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International audience; The functional properties of oxide heterostructures ultimately rely on how the electronic and structural mismatches occurring at interfaces are accommodated by the chosen materials combination. We discuss here LaMnO$_3$/LaNiO$_3$ heterostructures, which display an intrinsic interface structural asymmetry depending on the growth sequence. Using a variety of synchrotron-based techniques, we show that the degree of intermixing at the monolayer scale allows interface-driven properties such as charge transfer and the induced magnetic moment in the nickelate layer to be controlled. Further, our results demonstrate that the magnetic state of strained LaMnO$_3$ thin films dramatically depends on interface reconstructions.

A marked conductivity enhancement is reported in 6-11 unit cell LaNiO3 thin films. A maximal conductivity is also observed in ab initio calculations for films of the same thickness. In agreement with results from state of the art scanning transmission electron microscopy, the calculations also reveal a differentiated film structure comprising characteristic surface, interior, and heterointerface structures. Based on this observation, a three-element parallel conductor model is considered and leads to the conclusion that the conductivity enhancement for films of 6-11 unit cells, stems from the onset of intercompetition between the three local structures in the film depth. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

NdNiO3 and SmxCa1-xMnO3 perovskites thin films were synthesized by magnetron co-sputtering process and the thermochromic properties have been studied in the infrared range. At first, the study of the NdNiO3 nickelate has established the timeliness to synthesise the perovskite phase using a subsequent annealing crystallisation in air at moderate temperature (550°C). We have demonstrated a competitive crystallisation between the thermochromic orthorhombic phase and a non-thermochromic rhombohedral phase largely influenced by experimental parameters: deposition pressure, layer thickness, annealing temperature... The presence of these two phases allowed to clarify more precisely the oxidation mechanisms involved during the deposition process and the reduction mechanisms occurring during the heat treatment. At the same time, the SmxCa1-xMnO3 system was synthesised also as thin films. The first step includes developing the experimental conditions to obtain stoichiometric films using two separate targets. After structural analyses (SEM, XRD), the electrical and optical behaviours of films were analysed using the four probe configuration and the FTIR spectroscopy versus temperature respectively. As result, the metal-insulator transition of the Sm0,5Ca0,5MnO3 compound is close to the room temperature and the film shows an interesting optical contrast in the near and mid-infrared ranges. The optical transmittance measurements in the visible-near infrared range were used to evaluate the optical gap dependence with temperature. At room temperature, the optical gap is close to 0.7 eV. Finally, SmxCa1-xMnO3 manganite is a good candidate for thermal regulation and infrared furtivity; Des films minces de pérovskites NdNiO3 et SmxCa1-xMnO3 ont été synthétisés par co-pulvérisation magnétron et leurs propriétés thermochromes ont été étudiées dans le domaine infrarouge. Dans un premier temps, l'étude du nickelate NdNiO3 a montré la possibilité de synthétiser la phase pérovskite en utilisant un recuit de cristallisation sous air à basse température (550°C). Nous avons mis en évidence une cristallisation compétitive entre la phase orthorhombique thermochrome et une phase rhomboédrique non thermochrome qui est largement influencée par les paramètres d'élaboration : pression de dépôt, épaisseur des couches, température de recuit... L'obtention de ces deux phases a permis d'étudier plus précisément les mécanismes d'oxydation et de réduction intervenant lors du dépôt et lors du traitement thermique respectivement. Parallèlement à cette étude, des couches minces de SmxCa1-xMnO3, ont été également synthétisées par co-pulvérisation. Après une optimisation des conditions expérimentales permettant d'obtenir des films stoechiométriques et des analyses structurales (MEB, DRX), les comportements électriques et optiques des films ont été suivis en fonction de la température (mesure 4 pointes et IRTF). Le composé Sm0,5Ca0,5MnO3 présente une température de transition proche de la température ambiante ainsi qu'un contraste optique dans le très proche et le moyen infrarouge. Les mesures de la transmittance optique dans le domaine visible-proche infrarouge ont permis d'évaluer la dépendance du gap optique avec la température. A 20°C, celui-ci est proche de 0,7 eV. Le manganite SmxCa1-xMnO3 est donc un bon candidat pour la régulation thermique et la furtivité infrarouge

