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We present Mn2LiReO6 synthesised at high pressure in two different polymorphs. They are the only A-site manganites with an ideal +/7+ charge distribution over the B/B′ sites. The lower pressure polymorph crystallises in a polar R3 ordered-ilmenite type structure (OIL) with a calculated 26.9 μC cm−2 electrical polarisation. It shows a cluster spin glass behaviour due to Li/Mn anti-site mixing. The well-ordered higher-pressure double-perovskite polymorph (DPv, P21/n) develops weak-ferromagnetism with giant magnetic coercivity (HC = 6.9 T at 2 K) due to magnetocrystalline anisotropy enhanced by symmetry-restricted exchange interactions but in the absence of spin–orbit coupling.

International audience; Mixed anion compounds AFe3(SeO3)2F6 (A=K, Rb) have been synthesized by hydrothermal reactions. Prior results obtained for the isostructural Cs analog have revealed original 2D-ferrimagnetic (FI) blocks, antiferromagnetically (AFM) ordered around 120 K. Surprisingly, the whole series orders at a nearly constant TN temperature with similar field-induced magnetization steps, despite significant changes of the interlayer thickness. The metamagnetic transition from AFM to FI on oriented single crystals and Cp measurements prove an in-plane easy-magnetic axis confirmed by neutron powder diffraction (NPD). The ferrimagnetic alignment [Fe(1)↑−Fe(2)↓−Fe(1)↑] in each 2D subunit is robust, but the AFM-ordering between them is broken under weak field leaving field-aligned 2D-macrospins, with weak magnetocrystalline anisotropy. DFT+U calculations validate the exchange ratio Jinter/Jintra of 5.10-3 for all compounds and the relatively high TN value is well predicted by a modified random-phase approximation (RPA) like relation, established for Quasi-Low-Dimensional Heisenberg Antiferromagnets. However, our calculations of intralayer magnetic dipole-dipole (MDD) interactions indicate a non-neglectable values and suggest the latter to assist the in-plane spin orientation below TN. At TN, under field a 3D-Heisenberg model was deduced for the AFM →FI transition from the critical behavior using modified Arrott plots.

International audience; Cation ordering in ABX3 perovskites is important to structural, physical and chemical properties. Here we report discovery of CaCuFeReO6 with the tetragonal AA′BB′O6 double double perovskite structure that was previously only reported for A′=Mn compositions. CaCuFeReO6 occurs in the same phase field as CaCu3Fe2Re2O12 demonstrating that different A-cation ordered peroskites may be obtained in the same chemical system. CaCuFeReO6 has ferrimagnetic order of Fe, Re and Cu spins below TC=567 K, in contrast to Mn analogues where the Mn spins order separately at much lower temperatures. The magnetoresistance of CaCuFeReO6 displays low-field “butterfly” hysteresis with an unusual change from negative to positive values as field increases. Many more AA′BB′O6 double double perovskites may be accessible for A′=Cu and other divalent transition metals at high pressure, so the presently known phases likely represent only the “tip of the iceberg” for this family.

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.

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 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.

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.

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.

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.

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.

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

International audience; Oxygen-deficient Sr$_2$ScGaO$_5$ single crystals with a cubic perovskite structure were grown by the floating-zone technique. The transparent crystals of this pure 3D oxygen electrolyte are metastable at ambient temperature, showing one-sixth of all oxygen positions vacant. While neutron single-crystal diffraction, followed by maximum entropy analysis, revealed a strong anharmonic displacements for the oxygen atoms, a predominant formation of ScO$_6$ octahedra and GaO$_4$ tetrahedra is indicated by Raman spectroscopic studies, resulting in a complex oxygen defect structure with short-range order. Temperature-dependent X-ray powder diffraction (XPD) and neutron powder diffraction (NPD) studies reveal the cubic Sr$_2$ScGaO$_5$ to be thermodynamically stable only above 1400 °C, while the stable modification below this temperature shows the brownmillerite framework with orthorhombic symmetry. Cubic Sr$_2$ScGaO$_5$ remains surprisingly kinetically stable upon heating from ambient temperature to 1300 °C, indicating a huge inertia for the retransformation toward the thermodynamically stable brownmillerite phase. Ionic conductivity investigated by impedance spectroscopy was found to be 10$^{–4}$ S/cm at 600 °C, while oxygen 18O/16O isotope exchange indicates a free oxygen mobility to set in at around 500 °C.

