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This study proposes the full structural characterization of the most common MXene composition, Ti3C2Tx, which presents outstanding stability as anode for sodium ion batteries (100% of capacity retention after 530 cycles with charge efficiency > 99.7%). The structural investigation is carried out with a multi-technique approach that allows to explore both the short- and long- range structure, combining the analysis of X ray absorption spectroscopy, X-ray and neutron diffraction data, and TEM images. The diffraction data have been analyzed with the approach embodied in the Faults software, that accounts for the evaluation of extended defects, thus allowing to fit, for the first time, the MXene diffraction patterns. The analysis shows that the presence of static disorder on the termination sites induces a variability in the interlayer distance which affects the electrochemical properties.

Mixed-valent transition-metal compounds display complex structural, electronic and magnetic properties, which often intricately coexist. Here, we report the new ternary oxide GaV4O8, a structural sibling of skyrmion-hosting lacunar spinels. GaV4O8 contains a vanadium trimer and an original spin–orbital–charge texture that forms upon the structural phase transition at TS = 68 K followed by the magnetic transition at TN = 35 K. The texture arises from the coexistence of orbital molecules on the vanadium trimers and localized electrons on the remaining vanadium atoms. Such hybrid electrons create opportunities for novel types of spin, charge, and orbital order in mixed-valent transition-metal compounds.

Atomic arrangements in the mixed-valence oxide U

Control of cation ordering in ABX 3 perovskites is important to structural, physical and chemical properties. Here we show that thermal transformations of AA’BB’O 6 double double perovskites, where both A and B sites have 1:1 cation order, to (A 0.5 A’ 0.5 ) 2 BB’O 6 double perovskites with fully disordered A/A’ cations can be achieved under pressure in CaMnMnWO 6 and SmMnMnTaO 6 , enabling both polymorphs of each material to be recovered. This leads to a dramatic switch of magnetic properties from ferrimagnetic order in double double perovskite CaMnMnWO 6 to spin glass behaviour in the highly frustrated double perovskite polymorph. Comparison of double double and double perovskite polymorphs of other materials will enable effects of cation order and disorder on other properties such as ferroelectricity and conductivity to be explored.

Two $\mathrm{CaMn}B\mathrm{Sb}{\mathrm{O}}_{6}$ ($B=\mathrm{Cr}$ and Fe) high-pressure double double perovskites are reported. Spins in $\mathrm{Ca}\mathrm{Mn}\mathrm{Cr}\mathrm{Sb}{\mathrm{O}}_{6}$ order below ${T}_{\mathrm{C}}=49\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ into a collinear ferrimagnetic arrangement with spins in the $xy$ plane. $\mathrm{Ca}\mathrm{Mn}\mathrm{Fe}\mathrm{Sb}{\mathrm{O}}_{6}$ has the same ordered magnetic structure below ${T}_{\mathrm{C}1}=55$ K, but also shows a second magnetic transition at ${T}_{\mathrm{C}2}=21\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ where the spins reorient towards the $z$ axis. This may reflect a greater degree of magnetic disorder for $B=\mathrm{Fe}$ and A-B intersite charge transfer may also be significant.

The antiferromagnetic behavior of Fe3+ oxides of composition RE1.2Ba1.2Ca0.6Fe3O8, RE2.2Ba3.2Ca2.6Fe8O21, and REBa2Ca2Fe5O13 (RE = Gd, Tb) is highly influenced by the type of oxygen polyhedron around the Fe3+ cations and their ordering, which is coupled with the layered RE/Ba/Ca arrangement within the perovskite-related structure. Determination of the magnetic structures reveals different magnetic moments associated with Fe3+ spins in the different oxygen polyhedra (octahedron, tetrahedron, and square pyramid). The structural aspects impact on the strength of the Fe-O-Fe superexchange interactions and, therefore, on the Néel temperature (TN) of the compounds. The oxides present an interesting transition from three-dimensional (3D) to two-dimensional (2D) magnetic behavior above TN. The 2D magnetic interactions are stronger within the FeO6 octahedra layers than in the FeO4 tetrahedra layers., The antiferromagnetic behavior of Fe3+ oxides is highly influenced by the type of oxygen polyhedron around the Fe3+ cations and their ordering, which is coupled with the layered RE/Ba/Ca arrangement, within the perovskite-related structure in this striking (RE, Ba, Ca)3m+5nFe3m+5nO8m+13n (m, n) homologous series.

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

[EN] The BaZrCeYO-BaPrO perovskite system, of interest for high-temperature electrochemical applications involving mixed protonic-electronic conductivity, forms a solid-solution with a wide interval of Ba substoichiometry in the range Ba(CeZr)PrYO, 0 ≤ x ≤ 1. Structural phase transitions mapped as a function of temperature and composition by high-resolution neutron powder diffraction and synchrotron X-ray diffraction reveal higher symmetry for lower Pr content and higher temperatures, with the largest stability field observed for rhombohedral symmetry (space group, R3c). Rietveld refinement, supported by magnetic-susceptibility measurements, indicates that partitioning of the B-site cations over the A and B perovskite sites compensates Ba substoichiometry in preference to A-site vacancy formation and that multiple cations are distributed over both sites. Electron-hole transport dominates electrical conductivity in both wet and dry oxidizing conditions, with total conductivity reaching a value of 0.5 S cm for the x = 1 end-member in dry air at 1173 K. Higher electrical conductivity and the displacement of oxygen loss to higher temperatures with increasing Pr content both reflect the role of Pr in promoting hole formation at the expense of oxygen vacancies. In more reducing conditions (N) and at low Pr contents, conductivity is higher in humidified atmospheres (0.023 atm pHO) indicating a protonic contribution to transport, whereas the greater electron-hole conductivity with increasing Pr content results in lower conductivity in humidified N due to the creation of protonic defects and the consumption of holes., We thank the MINECO, Spain (ENE2015-66183-R and MAT2016-78362-C4-1-R), CSIC, Spain (i-link0743), and CAPES, Brazil (PVE, Proceso 88881.03418/2013-1). Access to the neutron facilities at the Institut Laue Langevin (Grenoble, France) and the National Synchrotron Light Laboratory (LNLS, Campinas, Brazil) under grant 5-24-55(D2B) and research proposal D10B-XRD1-16166, respectively, is gratefully acknowledged. We also thank the FCT, PTDC/CTM-EME/6319/2014, QREN, FEDER, and COMPETE Portugal and the European Social Fund, European Union. U.A. acknowledges the Universidad San Pablo for financial support. We would also like to thank Steven Kermorvant and Alexandre Bosser of the IUT, University of Rennes (France), for assistance.

