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This is the summary of the results obtained in 1999 on RENiO3.

Among standard high temperature cathode materials for SOFC devices, La0.8Sr0.2MnO3- (LSM) displays the least reactivity with the oxide-ion conductor La2Mo2O9 (LMO), yet reaction is observed at high processing temperatures, identified by X-ray diffraction (XRD) and FIB-SIMS studies after annealing at 1050°C and 1150°C. Additionally Sr and Mn solutions were deposited and annealed on LMO pellets, as well as a Mo solution on a LSM pellet. From these studies several reaction products were identified by XRD and located by FIB-SIMS on the surface of pelletized samples and by depth profiling it was evident that reactivity extended up to ~10 m from the surface region. When strontium was present, a SrMoO4-type scheelite phase was always observed as a reaction product, and when manganese was present, LaMnO3+ single crystals were observed on the surface of the LMO pellets. Depending on configuration and annealing temperature, additional phases such as La2MoO6 and La6MoO12 have also been detected. Reaction mechanisms and detailed reaction formulae are proposed to explain these observations. The strongest driving force for cationic diffusion appears to originate from Mo6+ and Mn3+ cations, rather than from Sr2+.

International audience; X-ray absorption near edge structure (XANES) measurements at the Ni K edge of PrNiO3 across the metal insulator transition show that a change in the local lattice structure accompanies such transition. Theoretical calculations of the XANES show that the observed features of the spectra are consistent with the orthorhombic structure, reported by neutron diffraction, in the metallic phase. However, for the insulating phase, a change in the local symmetry, consistent with a monoclinic structure, is required to reproduce the spectra. This local symmetry change yields to two nickel sites with different local environment and implies a strong electron-lattice interaction at the local level. This claim is also supported by the observation of a O-16-O-18 isotopic shift in the temperature at which the XANES exhibits the change attributed to the switching of the local symmetry from orthorhombic to monoclinic.

International audience

International audience; The crystal structure of the solid solutions La2Mo2-yCryO9 (y≤0.5) and La2Mo2-yWyO9 (y≤1.4) has been studied using X-ray and neutron powder diffraction. These two series of lanthanum molybdates, which belong to the LAMOX family of fast oxide-ion conductors, exhibit a different structural behavior depending on the substituting element. The Cr series follows a regular Vegard-type evolution of crystallographic parameters. However, the behavior of theWseries is different, the lattice constant varying in a nonlinear fashion with substitution level, resulting in a smaller cell volume for higher tungsten contents, despite the larger ionic radius of tungsten. Two main structural effects are evidenced: a variation in the distribution of ligands around tungsten, made apparent through the changing balance of oxygen O2 and O3 site occupancies along the series, and a nonlinear evolution of interatomic distances and angles involving the O1 oxygen site. An alternative structural description, based on [O1La3Mo] antitetrahedra, is proposed in order to better account for the transport properties of these materials.

A short review of our recent works and original results on the new LAMOX family of fast oxide-ion conductors based on La 2 Mo 2 O 9 are presented. The order/disorder phase transition of La 2 Mo 2 O 9 , accompanied by an increase of anion conductivity, is suppressed by most of the substituants which stabilize the highly conducting cubic phase down to room temperature. The series La 2-x R x Mo 2-y W y O 9 (R=Nd,Gd,Y) was studied with respect to the optimisation of anion conduction and stability against reduction. Tungsten substitution is a good way to mi nimize oxygen loss under reducing atmosphere, a drawback in applications such as electrolytes for solid oxide fuel cells. Ball-milling and dilatom etric studies allowed an efficient control and improvement of the samples relative density, for a better characterisation of the oxygen diffusion pro perties, a first account of which is presented here. The LAMOX family exhibits among the highest oxygen tracer diffusion coefficients. A specific curvature in the conductivity curves of cubic substituted phases is interpreted as resulting from a VTF-type conduction mechanism above a certain temperature , an indication of the existence of a dynamic disorder at high T, which would freeze while cooling down.

