Ce travail est consacré à l’élaboration par projection plasma à la pression atmosphérique, et à la caractérisation, de couches d’électrolyte en apatite de type La9Sr1Si6O26,5 pour les piles à combustibles à oxyde solide (SOFC) fonctionnant à température intermédiaire (700 °C). Dans un premier temps, ce travail a porté sur le choix des conditions opératoires permettant l’élaboration de dépôts denses d’apatite, caractéristique essentielle d’un électrolyte. Il est apparu que l’utilisation de mélanges plasmagènes à enthalpie et conductivité thermique élevées ainsi qu’un faible débit massique, soit favorable à l’obtention des dépôts les plus denses. Les deux techniques de frittage essayées (le frittage conventionnel et le frittage par « Spark Plasma Sintering ») ont conduit à la cristallisation et à la densification des dépôts, permettant ainsi d’améliorer leurs propriétés électrochimiques. L’augmentation de conductivité ionique est particulièrement notable pour les dépôts réalisés avec des conditions de projection très énergétiques et frittés de manière classique sous air. La conductivité ionique atteint 1,3.10-3 de Ω-1.cm-1 à 700 °C. Dans un second temps, les nickelates de type (La2, Nd2 et Pr2)NiO4+δ ont été étudiés comme matériau de cathode. Leurs compatibilités avec l’électrolyte en apatite ont été vérifiées et leurs températures de frittage ont été optimisées pour obtenir une microstructure suffisamment poreuse. Les meilleures performances électrochimiques sont ainsi obtenues pour les cathodes Pr2NiO4+δ frittées à 1100 °C dont l’ASR (Area-Specific Resistivity) est de 0,2 Ω.cm2 à 700 °C. / This study deals with the elaboration by atmospheric plasma spraying process and the characterization of La9Sr1Si6O26,5 lanthanum silicates electrolyte for SOFC cell working at intermediate temperature (700 °C). In a first step, this work has been devoted to choose the plasma spraying conditions to elaborate dense apatite coatings, fundamental characteristic of an electrolyte. It has been observed that the use of plasma mixture with high enthalpy and thermal conductivity, as well as low gas flow rate, favours dense apatite coatings formation. However, to ensure a sufficient density of the coating while keeping their composition, it is necessary to add a sintering step. The two sintering techniques tested (conventional sintering and Spark Plasma Sintering) have leaded to the coatings crystallization and densification, thus permitted to improve their electrochemical properties. Particularly, a high ionic conductivity increase has been observed for sintered coatings elaborated with high power plasma spraying conditions. The ionic conductivity reaches 1.3.10-3 de Ω-1.cm-1 at 700 °C. In a second step, (La2, Nd2 et Pr2)NiO4+δ type nickelates materials have been studied as cathode. Their compatibility with apatite electrolyte was demonstrated and their sintering temperature was optimized in order to obtain sufficiently porous microstructure. The best electrochemical results were reached with Pr2NiO4+δ cathodes sintered at 1100 °C presenting a 0.2 Ω.cm2 ASR (Area-Specific Resistivity) at 700 °C.

Les nickelates de terre – rare, présentent une transition métal - isolant abrupte dont la température dépend de la composition. Cette transition se traduit dans le domaine infrarouge par le passage d'un comportement opaque à transparent en fonction de la température. Cette propriété se traduit par des contrastes en émissivité et présente un intérêt dans le domaine de la furtivité infrarouge. Le travail présenté porte sur la validation d’une solution solide de nickelate de terre rare de formule Nd0,3Sm0,7NiO3 sous forme de pigments actifs pour la furtivité infrarouge. Dans un premier temps, nous avons cherché à optimiser le procédé de synthèse en nous intéressant à la pression d’oxygène durant le recuit sous pression. Nous avons caractérisé les propriétés structurales de chaque lot de poudre synthétisée..La seconde partie est consacrée aux propriétés thermo – optique de Nd0,3Sm0,7NiO3 sous différentes mis en forme (céramique, poudre, film hybride). La dernière partie à proposer des simulations de comportement optique de pigment de Nd0.3Sm0.7NiO3 dans différentes matrices. / The rare – earth nickelats exhibit a sharp metal – insulator transition whose temperature depends on the composition. This transition is responsible, in the infrared range, of the . This work deals with the validation of nd0,3Sm0,7NiO3 powders like active pigments for the infared furtivity. The first part consists of the optimisations of synthesis process by reducting the oxygen pressure during the annealing. We have characterized the structural properties of each batch synthetized. The second part presents the thermo – optical properties of …. The last part proposes some simulations of the optical behaviour of Nd0,3Sm0,7NiO3 in various matrix.