High resolution and high intensity neutron powder diffraction are used to determine the temperature dependence of the crystallographic and magnetic structure of the orthoferrite CeFeO3. The high temperature G x -type magnetic coupling of the Fe-sublattice described by the Γ4 (G x A y F z ) irreducible representation changes at the spin reorientation temperature T SR = 228 K to a G y -type coupling of Γ1 (A x G y C z ). The spin reorientation is of first order and sees a hysteresis of about 2.5 K at T SR. Below 35 K faint magnetic peaks reflecting C z type magnetic coupling appear and are argued to be related to the Ce-sublattice. Magnetic moments at 2 K amount to μ Fe = 4.15 μ B and μ Ce = 0.11 μ B. CeFeO3 is only the second RFeO3 compound after DyFeO3 showing this ground state magnetic structure of the Fe-sublattice. The orthorhombic structure Pbnm is kept over the whole temperature range.

International audience; New Ni3TeO6-type (NTO) and double perovskite (DPv) polymorphs of Co3TeO6 are synthesised at pressures of 15 GPa. A complex elliptic helical magnetic order is observed in the NTO polymorph (TN1 = 58 K) that reorientates (42 K) and further splits (TN2 = 23.5 K) creating a coexisting helix. Increasing Co content within the Mn3-xCoxTeO6 system changes the dominant DPv phase to NTO structural type and drastically modifies the magnetic behaviour. DPv Co3TeO6 is the first A-site double cobaltite.

The most common MXene composition Ti3C2Tx(T = F, O) shows outstanding stability as anode for sodium ion batteries (100% of capacity retention after 530 cycles with charge efficiency >99.7%). However, the reversibility of the intercalation/deintercalation process is strongly affected by the synthesis parameters determining, in turn, significant differences in the material structure. This study proposes a new approach to identify the crystal features influencing the performances, using a structural model built with a multitechnique approach that allows exploring the short-range order of the lamella. The model is then used to determine the long-range order by inserting defective elements into the structure. With this strategy it is possible to fit the MXene diffraction patterns, obtain the structural parameters including the stoichiometric composition of the terminations (neutron data), and quantify the structural disorder which can be used to discriminate the phases with the best electrochemical properties.

The first triple perovskite with Mn in A- and 1 : 2 B-site order Mn3MnNb2O9, prepared using high pressure phase transformation of the magnetodielectric Mn4MnNb2O9, is reported herein. It has a complex magnetic behaviour with a transition from a collinear AFM into an evolving incommensurate spin density wave (SDW) further stabilised into a lock-in structure dictated by the B-site order.

Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c01212; International audience; Atomic arrangements in the mixed-valence oxide U3O7 are refined from high-resolution neutron scattering data. The crystallographic model describes a long-range structural order in a U60O140 primitive cell (space group P42/n) containing distorted cuboctahedral oxygen clusters. By combining experimental data and electronic structure calculations accounting for spin−orbit interactions, we provide robust evidence of an interplay between charge localization and the magnetic moments carried by the uranium atoms. The calculations predict U3O7 to be a semiconducting solid with a band gap of close to 0.32 eV, and a more pronounced charge-transfer insulator behavior as compared to the well-known Mott insulator UO2. Most uranium ions (56 out of 60) occur in 9-fold and 10-fold coordinated environments, surrounding the oxygen clusters, and have a tetravalent (24 out of 60) or pentavalent (32 out of 60) state. The remaining uranium ions (4 out of 60) are not contiguous to the oxygen cuboctahedra and have a very compact, 8-fold coordinated environment with two short (2 × 1.93(3) Å) “oxo-type” bonds. The higher Hirshfeld charge and the diamagnetic character point to a hexavalent state for these four uranium ions. Hence, the valence state distribution corresponds to 24/60 × U(IV) + 32/60 U(V) + 4/60 U(VI). The tetravalent and pentavalent uranium ions are predicted to carry noncollinear magnetic moments (with amplitudes of 1.6 and 0.8 μB, respectively), resulting in canted ferromagnetic order in characteristic layers within the overall fluorite-related structure.

The R 3TiSb5 ternary compounds, with R a light rare earth (La to Sm) have been reported to crystallize with the anti-Hf5CuSn3-type hexagonal structure (Pearson’s symbol hP18; space-group P63/mcm, N. 193). An early article that reported possible superconductivity in some of these intermetallic phases (namely those with R = La, Ce, and Nd) caught our attention. In this work, we have now refined the crystal structure of the R 3TiSb5 compounds with R = Ce, Pr and Nd by Rietveld methods using high-resolution neutron powder diffraction data. The magnetic ground states of these intermetallics have been investigated by low-temperature magnetization and high-intensity neutron diffraction. We find two different magnetic transitions corresponding to two related magnetic structures at T N1 = 4.8 K (k 1 = [0, 1/2, 1/8]) and T N2 = 3.4 K (k 2 = [0, 0, 1/8]), respectively for Ce3TiSb5. However, the magnetic ordering appears to occur following a peculiar hysteresis: the k 2-type magnetic structure develops only after the k 1-type phase fraction has first slowly ordered with time and the size of the ordered Ce3+ magnetic moment has become large enough to induce the second magnetic transition. At T = 1.5 K the maximum amplitude of the Ce moment in the coexisting phases amounts to μ Ce = 2.15 μ B. For Nd3TiSb5 an antiferromagnetic ordering below T N = 5.2 K into a relatively simpler commensurate magnetic structure with a magnetic moment of μ Nd = 2.14(3) μ B and magnetic propagation vector of k = [0, 0, 0], was determined. No evidence of superconductivity has been found in Nd3TiSb5. Finally, Pr3TiSb5 does not show any ordering down to 1.5 K in neutron diffraction while an antiferromagnetic ground state is detected in magnetization measurements. There is no sign of magnetic contribution from Ti atoms found in any of the studied compounds.