NdNiO3 and SmxCa1-xMnO3 perovskites thin films were synthesized by magnetron co-sputtering process and the thermochromic properties have been studied in the infrared range. At first, the study of the NdNiO3 nickelate has established the timeliness to synthesise the perovskite phase using a subsequent annealing crystallisation in air at moderate temperature (550°C). We have demonstrated a competitive crystallisation between the thermochromic orthorhombic phase and a non-thermochromic rhombohedral phase largely influenced by experimental parameters: deposition pressure, layer thickness, annealing temperature... The presence of these two phases allowed to clarify more precisely the oxidation mechanisms involved during the deposition process and the reduction mechanisms occurring during the heat treatment. At the same time, the SmxCa1-xMnO3 system was synthesised also as thin films. The first step includes developing the experimental conditions to obtain stoichiometric films using two separate targets. After structural analyses (SEM, XRD), the electrical and optical behaviours of films were analysed using the four probe configuration and the FTIR spectroscopy versus temperature respectively. As result, the metal-insulator transition of the Sm0,5Ca0,5MnO3 compound is close to the room temperature and the film shows an interesting optical contrast in the near and mid-infrared ranges. The optical transmittance measurements in the visible-near infrared range were used to evaluate the optical gap dependence with temperature. At room temperature, the optical gap is close to 0.7 eV. Finally, SmxCa1-xMnO3 manganite is a good candidate for thermal regulation and infrared furtivity, Des films minces de pérovskites NdNiO3 et SmxCa1-xMnO3 ont été synthétisés par co-pulvérisation magnétron et leurs propriétés thermochromes ont été étudiées dans le domaine infrarouge. Dans un premier temps, l'étude du nickelate NdNiO3 a montré la possibilité de synthétiser la phase pérovskite en utilisant un recuit de cristallisation sous air à basse température (550°C). Nous avons mis en évidence une cristallisation compétitive entre la phase orthorhombique thermochrome et une phase rhomboédrique non thermochrome qui est largement influencée par les paramètres d'élaboration : pression de dépôt, épaisseur des couches, température de recuit... L'obtention de ces deux phases a permis d'étudier plus précisément les mécanismes d'oxydation et de réduction intervenant lors du dépôt et lors du traitement thermique respectivement. Parallèlement à cette étude, des couches minces de SmxCa1-xMnO3, ont été également synthétisées par co-pulvérisation. Après une optimisation des conditions expérimentales permettant d'obtenir des films stoechiométriques et des analyses structurales (MEB, DRX), les comportements électriques et optiques des films ont été suivis en fonction de la température (mesure 4 pointes et IRTF). Le composé Sm0,5Ca0,5MnO3 présente une température de transition proche de la température ambiante ainsi qu'un contraste optique dans le très proche et le moyen infrarouge. Les mesures de la transmittance optique dans le domaine visible-proche infrarouge ont permis d'évaluer la dépendance du gap optique avec la température. A 20°C, celui-ci est proche de 0,7 eV. Le manganite SmxCa1-xMnO3 est donc un bon candidat pour la régulation thermique et la furtivité infrarouge

We discuss two-magnon phonon-assisted infrared absorption in two-dimensional Heisenberg systems like the parent insulating high-T-c cuprates (spin 1/2) and nickelates (spin 1). The theoretical line shape is compared with experiments. In the case of the cuprates it explains the mid-infrared peaks observed in the insulator. In the case of the nickelates the predicted line shape is also shown to agree with the experiments. We discuss the possibility to observe these excitations in other experiments.

This paper is dedicated to the study of the relationship between the Nd 2 NiO 4+ δ powder micro-structural properties (especially particle size and crystallinity) and electrochemical properties when the oxide is used as SOFC cathode deposited on 8YSZ electrolyte coated with thin doped ceria. Nano-structured particles of Nd 2 NiO 4+ δ with controlled crystallinity, size and morphology have been synthesized using ultrasonic spray pyrolysis (USP). The series and polarization resistances measured on symmetrical half cells Nd 2 NiO 4+ δ /YDC/8YSZ/YDC/Nd 2 NiO 4+ δ are both found to be dependent on the cathode microstructure and present a similar evolution with temperature. The best results are obtained for highly crystalline cathode powders combined with a small particle size.