Nuclear fuel plates based on a γU-Mo/Al mixture are proposed for research reactors. In this work their thermal behavior in the [425; 550°C] temperature range has been studied mainly by neutron and high energy X-ray diffraction. Even if complementary studies will be necessary, the kinetics of first the growth of the interaction layer between γU-Mo and Al and second of the γU-Mo destabilization have been accurately measured. This basic work should be helpful for defining manufacturing conditions for fuel plates with optimized composition.

International audience; Double perovskite cobaltites were recently presented as promising cathode materials for solid oxide fuel cells. While an atomistic mechanism was proposed for oxygen diffusion in this family of materials, no direct experimental proof has been presented so far. We report here the first study that directly compares experimental and theoretical diffusion pathways of oxygen in an oxide, namely in the double cobaltite compound, NdBaCo2O5+x. Model-free experimental nuclear density maps are obtained from the maximum entropy method combined with Rietveld refinement against high resolution neutron diffraction data collected at 1173 K. They are then compared to theoretical maps resulting from classical molecular dynamics calculations. The analysis of 3D maps of atomic densities allows identifying unambiguously the pathways and the mechanisms involved in the oxide ion diffusion. It is shown that oxygen diffusion occurs along a complex trajectory between Nd- and Co-containing a,b planes. The study also reveals that Ba-containing planes act as a barrier for oxygen diffusion. The diffusion mechanism is also supported through the oxygen sites occupancy analysis that confir ms the increase of oxygen vacancies in the cobalt-planes on heating. The use of such combined experimental and theoretical analysis should be considered as a very powerful approach for materials design.

International audience; We have studied a ternary phase of the CePt3 +/- xGe composition, with the maximum values of x = 0.2. A series of samples was prepared by arc melting, Czochralski method and splat cooling, and characterized by scanning electron microscopy and X-ray diffraction. The arc-melted samples are well-crystalline and single-phase with slightly varying stoichiometry close to the nominal CePt3Ge, while the splat-cooled samples form an amorphous or a nanocrystalline constitution of the similar to CePt3Ge composition. Detailed measurements of magnetization, specific heat and electrical resistivity were performed in the temperature range 0.40-300 K, and magnetic fields up to 9 T. In addition, a delicate neutron diffraction experiment down to 70 mK was done, which excluded long-range magnetic order in the CePt3Ge compound. The specific heat data were tested to anisotropic Kondo model, and the resistivity behavior has been treated by means of electron localization effects. The experiments evidenced that the CePt3Ge compound can be considered as a Kondo lattice with low-temperature magnetic correlations, and the size effect seems to have minor role in control of the Kondo scattering phenomena in this compound.

International audience; T-La2CuO4 can be transformed into T?-La2CuO4 via a two-step topotactic reaction mechanism. First T-La2CuO4 is reduced by CaH2 at 200 °C to La2CuO3.5 which can be subsequently reoxidized to T?-La2CuO4 at 300 °C in air. La2CuO3.5 does not adopt the Sr2CuO3 type ?S-phase? structure, as taken for granted for almost 20 years, but constitutes an oxygen deficient T?-framework, with copper in 4-fold planar and 2-fold linear dumbbell coordination. Upon heating the T-La2CuO4 phase is reobtained from T?-La2CuO4 above 650 °C, confirming the ?T-phase? to be the thermodynamically stable modification at least at higher temperature.

The long-range ordering of a crystalline material can be accurately determined by analyzing the Bragg intensities and positions. In contrast, direct observation of short-range ordering in crystalline materials, which is increasingly considered of fundamental importance to unravel the structure-property relationships that underpin their technological applications, is a challenging task. In this study we have investigated the structure of LaSrAlO4, a representative example of compounds with the K2NiF4-type structure. By the combined use of synchrotron and neutron diffraction, pair distribution function analysis, Al-27 MQMAS NMR and atomistic simulations we have highlighted differences between the average and local structure of this material, directly determining the existence of at least two distinct Al sites. The results presented in this study confirm that La and Sr are, on average, randomly distributed throughout the structure, but demonstrate that the situation is considerably more complex at the short range

International audience; We provide a full description of the first single-crystal synthesis of the low-dimensional quantum spin compound (CuCl)LaNb(2)O(7) through the low-temperature topotactic ion exchange route. Very fast diffusion of ion-exchanged CuCl and Cs ions is observed. In addition, thorough structure determination at very low temperature is outlined following use of single-crystal X-ray diffraction and powder neutron diffraction, taking advantage of the better sensitivity of the latter method for describing O and Cl atoms. State-of-the-art calculations (Maximum Entropy Method) are used for visualising the fine structural features of this unusual pseudo-tetragonal superstructure. Our study unambiguously throws light on the long-standing controversy concerning the crystal structure of this compound, and allows for a relevant description of its magnetic properties. Incidentally, it is demonstrated that the low-temperature structural model tentatively proposed very recently by Tsirlin et al. (Phys. Rev. B, 2010, 82, 054107), according to powder synchrotron X-ray diffraction, is correct.