We report muon spin rotation (mu SR) and neutron diffraction on the rare-earth-based magnets (Mo2/3RE1/3)2AlC, also predicted as parent materials for two-dimensional (2D) derivatives, where the rare earth (RE) = Nd, Gd (only mu SR), Tb, Dy, Ho, and Er. By crossing information between the two techniques, we determine the magnetic moment (m), structure, and dynamic properties of all compounds. We find that only for RE = Nd and Gd the moments are frozen on a microsecond timescale. Out of these two, the most promising compound for a potential 2D high m magnet is the Gd variant, since the parent crystals are pristine with m = 6.5 +/- 0.5 mu B, Neel temperature of 29 +/- 1 K, and the magnetic anisotropy between out-of- and in-plane coupling is smaller than 10-8. This result suggests that magnetic ordering in the Gd variant is dominated by in-plane magnetic interactions and should therefore remain stable when exfoliated into 2D sheets. Funding Agencies|Israel Atomic Energy Commission Pazy Foundation grant; Knut and Alice Wallenberg (KAW) FoundationKnut & Alice Wallenberg Foundation; Flag-ERA JTC 2017 project "MORE-MXenes"

International audience; Two M2(SeO3)F2 fluoro-selenites (M = Mn 2+ , Ni 2+) have been synthesized using optimized hydrothermal reactions. Their 3D framework consists of 1D-[MO2F2] 4chains of edge-sharing octahedra with a rare topology of O-O and F-F alternating µ2 bridges. The inter-chain corner sharing connections are assisted by the mixed O vs. F anionic nature and develop a complex set of M-X-M super-exchanges calculated by LDA+U. Their interplay induce prominent in-chain antiferromagnetic frustration while the interchain exchanges are responsible for the cycloidal magnetic structure observed below TN ~21.5 K in the Ni 2+ case. For comparison the Mn 2+ compound develops a nearly collinear spin (canted) ordering below TN ~26 K with ferromagnetic chain-units.

Fe2O3 and Fe2O3:Ge nanofibers (NFs) were prepared via electrospinning and thoroughly characterized via several techniques in order to investigate the effects produced by germanium incorporation in the nanostructure and crystalline phase of the oxide. The results indicate that reference Fe2O3 NFs consist of interconnected hematite grains, whereas in Fe2O3:Ge NFs, constituted by finer and elongated nanostructures developing mainly along their axis, an amorphous component coexists with the dominant α-Fe2O3 and γ-Fe2O3 phases. Ge4+ ions, mostly dispersed as dopant impurities, are accommodated in the tetrahedral sites of the maghemite lattice and probably in the defective hematite surface sites. When tested as anode active material for sodium ion batteries, Fe2O3:Ge NFs show good specific capacity (320 mAh g−1 at 50 mA g−1) and excellent rate capability (still delivering 140 mAh g−1 at 2 A g−1). This behavior derives from the synergistic combination of the nanostructured morphology, the electronic transport properties of the complex material, and the pseudo-capacitive nature of the charge storage mechanism.

The development of new synthetic methodologies of perovskite oxynitrides is challenging but necessary for the search of new compounds and the investigation of new properties. Here, we report a new method of preparation of the perovskite LaTaON2 that has been investigated as a pigment and photocatalyst for water splitting. The synthesis proceeds through the solid-state reactions under N2 at 1500 °C between La2O3, LaN, and Ta3N5 or between LaN and TaON, which are completed after 3 h and lead to sintered, highly crystalline samples with particle sizes up to 1 μm. Nitrogen-deficient samples LaTaO1+xN2-x with x ≤ 0.35 are prepared by changing the N/O ratio in the mixture of reactants. Electron diffraction, synchrotron diffraction, and neutron diffraction studies on stoichiometric and nitrogen-deficient compounds indicate that they crystallize in the monoclinic space group I2/m with lattice parameters for LaTaON2 of a = 5.71458(7), b = 8.05987(10), c = 5.74772(6) Å, and β = 89.982(3)°. The three anion sites of the I2/m structure are partially occupied by oxygen and nitrogen, with a preference of nitride for two positions with occupancies of 77 and 88%. This anion distribution is different from that reported in previous studies of samples prepared by ammonolysis at lower temperature, suggesting that the synthesis conditions affect the anion order of this perovskite. Optical measurements indicate a band gap of about 1.9 eV, which is close to that observed in samples prepared by other methods. The determined dielectric permittivity for LaTaON2 εr ≈ 200, reported for the first time for a highly nitrided pseudocubic perovskite, is similar to that observed in perovskites with one nitrogen per formula., This work was supported by the Ministerio de Ciencia e Innovación, Spain [MAT2017-86616-R, MAT2017-85232-R, PID2019-107727RB-I00, PID2020-113805GB-I00, PID2020-118479RB-I00, and CEX2019-000917-S], from Generalitat de Catalunya (2017SGR1377 and 2017SGR581) and from CSIC through the i-LINK (LINKA20338) program. The authors thank Alba and ILL for beam time provision (experiments 2017092391 and 5-21-1113, respectively), Dr. François Fauth for assistance in data collection at Alba, and Dr. Bernat Bozzo (ICMAB-CSIC) for performing electrical measurements. The project was also supported by a 2020 Leonardo Grant for Researchers and Cultural Creators, BBVA Foundation. I.F. acknowledges the Ramón y Cajal contract RYC-2017-22531 and JRG acknowledges the AEAT predoctoral fellowship PRE2018-085204., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