4 páginas, 3 figuras.-- et al., Reversible electric-field control of the metal-insulator transition in the rare earth nickelate compound NdNiO3 is demonstrated. Using the electric double layer technique, giant modulations in the conductance as well as large shifts in the transition temperature are obtained., This work was supported by the Swiss National Science Foundation through the National Center of Competence in Research, Materials with Novel Electronic Properties, ‘MaNEP’, division II, Oxides and partly by NEDO and by Grant-in-Aid for Scientifi c Research (Grant No. 20740213) from MEXT.

Oxygen 1s x-ray-absorption measurements of La2–xSrxNiO4+δ (0≤x≤1.15) are presented, together with O 1s polarization-dependent x-ray absorption on single-crystal La1.85Sr0.15NiO4. It is concluded that the charge-compensating holes have mainly oxygen 2p character. The Ni 3d9 final states reached because of the p-d hybridization in the ground state are strongly polarized in the (a,b) plane. The Sr-induced O 2p holes are found to be distributed equally over in-plane px,py and out-of-plane pz orbitals. Possible explanations for this are presented. The intrinsic band gap of La2NiO4 is found to be at least 4 eV. The bremsstrahlung-isochromat-spectroscopy results show that the Sr-induced states are spread out over a large portion of the band gap and are not concentrated at the top of the valence band.

In the present work, 57Fe probe Mossbauer spectroscopy was developed to study the nickelates RNi0.98Fe0.02O3 (R = Sm,Eu,Gd,Dy) with the perovskite-like structure. The restoration method for a distribution function P(v) of the positions (v) involving individual Lorentzian lines has been used for processing and analysing the Mossbauer spectra. The P(v) profile for the nickelates, at T

In this thesis, we studied electronic excitations in strongly correlated materials, in particular the transition metal monoxides CoO, NiO and CuO, the cuprate La2CuO4 and the nickelate La2NiO4. Resonant Inelastic X-ray Scattering (RIXS) was used to study the dynamics of low energy (few eV) excitations in these charge transfer insulators. This was possible thanks to instrumental development allowing an overall energy resolution improvement of a factor 3 to reach 300 meV. In monoxides, we focused on localised crystal field excitations while in La2CuO4 and La2NiO4, our emphasis was on the measurement of charge transfer excitations. We observe a characteristic excitation identified as an excitonic feature (electron-hole bound state) in both materials with however a different dynamic behaviour. In La2CuO4, the exciton disperses while in La2NiO4 it is localised through antiferromagnetic correlations. A model invoking Zhang-Rice singlet formation in the cuprate has been proposed to explain the dynamics of the exciton in the antiferromagnetic background.; Cette thèse s'inscrit dans le cadre de l'étude des excitations électroniques dans des matériaux fortement corrélés, ici les monoxydes de métaux de transition CoO, NiO et CuO, le cuprate La2CuO4 et le nickelate La2NiO4. Nous avons utilisé la diffusion inélastique résonante des rayons X (RIXS) pour étudier la dynamique des excitations de basse énergie (quelques eV) dans ces isolants de transfert de charge. Cette étude a été possible grâce à un développement instrumental permettant une amélioration de la résolution expérimentale d'un facteur 3 pour atteindre 300 meV. Dans les monoxydes, nous avons étudié les excitations localisées du champ cristallin. Dans La2CuO4, et La2NiO4, nous avons plus particulièrement étudié les excitations de transfert de charge. Un exciton (état lié électron-trou) a été observé dans les deux matériaux avec cependant une dynamique différente. Dans La2CuO4, l'exciton a un comportement dispersif alors que dans La2NiO4, il est localisé par les corrélations antiferromagnétiques. Un modèle invoquant la formation d'un singulet de Zhang et Rice dans le cuprate a été proposé pour expliquer cette dynamique excitonique dans le réseau antiferromagnétique sous-jacent.

International audience; Mössbauer spectroscopy has been used for following the insulator -> metal transition versus temperature in ANiO3 perovskites (NdNiFe0.02O3 and LuNiFe0.02O3). The evolution of the number of 57Fe sites and the isomer-shift value at the transition bring important local information taking into account the role of the different factors (structural, geometrical, and bond covalency). Mössbauer measurements have been made as a function of temperature to study electronic and vibrational behavior of the 57Fe probe atoms in the nickelate structure.