The US navy has a critical need for air independent advanced electric power sources to replace batteries in unmanned undersea vehicles (UUVs). Solid oxide fuel cells (SOFCs) are being considered as one potential replacement option. However, SOFCs typically operate using atmospheric air as their oxidant which is not an option for this underwater application. For this application, pure pressurized oxygen would be used as the oxidant which motivates the search for a cathode material which would be optimal for a high oxygen partial pressure environments. Specifically, this research focuses on cathode materials which can exploit the unique operating conditions required for UUVs. The operation in 100% oxygen atmosphere rather than air provides a significant opportunity. This is because oxygen surface exchange and bulk transport through the cathode is mediated through point defects whose concentrations are sensitive to the partial pressure of oxygen in the atmosphere surrounding the cathode. Oxygen bulk transport along with oxygen surface exchange are the rate controlling steps in oxygen reduction and incorporation at the cathode. The focus of this research is to examine the relationship between oxygen partial pressure and its effect on SOFC cathode performance for two different families of cathode materials, namely strontium doped lanthanum manganite, and a relatively new class of cathode materials, rare-earth nickelates. The experimentally measured relationship between cathode polarization and oxygen partial pressure will be correlated with the underlying transport and surface exchange processes in both families of materials.

The need for cleaner and more efficient alternative energy sources is becoming urgent as concerns mount about climate change wrought by greenhouse gas emissions. Solid oxide fuel cells (SOFCs) are one of the most efficient options if the goal is to reduce emissions while still operating on fossil energy resources. One of the foremost problems in SOFCs that causes efficiency loss is the polarization resistance associated with the oxygen reduction reaction(ORR) at the cathodes. Hence, improving the cathode design will greatly enhance the overall performance of SOFCs. Lanthanum nickelate, La2NiO4+δ (LNO), is a mixed ionic and electronic conductor that has competitive surface oxygen exchange and transport properties and excellent electrical conductivity compared to perovskite-type oxides. This makes it an excellent candidate for solid oxide fuel cell (SOFC) applications. It has been previously shown that composites of LNO with Sm0.2Ce0.8O2-δ (SDC20) as cathode materials lead to higher performance than standalone LNO. However, in contact with lanthanide-doped ceria, LNO decomposes resulting in free NiO and ceria with higher lanthanide dopant concentration. In this study, the aforementioned instability of LNO has been addressed by compositional tailoring of LNO: lanthanide doped ceria (LnxCe1-xO2,LnDC)composite. By increasing the lanthanide dopant concentration in the ceria phase close to its solubility limit, the LNO phase has been stabilized in the LNO:LnDC composites. Electrical conductivity of the composites as a function of LNO volume fraction and temperature has been measured, and analyzed using a resistive network model which allows the identification of a percolation threshold for the LNO phase. The thermomechanical compatibility of these composites has been investigated with SOFC systems through measurement of the coefficients of thermal expansion. LNO:LDC40 composites containing LNO lower than 50 vol%and higher than 40 vol% were identified as being suitable to incorporate into full button cell configuration from the standpoint of thermomechanical stability and adequate electrical conductivity. Proof-of-concept performance comparison for SOFC button cells manufactured using LNO: La0.4Ce0.6O2-δ composite to the conventional composite cathode materials has also been provided. This thermodynamics-based phase stabilization strategy can be applied to a wider range of materials in the same crystallographic family, thus providing the SOFC community with alternate material options for high performance devices.