Thermochemical splitting of CO2 and H2O via two-step metal oxide redox cycles offers a promising approach to produce solar fuels. Perovskite-type oxides with the general formula ABO3 have recently gained attention as an attractive redox material alternative to the state-of-the-art ceria, due to their high structural and thermodynamic tunability. A novel Ce-substituted lanthanum strontium manganite perovskite-oxide composite, La3+0.48Sr2+0.52(Ce4+0.06Mn3+0.79)O2.55 (LSC25M75) is introduced, aiming to bridge the gap between ceria and perovskite oxide-based materials by overcoming their individual thermodynamic constraints. Thermochemical CO2 splitting redox cyclability of LSC25M75 evaluated with a thermogravimetric analyzer and an infrared furnace reactor over 100 consecutive redox cycles demonstrates a twofold higher conversion extent to CO than one of the best Mn-based perovskite oxides, La0.60Sr0.40MnO3. Based on complementary in situ high temperature neutron, synchrotron X-ray, and electron diffraction experiments, unprecedented structural and mechanistic insight is obtained into thermochemical perovskite oxide materials. A novel CO2 splitting reaction mechanism is presented, involving reversible temperature induced phase transitions from the n= 1 Ruddlesden–Popper phase (Sr1.10La0.64Ce0.26)MnO3.88 (I4/mmm, K2NiF4-type) at reduction temperature (1350°C) to the n= 2 Ruddlesden–Popper phase (Sr2.60La0.22Ce0.18)Mn2O6.6 (I4/mmm, Sr3Ti2O7-type) at re-oxidation temperature (1000°C) after the CO2 splitting step.

High resolution and high intensity neutron powder diffraction are used to determine the temperature dependence of the crystallographic and magnetic structure of the orthoferrite CeFeO

The first representative of a novel class of mixed‐anionic compounds, the sulfate hydride Na3SO4H, and the corresponding deuteride Na3SO4D were obtained from the solid‐state reaction of NaH or NaD with dry Na2SO4. Precise reaction control is required, because too harsh conditions lead to the reduction of sulfate to sulfide. A combined X‐ray and neutron diffraction study revealed that the compound crystallizes in the tetragonal space group P4/nmm with the lattice parameters a=7.0034(2) Å and c=4.8569(2) Å. The sole presence of hydride and absence of hydroxide ions is proven by vibrational spectroscopy and comparison with spectra predicted from quantum chemical calculations. 1H and 23Na MAS NMR spectra are consistent with the structure of Na3SO4H: a single 1H peak at 2.9 ppm is observed, while two peaks at 15.0 and 6.2 ppm for the inequivalent 23Na sites are observed. Elemental analysis and quantum chemical calculations further support these results., The novel sulfate hydride—the first representative of a new compound class—has been prepared by precise reaction control. A number of different methods, including neutron and X‐ray powder diffraction, solid state MAS NMR, vibrational spectroscopy, and density functional calculations, were applied.

Polycrystalline members of the ${\text{GaV}}_{4}{\text{S}}_{8\ensuremath{-}y}{\text{Se}}_{y}$ family of materials with small levels of substitution between $0\ensuremath{\le}y\ensuremath{\le}0.5$ and $7.5\ensuremath{\le}y\ensuremath{\le}8$ have been synthesized in order to investigate their magnetic and structural properties. Substitutions to the skyrmion hosting parent compounds ${\text{GaV}}_{4}{\text{S}}_{8}$ and ${\text{GaV}}_{4}{\text{Se}}_{8}$ are found to suppress the temperature of the cubic-to-rhombohedral structural phase transition that occurs in both end compounds and to create a temperature region around the transition where there is a coexistence of these two phases. Similarly, the magnitude of the magnetization and temperature of the magnetic transition are both suppressed in all substituted compounds until a glassy-like magnetic state is realized. There is evidence from the ac susceptibility data that skyrmion lattices with similar dynamics to those in ${\text{GaV}}_{4}{\text{S}}_{8}$ and ${\text{GaV}}_{4}{\text{Se}}_{8}$ are present in compounds with very low levels of substitution, $0lyl0.2$ and $7.8lyl8$, however, these states vanish at higher levels of substitution. The magnetic properties of these substituted materials are affected by the substitution altering exchange pathways and resulting in the creation of increasingly disordered magnetic states.

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

We report magnetic properties of a $3{d}^{9}$ $({\mathrm{Cu}}^{2+})$ magnetic insulator ${\mathrm{Cu}}_{2}{\mathrm{OSO}}_{4}$ measured on both powder and single crystal. The magnetic atoms of this compound form layers whose geometry can be described either as a system of chains coupled through dimers or as a kagome lattice where every third spin is replaced by a dimer. Specific heat and DC susceptibility show a magnetic transition at 20 K, which is also confirmed by neutron scattering. Magnetic entropy extracted from the specific heat data is consistent with an $S=1/2$ degree of freedom per ${\mathrm{Cu}}^{2+}$, and so is the effective moment extracted from DC susceptibility. The ground state has been identified by means of neutron diffraction on both powder and single crystal and corresponds to an $\ensuremath{\sim}{120}^{\ensuremath{\circ}}$ spin structure in which ferromagnetic intradimer alignment results in a net ferrimagnetic moment. No evidence is found for a change in lattice symmetry down to 2 K. Our results suggest that ${\mathrm{Cu}}_{2}{\mathrm{OSO}}_{4}$ represents a type of model lattice with frustrated interactions where interplay between magnetic order, thermal and quantum fluctuations can be explored.