The magnetic structure of the ternary intermetallic compound Ho6FeTe2 (hexagonal Zr6CoAs2 structure type, a ternary ordered variant of the Fe2P-type; space group P 6 ¯ 2 m , No 189) has been investigated by neutron diffraction and magnetization measurements. The neutron diffraction study shows that in between the temperatures TN ∼ 24 K and TC ∼ 8 K the magnetic moments of the Ho atoms adopt a flat helix ordering with elliptical envelope having a wave vector K = [0, 0, ∼1/9], the helix axis being coincident with the c-axis (with maximal magnetic moments of M H o 1 = 6.5 ( 7 ) μ B and M H o 2 = 8.0 ( 7 ) μ B , at 10 K). Below the TC ∼ 8 K the magnetic structure of Ho6FeTe2 changes to a ferromagnetic cone: superposed on the helix ordering within the basal plane a ferromagnetic ordering with K = [0, 0, 0] and moments pointing in the c-direction appears. At 2 K the total maximal magnetic moments amount to M H o 1 = 9.1 ( 7 ) μ B and M H o 2 = 9.6 ( 7 ) μ B with a ferromagnetic component on the H o 1 site of M c H o 1 = 2.2 ( 5 ) μ B and on the H O 2 site of M c H o 2 = 5.2 ( 5 ) μ B . No local moment was detected on the iron atomic site in this compound. Magnetization measurements confirm the first magnetic transition at ∼26 K and the second at ∼10 K. The magnetization vs. field data obtained at 2.5 K show soft ferromagnetic behavior, with a saturation magnetization of 7.9 μB/Ho3+.

International audience; A new iron oxophosphate of composition Rb7Fe7(PO4)8O2·2H2O has been synthesized and studied by X-ray diffraction, TG and DTA analysis, magnetic susceptibility, neutron diffraction, Mössbauer spectroscopy and ionic conductivity. This compound crystallizes in the monoclinic system with the P21/c space group and the unit cell parameters a = 8.224(8) Å, b = 22.162(6) Å, c = 9.962(6) Å and β = 109.41(8)°. Its structure is built up from Fe7O32 clusters of edge- and corner-sharing FeO5 and FeO6 polyhedra. Neighboring clusters are connected by the phosphate tetrahedra to form a three-dimensional framework. The Rb+ cations and the water molecules are occupying intersecting tunnels parallel to a and c. The presence of water molecules was confirmed by TG and DTA analysis. The magnetic susceptibility measurements have shown the existence of antiferromagnetic ordering below 22 K with a weak ferromagnetic component. Additionally, these measurements show evidence for a strong magnetic frustration characterized by |θ/TN| ≈ 12. Powder neutron diffraction study confirms the presence of a long range antiferromagnetic order coupled to a weak ferromagnetic component along the b-axis. The strongly reduced magnetic moments extracted from the refinement support the existence of a magnetically frustrated ground state. The Mössbauer spectroscopy results confirmed the presence of only Fe3+ ions in both five and six coordination. The ionic conductivity measurements led to activation energy of 0.81 eV, a value that agrees with the obtained for other rubidium phosphates.

14 SPIN-LADDER IRON OXIDE: Sr3Fe2O5 H. Kageyama, T. Watanabe, Y. Tsujimoto, A. Kitada, Y. Sumida, K. Kanamori, K. Yoshimura, N. Hayashi, S. Muranaka, M. Takano , M. Ceretti, W. Paulus, C. Ritter, G. Andre Department of Chemistry, Graduate School of Science, Kyoto University, Japan Graduate School of Human and Environmental Studies, Kyoto University, Japan 3 Institute for Chemical Research, Kyoto University, Uji, Japan 4 Institute for Integrated Cell-Materials Sciences and Research Institute for Production Development, Japan 5 University of Rennes1, Sciences Chimiques de Rennes UMR CNRS 6226, Campus de Beaulieu, Rennes 6 Institute Laue Langevin, BP 156, 38042, Grenoble, France 7 Laboratoire Leon Brillouin, CEA-CNRS Saclay, 91191, Gif-sur-Yvette, France