K. $\mathrm{N}{\mathrm{i}}_{2}\mathrm{CoTe}{\mathrm{O}}_{6}$ and $\mathrm{NiC}{\mathrm{o}}_{2}\mathrm{Te}{\mathrm{O}}_{6}$ undergo antiferromagnetic phase transitions at ${T}_{\mathrm{N}}=55$ and 52 K, and spin-flop transitions at an external magnetic field of approximately 8 and 4 T, respectively. Both compounds present an incommensurate antiferromagnetic helical structure with spins lying in the $ab$ plane, in contrast to the collinear one along the $c$ axis in $\mathrm{N}{\mathrm{i}}_{3}\mathrm{Te}{\mathrm{O}}_{6}$. Moreover, dielectric anomalies are observed at their antiferromagnetic phase transitions, suggesting a magnetoelectric behavior. Spin and lattice dynamics studies by a combination of infrared, Raman, and terahertz spectroscopies were performed. Below ${T}_{\mathrm{N}}$, in both $\mathrm{N}{\mathrm{i}}_{2}\mathrm{CoTe}{\mathrm{O}}_{6}$ and $\mathrm{NiC}{\mathrm{o}}_{2}\mathrm{Te}{\mathrm{O}}_{6}$, low-frequency spin excitations extremely sensitive to external magnetic field were observed. At least one of these magnons was simultaneously seen in Raman and THz spectra of both compounds, therefore we propose to assign them to electromagnons.

[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.

Y2CoRuO6 was synthesized as a B-site ordered double perovskite with distorted monoclinic P21/n symmetry and an average tilting of the CoO6 and RuO6 octahedra of ψ = 19.7°. DC magnetization measure...

Transition-metal-only perovskite oxides can introduce additional magnetic functionality with robust magnetoelectric properties but are rare. In this work we prepared a new transition-metal-only perovskite Mn2(Fe0.8Mo0.2)MoO6 at high pressure and temperature. Uniquely, Mn2(Fe0.8Mo0.2)MoO6 was discovered as a line phase upon composition modulation that was motivated from the above-room-temperature multiferroic Mn2FeMoO6 corundum phase. It exhibits ferrimagnetic Fe–Mo sublattice (TC = 194 K) and Mn sublattice antiferromagnetic (Tm ∼ 45 K) transitions. Below Tm the two sublattice orderings are coupled and give rise to canted components in both. A first-order field-induced transition is also observed below 45 K. Mn2(Fe0.8Mo0.2)MoO6 is a Mott variable range hopping semiconductor. These findings for the first time show that either an exotic perovskite or a corundum phase can be achieved by composition modulation besides the pressure effect.

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.

Perovskites (ABO

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.

New A2Fe(Mn0.5W0.5)O6 (A = Ca, Sr, Ba) double perovskite oxides have been prepared by ceramic techniques. X-ray diffraction (XRD) complemented with neutron powder diffraction (NPD) indicate a structural evolution from monoclinic (space group P21/n) for A = Ca to cubic (Fm-3m) for A = Sr and finally to hexagonal (P63/mmc) for A = Ba as the perovskite tolerance factor increases with the A2+ ionic size. The three oxides present different tilting schemes of the FeO6 and (Mn,W)O6 octahedra. NPD data also show evidence in all cases of a considerable anti-site disordering, involving the partial occupancy of Fe positions by Mn atoms, and vice-versa. Magnetic susceptibility data show magnetic transitions below 50 K characterized by a strong irreversibility between ZFC and FC susceptibility curves. The A = Ca perovskite shows a G-type magnetic structure, with weak ordered magnetic moments due to the mentioned antisite disordering. Interesting magnetostrictive effects are observed for the Sr perovskite below 10 K. Fil: García Ramos, Crisanto A.. Institute for Nuclear Research and Nuclear Energy; Bulgaria. Instituto de Ciencia de Materiales de Madrid; España. Consejo Superior de Investigaciones Científicas; España Fil: Larregola, Sebastian Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis. Instituto de Investigaciones en Tecnología Química. Universidad Nacional de San Luis. Facultad de Química, Bioquímica y Farmacia. Instituto de Investigaciones en Tecnología Química; Argentina Fil: Retuerto, María. Consejo Superior de Investigaciones Científicas. Instituto de Catálisis y Petroleoquímica; España Fil: Fernández Díaz, María Teresa. Institut Laue Langevin; Francia Fil: Krezhov, Kiril. Institute for Nuclear Research and Nuclear Energy; Bulgaria Fil: Alonso, José Antonio. Instituto de Ciencia de Materiales de Madrid; España. Consejo Superior de Investigaciones Científicas; España