The potential of the perovskite system Nd

Several previous studies reported that a one-dimensional Heisenberg chain model is inadequate in describing the magnetic properties of the low-dimensional quantum antiferromagnet $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{CuV}}_{2}{\mathrm{O}}_{6}$, but the origin for this observation has remained unclear. We have reinvestigated the magnetic properties of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{CuV}}_{2}{\mathrm{O}}_{6}$ and found that our anisotropic magnetic susceptibility, neutron-powder diffraction, and electron paramagnetic spin-resonance measurements are in good agreement with extensive density-functional theory ($\mathrm{DFT}+U$) total energy calculations which indicate that the correct spin lattice model for $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{CuV}}_{2}{\mathrm{O}}_{6}$ is rather a $S=1/2$ 2D-Heisenberg antiferromagnetic lattice. The magnetic susceptibility data are well described by a rectangular Heisenberg antiferromagnet with anisotropy ratio $\ensuremath{\alpha}\ensuremath{\sim}$ 0.7 consistent with the DFT results. Quantum Monte Carlo simulations of the magnetic susceptibilities for a rectangular lattice Heisenberg antiferromagnet were performed in the anisotropy range 0.5 $\ensuremath{\le}\ensuremath{\alpha}\ensuremath{\le}$ 1.0. The results of the Quantum Monte Carlo calculations were cast into a Pad\'e approximant which was used to fit the temperature-dependent magnetic susceptibility data. Neutron-powder-diffraction measurements were used to conclusively solve the collinear antiferromagnetic structure of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{CuV}}_{2}{\mathrm{O}}_{6}$ below the N\'eel temperature of $\ensuremath{\sim}22.4$ K.

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

The GaV$_4$S$_{8-y}$Se$_y$ $(y = 0$ to $8)$ family of materials have been synthesized in both polycrystalline and single crystal form, and their structural and magnetic properties thoroughly investigated. Each of these materials crystallizes in the $F\bar{4}3m$ space group at ambient temperature. However, in contrast to the end members GaV$_4$S$_8$ and GaV$_4$Se$_8$, that undergo a structural transition to the $R3m$ space group at 42 and 41 K respectively, the solid solutions $(y = 1$ to $7)$ retain cubic symmetry down to 1.5 K. In zero applied field the end members of the family order ferromagnetically at 13 K (GaV$_4$S$_8$) and 18 K (GaV$_4$Se$_8$), while the intermediate compounds exhibit a spin-glass-like ground state. We demonstrate that the magnetic structure of GaV$_4$S$_8$ shows localization of spins on the V cations, indicating that a charge ordering mechanism drives the structural phase transition. We conclude that the observation of both structural and ferromagnetic transitions in the end members of the series in zero field is a prerequisite for the stabilization of a skyrmion phase, and discuss how the absence of these transitions in the $y = 1$ to $7$ materials can be explained by their structural properties., Main text: 14 pages, 5 figures, 1 table. Supplementary information: 10 pages, 6 figures, 6 tables

Non-centrosymmetric polar compounds have important technological properties. Reported perovskite oxynitrides show centrosymmetric structures, and for some of them high permittivities have been observed and ascribed to local dipoles induced by partial order of nitride and oxide. Reported here is the first hexagonal perovskite oxynitride BaWON

Spin liquids are highly correlated yet disordered states formed by the entanglement of magnetic dipoles$^1$. Theories typically define such states using gauge fields and deconfined quasiparticle excitations that emerge from a simple rule governing the local ground state of a frustrated magnet. For example, the '2-in-2-out' ice rule for dipole moments on a tetrahedron can lead to a quantum spin ice in rare-earth pyrochlores - a state described by a lattice gauge theory of quantum electrodynamics$^{2-4}$. However, f-electron ions often carry multipole degrees of freedom of higher rank than dipoles, leading to intriguing behaviours and 'hidden' orders$^{5-6}$. Here we show that the correlated ground state of a Ce$^{3+}$-based pyrochlore, Ce$_2$Sn$_2$O$_7$, is a quantum liquid of magnetic octupoles. Our neutron scattering results are consistent with the formation of a fluid-like state of matter, but the intensity distribution is weighted to larger scattering vectors, which indicates that the correlated degrees of freedom have a more complex magnetization density than that typical of magnetic dipoles in a spin liquid. The temperature evolution of the bulk properties in the correlated regime below 1 Kelvin is well reproduced using a model of dipole-octupole doublets on a pyrochlore lattice$^{7-8}$. The nature and strength of the octupole-octupole couplings, together with the existence of a continuum of excitations attributed to spinons, provides further evidence for a quantum ice of octupoles governed by a '2-plus-2-minus' rule. Our work identifies Ce$_2$Sn$_2$O$_7$ as a unique example of a material where frustrated multipoles form a 'hidden' topological order, thus generalizing observations on quantum spin liquids to multipolar phases that can support novel types of emergent fields and excitations.

In the heavy-fermion system $Yb_2Pd_2In_{1-x}Sn_x$, the interplay of crystal-field splitting, Kondo effect, and Ruderman-Kittel-Kasuya-Yosida interactions leads to complex chemical-, pressure-, and magnetic-field phase diagrams, still to be explored in full detail. By using a series of techniques, we show that even modest changes of parameters other than temperature are sufficient to induce multiple quantum-critical transitions in this highly susceptible heavy-fermion family. In particular, we show that, above $\sim 10$ kbar, hydrostatic pressure not only induces an antiferromagnetic phase at low temperature, but it likely leads to a reorientation of the Yb magnetic moments and/or the competition among different antiferromagnetic configurations.

TiH2 and ZrH2 are the most frequently used blowing agents for foaming low-melting alloys based on aluminium or zinc. In order to tailor the temperature range of decomposition such hydrides are often pre-annealed in Ar or air prior to their use. The decomposition behaviour of such compounds is usually studied by thermogravimetry and calorimetry. We applied neutron diffraction to gather more information about the decomposition of treated and untreated hydrides. In order to avoid strong incoherent scattering effects from hydrogen (1H), we used deuterated (2H) variants and could obtain high-quality diffraction patterns. In this way, we were able to quantify the amount of hydrogen for different decomposition stages as well as changes of the lattice structure for a range of Ti- and Zr-based blowing agents.