Version Russe; International audience

International audience; A study of perovskite-like nickelates NdNi0.98Fe0.02O3 and SmNi0.98Fe0.02O3 by Mössbauer spectroscopy in the magnetically ordered temperature...

International audience; The ANi0.98Fe0.02O3 nickelates (A = Pr, Nd, Sm, Lu, Y, Tl) were studied above the temperature of their magnetic ordering (TN) by 57Fe Mössbauer spectroscopy. Orthorhombic nickelates (A = Pr, Nd, Sm) have the only type of Fe(III) position in the symmetric octahedral oxygen surroundings. The 57Fe spectra of monoclinic nickelates (A = Lu, Y, Tl) show two components, which can be assigned to the dopant trivalent iron cations substituted for nickel in the Ni(3frame0')+O6 and Ni(3+)+O6 polyhedra. The data obtained are consistent with the results of neutron diffraction studies of ANiO3 , assuming the 2Ni3+ frame1 Ni(3frame2')+ + Ni(3+)+ disproportionation for monoclinic nickelates (A = Y, Ho-Lu) at T < TIM .

By comparing the optical conductivities of La_{1.67}Sr_{0.33}NiO_{4} (LSNO), Sr_{1.5}La_{0.5}MnO_4 (SLMO), Nd_2CuO_{4-y} (NCO), and Nd_{1.96}Ce_{0.04}CuO_{4} (NCCO), we have identified a peculiar behavior of polarons in this cuprate family. While in LSNO and SLMO small polarons localize into ordered structures below a transition temperature, in those cuprates the polarons appear to be large, and at low T their binding energy decreases. This reflects into an increase of the polaron radius, which may trigger coherent transport., Comment: File latex, 15 p. incl. 4 Figs. epsf, to appear on the Journal of Superconductivity - Proc. "Stripes 1996" - Roma Dec 1996

We discuss two-magnon phonon-assisted infrared absorption in two-dimensional Heisenberg systems like the parent insulating high-T-c cuprates (spin 1/2) and nickelates (spin 1). The theoretical line shape is compared with experiments. In the case of the cuprates it explains the mid-infrared peaks observed in the insulator. In the case of the nickelates the predicted line shape is also shown to agree with the experiments. We discuss the possibility to observe these excitations in other experiments.

We discuss two-magnon phonon-assisted infrared absorption in two-dimensional Heisenberg systems like the parent insulating high-T-c cuprates (spin 1/2) and nickelates (spin 1). The theoretical line shape is compared with experiments. In the case of the cuprates it explains the mid-infrared peaks observed in the insulator. In the case of the nickelates the predicted line shape is also shown to agree with the experiments. We discuss the possibility to observe these excitations in other experiments.

Oxygen 1s x-ray-absorption measurements of La2–xSrxNiO4+δ (0≤x≤1.15) are presented, together with O 1s polarization-dependent x-ray absorption on single-crystal La1.85Sr0.15NiO4. It is concluded that the charge-compensating holes have mainly oxygen 2p character. The Ni 3d9 final states reached because of the p-d hybridization in the ground state are strongly polarized in the (a,b) plane. The Sr-induced O 2p holes are found to be distributed equally over in-plane px,py and out-of-plane pz orbitals. Possible explanations for this are presented. The intrinsic band gap of La2NiO4 is found to be at least 4 eV. The bremsstrahlung-isochromat-spectroscopy results show that the Sr-induced states are spread out over a large portion of the band gap and are not concentrated at the top of the valence band.

International audience; Hyperfine interactions of 57Fe probe atoms in the perovskite structures of the NdNi0.98Fe0.02O3 and LuNi0.98Fe0.02O3 nickelates were studied by Mössbauer spectroscopy. The cardinal difference between the 57Fe spectra of these nickelates was shown to be associated with the specifics of their local crystal structure. Mea- suring the Mössbauer spectra on both sides from the point of the insulator–metal phase transition (TIM) made it possible to comprehensively study the dynamic behavior of dopant iron atoms. Different models were considered to account for the abrupt change in the chemical shift of the 57Fe atoms in the structure of NdNi0.98Fe0.02O3 at TTIM (148 K).