Single layered perovskites with the chemical formula La2−xSrxTO4 (T = transition metal) exhibit a variety of intriguing ordering phenomena. The most outstanding is the occurrence of high temperature superconductivity in La2−xSrxCuO4, which can be considered as the prototype system for the more complex cuprates. Some cuprates show incommensurate static charge order at low temperatures [38–40]. For others it is believed that charges are dynamically correlated [39, 147, 259]. Such effects are difficult to measure if the charges fluctuate. In contrast to the cuprate La2−xSrxCuO4 the isostructural nickelates and cobaltates remain insulating over a wide doping range [112, 134, 135, 138]. While incommensurate charge stripe order is long known for the nickelates, recently also evidence for charge stripes in cobaltates has been published [174]. Single crystal rods, with ≈10cm length and ≈0.8cm diameter, have been grown by the traveling solvent floating zone technique using an optical four mirror furnace. We investigated strontium doped nickelates in the range 0.15 ≤ x ≤ 0.22. In addition, also co-doped nickelates have been investigated. A large number of samples with different doping concentrations enabled us to systematically characterize the sample properties. Powder X-ray diffraction measurements were used to determine the lattice parameters. For the nickelates we could confirm the doping dependence of the lattice constants reported in literature [202]. The main interest for the cobaltate system was in the strontium doping range 1/3 ≤ x ≤ 1/2. It was previously reported that the ab-lattice parameter exhibits an anomalous peak around a Sr doping x ≈ 1/3 [140]. We could not confirm such an anomaly for our samples and, instead, we observe a strictly monotonic doping dependence of the lattice parameters which we attribute to the close to perfect stoichiometry of our samples. Samples with the 214-layered perovskite structure can be synthesized over a wide range of oxygen off-stoichiometry. However, the oxygen content can have similarly strong influence on the sample properties as strontium doping. It is therefore essential for data interpretation to determine the oxygen off stoichiometry. EDX and WDX measurements were used to confirm the oxygen content in our nickelates to be nearly stoichiometric. The oxygen content determination of the cobaltates is somewhat more difficult. Thermogravimetry measurements in a flow of Ar/H2 confirmed a nearly stoichiometric oxygen content δ in La2−xSrxCoO4+δ for all samples. We used neutron diffraction measurements to determine the magnetic order in our nickelate samples. In stripe ordered nickelates a small titanium co-doping of the order of 5% is suficcient to supress the incommensurate magnetism and restore antiferromagnetic order. Within the series of zinc co-doped nickelates three samples exhibit an incommensurability epsilon ≈ 1/8, indicating the stabilization of an intermediate stripe pattern with an eightfold unit cell. Compared to the epsilon ≈ 1/3 regime the correlation length is greatly reduced. The magnon dispersion of two samples within the intermediate stripe phases with epsilon ≈ 1/8 and epsilon ≈ 1/4 has been measured with neutron spectroscopy. The observed dispersion neither resembles the one in the undoped nor the 1/3 strontium doped samples. Despite the amount of disorder in our co-doped nickelate materials there are no clear signs for the emergence of hourglass spectra which is most likely caused by a strong exchange interaction across the holes. We investigated the charge and magnetic order in the incommensurate regime of La2−xSrxCoO4 with doping 0.33 ≤ x ≤ 0.5 by elastic neutron scattering and hard X-ray synchrotron measurements. In contrast to the established opinion that this phase is characterized by charge stripe order we were able to show that no charge stripes are present. Instead we found that checkerboard charge order, which is most stable at x = 1/2, persists to a much lower doping than previously thought. The absence of charge stripes is also in agreement with the dispersion of the top most Co-O bond stretching phonon mode. Charge order can induce an anomaly in this branch according to the modulation vector ~q. We observed a softening at ~q = (1/2 1/2 0), which is consistent with our expectations for a checkerboard charge ordered phase. Inelastic neutron measurements revealed an additional high energy part of the hourglass dispersion which has not been reported so far. The entire lowenergy spin excitations that belong to the classical hour-glass dispersion are mostly in-plane excitations, the newly discovered high-energy magnon mode arises from out-of-plane excitations. The resemblance between the low energy excitations below the neck of the hourglass with the excitations in La1.5Sr0.5CoO4 and similarly between the high energy excitations with those observed in La2CoO4 suggests that the observed dispersion is not a single dispersion, but instead consists of two dispersions with distinct origin. In this model the low-energy dispersion arises mainly from magnetic excitations of hole doped regions and the high-energy part would be connected to magnetic excitations within the undoped islands. The absence of charge stripe order in the insulating cobaltates in combination with an unmagnetic low spin state for Co+3 requires a different explanation for the presence of incommensurate magnetic order. We propose a picture on the basis of the ideal checkerboard charge order of the half doped reference system. Decreasing the strontium concentration requires the replacement of Co+3 by Co+2, effectively resulting in the competition between the antiferromagnetic order of the undoped and the antiferromagnetic order of the half doped compound. The induced frustration can be released by a twisting of magnetic moments away from their antiferromagnetic orientation, ultimately leading to the observed incommensurate magnetic order.