Non‐centrosymmetric polar compounds have important technological properties. Reported perovskite oxynitrides show centrosymmetric structures, and for some of them high permittivities have been observed and ascribed to local dipoles induced by partial order of nitride and oxide. Reported here is the first hexagonal perovskite oxynitride BaWON2, which shows a polar 6H polytype. Synchrotron X‐ray and neutron powder diffraction, and annular bright‐field in scanning transmission electron microscopy indicate that it crystalizes in the non‐centrosymmetric space group P63mc, with a total order of nitride and oxide at two distinct coordination environments in cubic and hexagonal packed BaX3 layers. A synergetic second‐order Jahn–Teller effect, supported by first principle calculations, anion order, and electrostatic repulsions between W6+ cations, induce large distortions at two inequivalent face‐sharing octahedra that lead to long‐range ordered dipoles and spontaneous polarization along the c axis. The new oxynitride is a semiconductor with a band gap of 1.1 eV and a large permittivity., This work was supported by the Ministerio de Ciencia e Innovación, Spain (MAT2017‐86616‐R, MAT2017‐85232‐R, SEV‐2015‐0496, MDM‐2017‐0767, PGC2018‐096955‐B‐C44 and PGC2018‐093863‐B‐C22) and the Generalitat de Catalunya (2017SGR1377, 2017SGR581, 2017SGR1506 and 2017SGR1289). We thank Alba and ILL (experiments 2018082958 and 5‐23‐721, https://doi.org/10.5291/ILL‐DATA.5‐23‐721) for beam time provision, and Dr. François Fauth for assistance in data collection at Alba. I.F and J.G. acknowledge the Ramon y Cajal contracts RYC‐2017‐22531 and RYC‐2012‐11709. S.C. was supported by FONDECYT, Chile (project 11171063). ICTS‐CNME at UCM is acknowledged for offering access to STEM and expertise.

International audience; We report a perovskite oxide with a new type of polar structure, CuNbO3, thanks to the use of high-pressure and high-temperature synthesis. The perovskite-type CuNbO3 was previously obtained under high-pressure and high-temperature conditions, but its crystal structure has not been solved yet. Our structural analysis reveals that CuNbO3 perovskite crystallizes in a polar monoclinic space group (Pc) with a root 2 a(pc) x root 2 a(pc ) x 2a(pc) unit cell (a(pc) being the pseudo-cubic lattice parameter), which VT, is one maximal nonisomorphic subgroup of the polar rhombohedral space group, R3c. This compound exhibits a remarkable "noncollinear ferrielectric" structure due to parallel displacements of Cu+ and antiparallel displacements of Nb5+ along different axes, representing a new type of polar phase in the perovskite structure. We also observe that the noncollinear ferrielectric Pc s tructure transforms around 470 K into the collinear ferroelectric R3c structure that features parallel displacements of Cu+ and Nb5+ in the same direction. The present work extends the accessible composition range of the perovskite niobate series and demonstrates the role of A-O covalency in determining their crystal structures.

We study the crystal structure of the tetragonal iron selenide FeSe and its nematic phase transition to the low-temperature orthorhombic structure using synchrotron x-ray and neutron scattering analyzed in both real and reciprocal space. We show that in the local structure the orthorhombic distortion associated with the electronically driven nematic order is more pronounced at short length scales. It also survives up to temperatures above 90 K where reciprocal-space analysis suggests tetragonal symmetry. Additionally, the real-space pair distribution function analysis of the synchrotron x-ray diffraction data reveals a tiny broadening of the peaks corresponding to the nearest Fe-Fe, nearest Fe-Se, and the next-nearest Fe-Se bond distances as well as the tetrahedral torsion angles at a short length scale of 20 angstr\"om. This broadening appears below 20 K and is attributed to a pseudogap. However, we did not observe any further reduction in local symmetry below orthorhombic down to 3 K. Our results suggest that the superconducting gap anisotropy in FeSe is not associated with any symmetry-lowering short-range structural correlations., Comment: 9 pages, 6 figures

Frustrated magnetic materials can show unconventional correlations such as quantum spin liquid states and monopole excitations in spin ices. These phenomena are observed on uniformly frustrated lattices such as triangular, kagome or pyrochlore types, where all nearest neighbour interactions are equivalent. Here we report incommensurate long-range spin amplitude waves in the spinels Fe2GeO4 and γ-Fe2SiO4 at low temperatures, which indicate that the degree of frustration may itself be a fluctuating quantity that can spontaneously order without a lattice distortion as a ‘frustration wave’. Fe2GeO4 with propagation vector (2/3 + δ 2/3 + δ 0) has ordered Fe2+ moments that vary between fully saturated 4 μB and 0 values, consistent with a frustration wave order. γ-Fe2SiO4 has a more complex (¾ + δ ¾ + δ 0) order that coexists with an ordered spin ice phase. Dynamic orbital fluctuations are proposed to give rise to locally correlated patterns of ferromagnetic and antiferromagnetic interactions consistent with the observed orders.