This PhD work is dedicated to stability and ageing studies of Praseodymium based nickelates ascathodes for Solid Oxide Fuel Cells (SOFCs). With this respect Ln2NiO4+δ (Ln=La, Pr or Nd)compounds with the K2NiF4 type structure act as alternative cathode materials for IT-SOFC due totheir mixed ionic and electronic conductivity (i.e. MIEC properties). Pr2NiO4+δ shows excellentelectrochemical properties at intermediate temperature (i.e. low polarization resistance Rp value, Rp= 0.03 Ω.cm² at 700 °C), while La2NiO4+δ exhibits higher chemical stability. So, the properties ofLa2-xPrxNiO4+δ nickelates were investigated with the aim to find best compromise between chemicalstability and electrochemical performances. After synthesis, the physical and chemical properties aswell as their transport and electrochemical properties have been determined. Measurements of thepolarization resistance of symmetrical half-cells have been carried out by impedance spectroscopy.Then, the chemical stability and the electrochemical performance of the materials have been studiedfor duration up to one month. As an interesting point, even after complete dissociation of Pr2NiO4+δinto PrNiO3-δ,Pr4Ni3O10+δ and Pr6O11, the polarization resistance does not show significant change.So finally, two new materials PrNiO3-δ and Pr4Ni3O10+δ were investigated as SOFCs cathodeshowing very promising results for Pr4Ni3O10+δ in symmetrical cell (Rp (Pr4Ni3O10+δ) = Rp(Pr2NiO4+δ) = 0.15 Ω.cm² à 600 ° C) and complete cell (1.6 W.cm-2 at 800 °C)., Ce travail de thèse est consacré à l’étude des nickelates La2-xPrxNiO4+δ, comme nouveaux matériauxde cathodes pour piles à combustible haute température, SOFC, et en particulier à la caractérisationde leur stabilité chimique et leur comportement en fonctionnement. En effet, du fait de leurpropriété de conduction mixte ionique et électronique, MIEC, les nickelates de structure typeK2NiF4, Ln2NiO4+δ (Ln = La, Pr, Nd), correspondant au terme n = 1 de la série de Ruddlesden-Popper (An+1MnO(3n+1)), sont des matériaux prometteurs pour des fonctionnements à températureintermédiaire, IT-SOFC (T < 800 °C). Compromis entre la stabilité chimique de La2NiO4+δ et lestrès bonnes performances électrochimiques de Pr2NiO4+δ, les phases La2-xPrxNiO4+δ, ont étésynthétisées et leurs propriétés physico-chimiques, de transport et électrochimiques ont étédéterminées. L’étude approfondie des caractéristiques des électrodes par spectroscopied’impédance en cellules symétriques a été réalisée à courant nul et sous polarisation anodique etcathodique sur des périodes d’un mois. De façon surprenante, même après la dissociation complètede Pr2NiO4+δ en PrNiO3-δ, Pr4Ni3O10+δ et Pr6O11, la résistance de polarisation ne montre pas dechangement significatif. L’étude de PrNiO3-δ et Pr4Ni3O10+δ, comme matériau de cathode pour pilesà combustible, démontre l’excellent comportement de la phase Pr4Ni3O10+δ et ceci en cellulesymétrique (Rp (Pr4Ni3O10+δ) = Rp (Pr2NiO4+δ) = 0.15 Ω.cm² à 600 ° C) et cellule complète (1.6W.cm-2 at 800 °C).

In rare earth nickelates, the metal-to-insulator transition observed as a function of temperature can be described using an electronic and a structural order parameter. The electronic one is linked to the electronic disproportionation observed below the transition temperature and the structural one characterizes the breathing mode that develops in the low temperature phase. Here SmNiO3/NdNiO3 superlattices are grown with insulating LaAlO3 spacer layers to study the unusual coupling of metal-to- insulator transitions observed at SmNiO3/NdNiO3 interfaces and determine the role of the two order parameters. Temperature-dependent transport measurements reveal that a single unit cell of LaAlO3 inserted between SmNiO3 and NdNiO3 leads to a complete decoupling of the metal-to-insulator transitions suggesting that the order parameter controlling the coupling is the electronic one.