We present a structural and spectroscopic study of the compound $\mathrm{N}{\mathrm{i}}_{2}\mathrm{MnTe}{\mathrm{O}}_{6}$, closely related to the polar antiferromagnet $\mathrm{N}{\mathrm{i}}_{3}\mathrm{Te}{\mathrm{O}}_{6}$ known to show a colossal magnetoelectric effect and pronounced elementary magnetoelectric excitations. We prepared single crystals and polycrystalline samples of $\mathrm{N}{\mathrm{i}}_{2}\mathrm{MnTe}{\mathrm{O}}_{6}$ showing the same polar structure as $\mathrm{N}{\mathrm{i}}_{3}\mathrm{Te}{\mathrm{O}}_{6}$ from room temperature down to 4 K with the $R3$ space-group symmetry. Magnetic and dielectric measurements have indicated an antiferromagnetic phase transition at ${T}_{\mathrm{N}}\ensuremath{\approx}70\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, almost 20 K higher than that of $\mathrm{N}{\mathrm{i}}_{3}\mathrm{Te}{\mathrm{O}}_{6}$. Extensive infrared, Raman, and terahertz spectroscopy experiments were employed for investigating lattice and spin excitations, revealing all phonons predicted by the factor group analysis. Terahertz spectra below ${T}_{\mathrm{N}}$ reveal one new excitation, which is strongly influenced by external magnetic field, thus assigned to a magnon.

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.

Pb2MReO6 (M = Co, Cr) perovskites were prepared by high pressure–high temperature method. Rietveld refinements of synchrotron powder X-ray diffraction show that the crystal structure of Pb2CoReO6 is trigonal (space group R-3) with almost complete ordering of Co and Re cations, while Pb2CrReO6 (space group Pm-3m) is a cubic perovskite with one single site for Cr and Re atoms. The difference between the symmetry and the degree of order was further clarified by X-ray absorption spectroscopy that establishes formal oxidation states in these phases as Pb2Co2+Re6+O6 and Pb2Cr3+Re5+O6. Pb2CrReO6 is a simple perovskite with a high magnetic ordering temperature of 643 K. Pb2CoReO6 is a double perovskite with −23% high field negative magnetoresistance at 10 K and 9 T. First-principles calculations of Pb2CoReO6 indicate a half metallic electronic structure.

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.

The AA′3B4O12-type quadruple perovskite LaCu3Ir4O12 prepared at high pressure (9 GPa) and temperature (1523 K) crystallizes in cubic symmetry (Im3, a = 7.52418(3) A) with square planar CuO4 and octahedral IrO6 coordination as established from synchrotron powder X-ray diffraction studies. Both crystal structure and X-ray absorption near edge spectroscopy analyses indicate formal oxidation states of LaCu2+3Ir3.75+4O12. The temperature dependence of resistivity of LaCu3Ir4O12 is metallic down to 10 K, with Femi-liquid behavior above T* ∼ 155 K, and non-Fermi-liquid behavior below T*. The two-fluid behavior of magnetic susceptibility and the dramatic downturn of the resistivity below T* indicate strong Cu2+ 3d and Ir3.75+ 5d orbital hybridization below T*, also supported by an enhanced electronic specific heat coefficient at low temperature. Theoretical calculations are in good agreement with the experimental results and show that the electronic structure of LaCu3Ir4O12 is different from that of CaCu2+3Ir4+4...