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

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

A complete solid solution forms between the perovskite proton conductor BaZr0.7Ce0.2Y0.1O3-δ (BZCY72) and BaPr0.9Y0.1O3-δ (BPY) on synthesis by the Pechini method and high-temperature annealing. Phase fields of selected members of the Ba(Zr0.7Ce0.2)1-(x/0.9)PrxY0.1O3-δ series were studied as a function of composition and temperature by high-resolution neutron powder diffraction revealing symmetry changes in the sequence Pnma → Imma → → Pm3m. Higher symmetry is favoured for low Pr contents and high temperatures, as consideration of tolerance factor suggests. A volume contraction, ascribed to dehydration, is observed by synchrotron X-ray diffraction on heating in air for lower x. Magnetic measurements and structural data support the presence of Pr in the IV valence state on the perovskite B site. Thermogravimetric analysis in CO2 to ∼1253 K indicates better chemical stability for x ≤ 0.445, whereas decomposition occurred for higher x. Electrical conductivity increases by over two orders of magnitude in dry air at lower temperature from x = 0.225 to 0.675; total conductivity reaches a value of 0.4 S cm-1 at 1173 K for x = 0.675. The series exhibits electron-hole transport with a positive pO2 dependence which increases with temperature, consistent with participation of oxygen vacancies in charge compensation of the Y3+ acceptor dopant. The activation energy for thermally activated hole hopping in air in the range 523-773 K decreases from ∼1 eV for BZCY72 to ∼0.4 eV for x = 0.675. Conductivity is generally lower in humidified N2 and air (pH2O ≈ 0.023 atm) than the corresponding dry atmospheres, consistent with consumption of holes by less mobile protonic species; however for x ≤ 0.225 the lower concentration of electron holes concomitant with higher oxygen-vacancy content in N2 results in slightly higher conductivity in wet conditions due to hydration of vacancies. Fil: Heras Juaristi, Gemma. Instituto de Cerámica y Vidrio de Madrid; España Fil: Amador, Ulises. Universidad San Pablo Ceu; España Fil: Fuentes, Rodolfo Oscar. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica; Argentina Fil: Chinelatto, Adilson L.. Universidade Estadual de Ponta Grossa; Brasil Fil: Romero De Paz, Julio. Universidad Complutense de Madrid; España Fil: Ritter, Clemens. Institut Laue Langevin; Francia Fil: Fagg, Duncan P.. Universidade de Aveiro; Portugal Fil: Pérez Coll, Domingo. Instituto de Cerámica y Vidrio de Madrid; España Fil: Mather, Glenn C.. Instituto de Cerámica y Vidrio de Madrid; España

We have studied structural properties of La 3 Ni 2 B 2 N 3− δ samples with distinctly different values of the superconducting transition temperature by means of powder neutron diffractometry and specific heat measurements. The refinement of lattice site occupations reveals full occupations for all sites in the La 3 Ni 2 B 11 2 N 3 structure with space group I 4/ mmm except for nitrogen site N(2). For samples with T c = 13.0 K and 13.7 K we obtain for the N(2) site distinctly different occupation factors of 0.90 and 0.93, respectively. The latter confirms a direct relation between the nitrogen stoichiometry and the superconducting transition temperature. Based on the analysis of temperature dependent lattice parameters, atomic displacement factors, lattice heat capacity data and ab initio phonon density of states calculations we discuss the thermal expansion and vibrational properties of superconducting La 3 Ni 2 B 2 N 3− δ .

Acknowledgements We acknowledge the University of Aberdeen for provision of a studentship for Harriet Hopper and the Higher Education Commission of Pakistan for providing a visiting fellowship for Dr. Falak Sher.

The electrical and structural properties of the series Ba3Mo1–xNb1+xO8.5–x/2 (x = 0.0, 0.1, 0.2, 0.3) have been determined. Ba3Mo1–xNb1+xO8.5–x/2 crystallizes in a hybrid of the 9R hexagonal perovskite and palmierite structures, in which (Mo/Nb)O4 and (Mo/Nb)O6 units coexist within the structure. Nb substitutes preferentially at the octahedral site so that the ratio of (Mo/Nb)O4 tetrahedra to (Mo/Nb)O6 octahedra decreases with increasing x resulting in a reduction in the magnitude of the ionic conductivity from 1.3 × 10–6 S cm–1 for x = 0.0 to 1.1 × 10–7 S cm–1 for x = 0.3 at 300 °C. However, upon heating the conductivities of the solid solution converge, which suggests that the unusual thermal structural rearrangement previously reported for Ba3MoNbO8 preserves the high temperature conductivity. The results demonstrate that the presence of (Mo/Nb)O4 tetrahedra with nonbridging apical oxygen atoms is an important prerequisite for the ionic conduction observed in the Ba3MoNbO8.5 system.

Members of the perovskite solid solution BaZr1-xPrxO3-δ (0.2 ≤ x ≤ 0.8) with potential high-temperature electrochemical applications were synthesized via mechanical activation and high-temperature annealing at 1250 °C. Structural properties were examined by Rietveld analysis of neutron powder diffraction and Raman spectroscopy at room temperature, indicating rhombohedral symmetry (space group R3c) for members x = 0.2 and 0.4 and orthorhombic symmetry (Imma) for x = 0.6 and 0.8. The sequence of phase transitions for the complete solid solution from BaZrO3 to BaPrO3 is Pm3m → R3c → Imma → Pnma. The structural data indicate that Pr principally exists as Pr4+ on the B site and that oxygen content increases with higher Pr content. Electrical-conductivity measurements in the temperature range of 250-900 °C in dry and humidified (pH2O ≈ 0.03 atm) N2 and O2 atmospheres revealed an increase of total conductivity by over 2 orders of magnitude in dry conditions from x = 0.2 to x = 0.8 (σ ≈ 0.08 S cm-1 at 920 °C in dry O2 for x = 0.8). The conductivity for Pr contents x > 0.2 is attributable to positively charged electronic carriers, whereas for x = 0.2 transport in dry conditions is n-type. The change in conduction mechanism with composition is proposed to arise from the compensation regime for minor amounts of BaO loss changing from predominantly partitioning of Pr on the A site to vacancy formation with increasing Pr content. Conductivity is lower in wet conditions for x > 0.2 indicating that the positive defects are, to a large extent, charge compensated by less mobile protonic species. In contrast, the transport mechanism of the Zr-rich composition (x = 0.2), with much lower electronic conductivity, is essentially independent of moisture content.

We report results of detailed structural investigations on the system La2-xSrxCuRuO6 (0 0 respectively, neutron diffraction patterns collected at 10 K and 30 K across the characteristic transition temperature suggested by the magnetic data, do not show any extra peaks or extra intensity at lower temperature as compared to the high temperature, thus, establishing the absence of any long-range magnetic ordering in these samples. (C) 2016 Elsevier B.V. All rights reserved.