High performance miniaturized passives are of great importance for advanced nanoelectronic packages for several applications including efficient power delivery. Low cost thin film capacitors fabricated directly on package (and/or on-chip) are an attractive approach towards realizing such devices. This thesis aims to explore fundamental frequency dependent dielectric and insulating properties of thin film high-k dielectric constant in the perovskite and perovskite-related complex oxides. Throughout this thesis, we have successfully observed the role of structure, strain and oxygen stoichiometry on the dielectric properties of thin film complex oxides, allowing a greater understanding of processing conditions and polarization mechanisms. In the first section of the thesis, we explore novel processing methods in the conventional ferroelectric, barium strontium titanate, \(Ba_{1-x}Sr_xTiO_3 (BST)\), using ultraviolet enhanced oxidation techniques in order to achieve improvements in the dielectric properties. Using this method, we also explore the growth of BST on inexpensive non-noble metals such as Ni which presents technical challenges due to the ability to oxidize at high temperatures. We observe a significant lowering of the dielectric loss while also lowering the process temperature which allows us to maintain an intimate interface between the dielectric layer and the metal electrode. The second section of this thesis explores the novel dielectric material, Lanthanum Strontium Nickelate, \(La_{2-x}Sr_xNiO_4 (LSNO)\), which exhibits a colossal dielectric response. For the first time, we report on the colossal dielectric properties of polycrystalline and epitaxial thin film LSNO. We observe a significant polarization dependence on the microstructure due to the grain/grain boundary interaction with charged carriers. We next grew epitaxial films on various insulating oxide substrates in order to decouple the grain boundary interaction. Here we observed substrate dependent dielectric properties associated with induced strain. We also observe, due to the p-type carriers in LSNO, pn junction formation when grown epitaxially on the conducting oxide degenerate n-type Nb-doped \(SrTiO_3\). Finally we explore the growth mechanism of epitaxial LSNO as a function of high oxygen content. Due to the ability for LSNO to take in interstitial oxygen, a reoriented growth is observed at a critical thickness, thereby allowing us to vary anisotropy as a function of deposition conditions. / Engineering and Applied Sciences

Single layered perovskites with the chemical formula La2−xSrxTO4 (T = transition metal) exhibit a variety of intriguing ordering phenomena. The most outstanding is the occurrence of high temperature superconductivity in La2−xSrxCuO4, which can be considered as the prototype system for the more complex cuprates. Some cuprates show incommensurate static charge order at low temperatures [38–40]. For others it is believed that charges are dynamically correlated [39, 147, 259]. Such effects are difficult to measure if the charges fluctuate. In contrast to the cuprate La2−xSrxCuO4 the isostructural nickelates and cobaltates remain insulating over a wide doping range [112, 134, 135, 138]. While incommensurate charge stripe order is long known for the nickelates, recently also evidence for charge stripes in cobaltates has been published [174]. Single crystal rods, with ≈10cm length and ≈0.8cm diameter, have been grown by the traveling solvent floating zone technique using an optical four mirror furnace. We investigated strontium doped nickelates in the range 0.15 ≤ x ≤ 0.22. In addition, also co-doped nickelates have been investigated. A large number of samples with different doping concentrations enabled us to systematically characterize the sample properties. Powder X-ray diffraction measurements were used to determine the lattice parameters. For the nickelates we could confirm the doping dependence of the lattice constants reported in literature [202]. The main interest for the cobaltate system was in the strontium doping range 1/3 ≤ x ≤ 1/2. It was previously reported that the ab-lattice parameter exhibits an anomalous peak around a Sr doping x ≈ 1/3 [140]. We could not confirm such an