International audience

International audience; We performed a photoemission investigation of the electronic structure of PrNiO3 and NdNiO3 through the first order (paramagnetic) metal to (antiferromagnetic) insulator transition. Surprisingly, the data reveal a temperature-dependent loss of spectral weight near the chemical potential, which extends well below the coincident metal-insulator (T-MI) and magnetic (T-Neel) transition temperatures. This is in contrast with the behavior in SmNiO3 and EuNiO3, where the two transitions are separate. The spectral properties clearly indicate two distinct regimes for RNiO3 with T-MI = T-Neel and T-MI > T-Neel.

International audience; The observation of unusually large, positive and rare-earth dependent O-16-O-18 isotope shifts in the metal-insulator transition temperature T-MI Of RNIO3 perovskites is reported (Delta T-MI less than or equal to +10.3 K). The results clearly indicate that the mechanism of the transition involves a strong electron-lattice interaction. A simple model based on the existence of Jahn-Teller polarons in a stoichiometric, metallic, charge-transfer system is used to account for the evolution of Delta T-MI along the series.

International audience; Neutron-powder-diffraction experiments on 154SmNiO3 and 153EuNiO3 at different temperatures have been performed. At variance with the case of Pr and Nd nickelates, the metal-insulator transition temperature in these compounds are different from the Néel temperature (TM−I≈400K, TN≈230K for Sm and TM−I≈480K, TN≈220K for Eu) so that the magnetic ordering develops upon cooling in an insulating matrix. With decreasing temperature the crystal structure undergoes a 0.15% lattice expansion at TM−I. The crystal symmetry Pbnm does not change within the experimental error, but the Ni-O average distance increases and the superexchange angle Ni-O-Ni decreases slightly in the insulating regime. The magnetic structure of both compounds has the same propagation vector k=(1/2,0,1/2) as observed in Pr and Nd nickelates. This feature leads us to speculate that the orbital ordering responsible for the magnetic structure is a characteristic of the insulating state of the RNiO3 series (at least for light rare earths R). A search for small nuclear superstructure peaks characterizing the orbital ordering has been carried out without success. The consequences of our experimental results are discussed in the context of current ideas about superexchange in oxides.

International audience; Resistivity and susceptibility measurements on the perovskite Nd0.7La0.3NiO3 reveal the coincidence of magnetic and metal-insulator transitions at TMI=110 K. Muon-spin-relaxation (μSR) has been used to characterize the magnetic order in the low-temperature insulating phase. Two magnetically inequivalent muon sites exist there below TMI. It is found that the crystallographically equivalent rare-earth planes split into two magnetically inequivalent planes below TMI, one plane containing ordered Nd ions, the other remaining paramagnetic down to low temperatures. This behavior is a consequence of the unusual magnetic order of the Ni ions below TMI, with the μSR data confirming the disappearance of the inversion center at the Ni site in the magnetic insulating regime of the RNiO3 perovskites.

International audience; The electronic ground state and magnetic properties of the wide-gapped antiferromagnetic insulator LaFeO3 have been investigated by 57Fe Mössbauer spectroscopy at pressures up to 63 GPa and temperatures of 7–300 K. Two separate magnetic-electronic phase transitions have been identified in the measured pressure range. At 300 K a new nonmagnetic phase begins to evolve at ∼30 GPa and coexists in ever increasing abundance with the original magnetic phase at pressures up to ∼45 GPa. In the range 45–55 GPa and 300 K the spectrum is comprised solely of a nonmagnetic phase having a single Fe site. Spectra recorded at 48 GPa and temperatures down to 7 K exhibit features of paramagnetic hyperfine structure. The results are consistent with a progressive transition from a magnetically ordered state to that of a spin-disordered state in the range 30–45 GPa. At 300 K and higher pressures of 55–63 GPa the nonmagnetic spectra show features of two sites with similar isomer shifts but different quadrupole splittings. Pressure evolution of the hyperfine interaction parameters and the magnetic transition at 30–45 GPa may be explained by a change of the original Fe high-spin state, namely, spin crossover to a low-spin configuration. The two sites at 55–63 GPa have been attributed to the coexistence of different charge states at crystallographically equivalent sites. These distinct charge states are supposed to represent both low-spin Fe(III) and Fe(II) as a result of fluctuations across a ligand-to-metal charge-transfer gap Δ∼kBT that has been reduced under high pressure.