The perovskite system Pb(Fe1-xMnx)2/3W1/3O3 (0 ≤ x ≤ 1, PFMWO) has been prepared by conventional solid-state reaction under different sintering conditions. Structures and phase composition as well as thermal, magnetic and dielectric properties of the compounds have been systematically investigated experimentally and by first-principles density functional calculations. A clean perovskite phase is established at room temperature for compositions 0 ≤ x ≤ 0.4. Rietveld refinements of X-ray and neutron powder diffraction patterns demonstrate that the compounds crystallize in space group Pm-3m (0 ≤ x ≤ 0.4). The degree of ordering of the Fe and W/Mn cations was found to depend on the concentration of Mn. First-principles calculations suggest that the structural properties of PFMWO are strongly influenced by the Jahn-Teller effect. The PFMWO compounds behave as relaxor ferroelectrics at weak Mn-doping with a dielectric constant that rapidly decreases with increasing Mn content. A low temperature antiferromagnetic G-type order with propagation vector k = (1/2,1/2,1/2) is derived from neutron powder diffraction data for the samples with x ≤ 0.4. However with increasing doping concentration, the magnetic order is perturbed. First-principles calculations show that the dominant exchange coupling is antiferromagnetic and occurs between nearest neighbor Fe atoms. When the system is doped with Mn, a relatively weak ferromagnetic (FM) interaction between Fe and Mn atoms emerges. However, due to the presence of this FM interaction, the correlation length of the magnetic order is greatly shortened already at rather low doping levels.

By using a high-pressure reaction, we prepared a new oxynitride ZnTaO2N that crystallizes in a centrosymmetric (R (3) over barc) high-temperature LiNbO3-type structure (HTLN-type). The stabilization of the HTLN-type structure down to low temperatures (at least 20 K) makes it possible to investigate not only the stability of this phase, but also the phase transition to a noncentrosymmetric (R3c) LiNbO3-type structure (LN-type) which is yet to be clarified. Synchrotron and neutron diffraction studies in combination with transmission electron microscopy show that Zn is located at a disordered 12c site instead of 6a, implying an order disorder mechanism of the phase transition. It is found that the dosed d-shell of Zn2+, as well as the high-valent Ta5+ ion, is responsible for the stabilization of the HTLN-type structure, affording a novel quasitriangular ZnO2N coordination. Interestingly, only 3% Zn substitution for MnTaO2N induces a phase transition from LN- to HTLN-type structure, implying the proximity in energy between the two structural types, which is supported by the first-principles calculations.

We study magnetic behavior of the ${\mathrm{Yb}}^{3+}$ ions on a frustrated pyrochlore lattice in the spinel ${\mathrm{CdYb}}_{2}{\mathrm{Se}}_{4}$. The crystal-electric-field parameters deduced from high-energy inelastic neutron scattering reveal a well-isolated ytterbium ground-state doublet with a weakly Ising character. Magnetic order studied by powder neutron diffraction evolves from the XY-type antiferromagnetic ${\mathrm{\ensuremath{\Gamma}}}_{5}$ state to a splayed icelike ferromagnet (both with $\mathbf{k}=0$) in applied magnetic field with ${B}_{c}=3\phantom{\rule{0.28em}{0ex}}\mathrm{T}$. Low-energy inelastic neutron scattering identifies weakly dispersive magnetic bands around 0.72 meV starting at $|\mathbf{Q}|=1.1\phantom{\rule{4pt}{0ex}}{\AA{}}^{\ensuremath{-}1}$ at zero field, which diminish with field and vanish above 3 T. We explain the observed magnetic behavior in framework of the nearest-neighbor anisotropic exchange model for effective $S=\frac{1}{2}$ Kramers doublets on the pyrochlore lattice. The estimated exchanges position the ${\mathrm{CdYb}}_{2}{\mathrm{Se}}_{4}$ spinel close to the phase boundary between the ${\mathrm{\ensuremath{\Gamma}}}_{5}$ and splayed ferromagnet states, similar to the Yb pyrochlores, suggesting an important role of the competition between these phases.

Ozone oxidation has allowed the stabilization of a very high iron oxidation state in the FeSr

Materials with the formula Sr2CoNb1−xTixO6−δ (x=1.00, 0.70; δ=number of oxygen vacancies) present a cubic perovskite-like structure. They are easily and reversibly reduced in N2 or Ar and re-oxidized in air upon heating. Oxidation by water (wet N2), involving splitting of water at a temperature as low as 700 °C, produces hydrogen. Both compounds displayed outstanding H2 production in the first thermochemical cycle, the Sr2CoNb0.30Ti0.70O6−δ material retaining its outstanding performance upon cycling, whereas the hydrogen yield of the x=1 oxide showed a continuous decay. The retention of the materials’ ability to promote water splitting correlated with their structural, chemical, and redox reversibility upon cycling. On reduction/oxidation, Co ions reversibly changed their oxidation state to compensate the release/recovery of oxygen in both compounds. However, in Sr2CoTiO6−δ, two phases with different oxygen contents segregated, whereas in Sr2CoNb0.30Ti0.70O6−δ this effect was not evident. Therefore, this latter material displayed a hydrogen production as high as 410 μmolurn:x-wiley:18645631:media:cssc201901484:cssc201901484-math-0001 g−1perovskite after eight thermochemical cycles at 700 °C, which is among the highest ever reported, making this perovskite a promising candidate for thermosolar water splitting in real devices. We thank Agencia Estatal de Investigación (AEI)/Fondo Europeo de Desarrollo Regional (FEDER/UE)for funding the project MAT2016-78632-C4-1-R. We acknowledge CSIC, ILL, and ESRF for financial support and facilitated access to the BM25-SpLine line at ESRF and D2B diffractometer at ILL. The authors also thank "Comunidad de Madrid" and the European Structural Funds for their financial support to the ALCCONES (S2013/MAE-2985) and MATERYENER3-CM (S2013/MIT-2753) projects.