anomaly for our samples and, instead, we observe a strictly monotonic doping dependence of the lattice parameters which we attribute to the close to perfect stoichiometry of our samples. Samples with the 214-layered perovskite structure can be synthesized over a wide range of oxygen off-stoichiometry. However, the oxygen content can have similarly strong influence on the sample properties as strontium doping. It is therefore essential for data interpretation to determine the oxygen off stoichiometry. EDX and WDX measurements were used to confirm the oxygen content in our nickelates to be nearly stoichiometric. The oxygen content determination of the cobaltates is somewhat more difficult. Thermogravimetry measurements in a flow of Ar/H2 confirmed a nearly stoichiometric oxygen content δ in La2−xSrxCoO4+δ for all samples. We used neutron diffraction measurements to determine the magnetic order in our nickelate samples. In stripe ordered nickelates a small titanium co-doping of the order of 5% is suficcient to supress the incommensurate magnetism and restore antiferromagnetic order. Within the series of zinc co-doped nickelates three samples exhibit an incommensurability epsilon ≈ 1/8, indicating the stabilization of an intermediate stripe pattern with an eightfold unit cell. Compared to the epsilon ≈ 1/3 regime the correlation length is greatly reduced. The magnon dispersion of two samples within the intermediate stripe phases with epsilon ≈ 1/8 and epsilon ≈ 1/4 has been measured with neutron spectroscopy. The observed dispersion neither resembles the one in the undoped nor the 1/3 strontium doped samples. Despite the amount of disorder in our co-doped nickelate materials there are no clear signs for the emergence of hourglass spectra which is most likely caused by a strong exchange interaction across the holes. We investigated the charge and magnetic order in the incommensurate regime of La2−xSrxCoO4 with doping 0.33 ≤ x ≤ 0.5 by elastic neutron scattering and hard X-ray synchrotron measurements. In contrast to the established opinion that this phase is characterized by charge stripe order we were able to show that no charge stripes are present. Instead we found that checkerboard charge order, which is most stable at x = 1/2, persists to a much lower doping than previously thought. The absence of charge stripes is also in agreement with the dispersion of the top most Co-O bond stretching phonon mode. Charge order can induce an anomaly in this branch according to the modulation vector ~q. We observed a softening at ~q = (1/2 1/2 0), which is consistent with our expectations for a checkerboard charge ordered phase. Inelastic neutron measurements revealed an additional high energy part of the hourglass dispersion which has not been reported so far. The entire lowenergy spin excitations that belong to the classical hour-glass dispersion are mostly in-plane excitations, the newly discovered high-energy magnon mode arises from out-of-plane excitations. The resemblance between the low energy excitations below the neck of the hourglass with the excitations in La1.5Sr0.5CoO4 and similarly between the high energy excitations with those observed in La2CoO4 suggests that the observed dispersion is not a single dispersion, but instead consists of two dispersions with distinct origin. In this model the low-energy dispersion arises mainly from magnetic excitations of hole doped regions and the high-energy part would be connected to magnetic excitations within the undoped islands. The absence of charge stripe order in the insulating cobaltates in combination with an unmagnetic low spin state for Co+3 requires a different explanation for the presence of incommensurate magnetic order. We propose a picture on the basis of the ideal checkerboard charge order of the half doped reference system. Decreasing the strontium concentration requires the replacement of Co+3 by Co+2, effectively resulting in the competition between the antiferromagnetic order of the undoped and the antiferromagnetic order of the half doped compound. The induced frustration can be released by a twisting of magnetic moments away from their antiferromagnetic orientation, ultimately leading to the observed incommensurate magnetic order.