LaNiO perovskite is a promising electrocatalyst for the oxygen reduction reaction (ORR). In this work we study the effects of several key structural and morphological features on the performance of LaNiO perovskites for the ORR. We have prepared a series of LaNiO oxides via soft chemistry and calcined the final materials at different temperatures from 500 to 1000 °C. With the aim of obtaining morphological and structural information relevant to their performance the prepared oxides have been thoroughly characterized by BET, Power Neutron Diffraction (PND), XRD, HRTEM, STEM-HAADF and XPS. We have analysed the effect of extrinsic (surface area and particle size) and intrinsic factors (formation of oxygen vacancies, Ni oxidation states and occupancy of the e levels) on the current densities and specific activity, respectively, of several samples of LaNiO. The catalysts with higher catalytic activity are those in which the perovskite structure is properly formed, but display high surface areas., María Retuerto acknowledges the Juan de la Cierva program of the Spanish Ministry of Economy and Competitiveness for a grant (FPDI-2013-17582). Economic support from projects ENE2013-42322-R from the Spanish Ministry of Economy and Competitiveness, and project 201480E122 from the CSIC are also acknowledged. Project S2013/MAE-2882 is also acknowledged. J. A. Alonso and M. Teresa Fernández-Diaz thank the financial support of the Spanish Ministry of Science and Innovation to the project MAT2013-41099-R. Amaru González acknowledges the SECAT for a grant.

Inelastic neutron scattering has been used to study the magneto-elastic excitations in the multiferroic manganite hexagonal YMnO$_3$. An avoided crossing is found between magnon and phonon modes close to the Brillouin zone boundary in the $(a,b)$-plane. Neutron polarization analysis reveals that this mode has mixed magnon-phonon character. An external magnetic field along the $c$-axis is observed to cause a linear field-induced splitting of one of the spin wave branches. A theoretical description is performed, using a Heisenberg model of localized spins, acoustic phonon modes and a magneto-elastic coupling via the single-ion magnetostriction. The model quantitatively reproduces the dispersion and intensities of all modes in the full Brillouin zone, describes the observed magnon-phonon hybridized modes, and quantifies the magneto-elastic coupling. The combined information, including the field-induced magnon splitting, allows us to exclude several of the earlier proposed models and point to the correct magnetic ground state symmetry, and provides an effective dynamic model relevant for the multiferroic hexagonal manganites., Comment: 12 pages, 10 figures

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.

Recently CsTlCl3 and CsTlF3 perovskites were theoretically predicted to be potential superconductors if they are optimally doped. The synthesis of these two compounds, together with a complete characterization of the samples are reported. CsTlCl3 is obtained as orange crystals in two different polymorphs: a tetragonal (I4/m) and a cubic (Fm-3m) phase. CsTlF3 is formed as a light brown powder, also as a double cubic perovskite (Fm-3m). In all three CsTlX3 phases Tl1+ and Tl3+ were located in two different crystallographic positions that accommodate their different bond lengths. In CsTlCl3 some Tl vacancies are found in the Tl1+ position. The charge ordering between Tl1+ and Tl3+ was confirmed by x-ray absorption and Raman spectroscopy. The Raman spectroscopy of CsTlCl3 under high pressure (58 GPa) did not indicate any phase transition to a possible single Tl2+ state. However, the highly insulating material becomes less resistive with increasing high pressure, while undergoing a change in the optical properties, from transparent to deeply opaque red, indicative of a decrease of the band gap. The theoretical design and experimental validation of the existence of CsTlF3 and CsTlCl3 cubic perovskites is the necessary first step in confirming the theoretical prediction of superconductivity in these materials.

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.