Philosophiae Doctor - PhD / Very few systems allow the study of the relationship between structural changes and physical properties in such a clear way as rare earth nickelate ReNi03 perovskites (Re (rare earth) = Pr, Nd, Sm and Gd). Synthesized for the first time by Demazeau et al [1] in 1971 and completely forgotten for almost twenty years, these compounds have regained interest since the discovery of high-temperature superconductivity and giant magnetoresistive effects in other perovskite-related systems. Due to its Metal-Insulator Transition (MIT) and thermochromic properties, the rare earth nickelate perovskite ReNi03 has received a great deal of attention for the past ten years in their thin films form [12]. Such unusual electronic and optical features are all the more interesting since the metal-insulator transition temperature (TMn) can be tuned by changing the Re cation: LaNi03 is metallic. No minimum of the metallic conductivity of Sm0 . ssNd 0.45Ni03, as observed by Gire et al [12] (entropic effect), was reported by Ambrosini and Hamet [11]. It has been suggested by Obradors et al. [13] that changing the rare earth cation in the ReNi03 system, acts as internal chemical pressure (increasing internal pressure by substituting the rare earth cation with another one of larger ionic radius) which can lead, as for the isostatic pressure experiment, to a tunability of the metal-insulator transition temperature [14, 15]. Obradors et al [13] reported on a decrease of T MIT upon increasing isostatic pressure but with remaining metallic properties of PrNi03 and NdNi03 (same magnitude and thermal dependence of the electrical resistivity)

This work presents a study on stoichiometry and structure in perovskite-type oxide thin films and investigates the role of growth–induced defects on the properties of materials. It also explores the possibility to grow thin films with properties close or similar to the ideal bulk parent compound. A novel approach to the growth of thin films, atomic layer-by-layer (ALL) laser molecular beam epitaxy (MBE) using separate oxide targets is introduced to better control the assembly of each atomic layer and to improve interface perfection and stoichiometry. It also is a way to layer materials to achieve a new structure that does not exist in nature. This thesis is divided into three sections. In the first part, we use pulsed laser deposition (PLD) to grow LaAlO3 (LAO) thin films on SrTiO3 (STO) and LAO substrates in a broad range of laser energy density and oxygen pressure. Using x-ray diffraction (θ-2θ scan and reciprocal space mapping), transmission electron microscopy (TEM) and x-ray fluorescence (XRF) we studied stoichiometry and structure of LAO films as a function of growth parameters. We show deviation from bulk–like structure and composition when films are grown at oxygen pressures lower than 10-2 Torr. We conclude that the discussion of LAO/STO interfacial properties should include the effects of growth–induced defects in the LAO films when the deposition is conducted at low oxygen pressures, as is typically reported in the literature. In the second part, we describe a new approach to atomically layer the growth of perovskite oxides: (ALL) laser MBE, using separate oxide targets to grow materials as perfectly as possible starting from the first atomic layer. We use All laser MBE to grow Ruddlesden–Popper (RP) phase Lan+1NinO3n+1 with n = 1, 2, 3 and 4 and we show that this technique enables us to construct new layered materials (n=4). In the last and main section of this thesis, we use All laser MBE from separate oxide targets to build the LaNiO3 (LNO) films as near perfectly as possible by depositing one atomic layer at a time. We study the thickness dependent metal-insulator transition (MIT) in ultrathin LNO films on an LAO substrate. In LNO, the MIT occurs in thin films and superlattices that are only a few unit cells in thickness, the understanding of which remains elusive despite tremendous effort devoted to the subject. Quantum confinement and structure distortion have been evoked as the mechanism of the MIT; however, first-principle calculations show that LaNiO3 remains metallic even at one unit cell thickness. Here, we show that thicknesses of a few unit cells, growth–induced disorders such as cation stoichiometry, oxygen vacancies, and substrate-film interface quality will impact the film properties significantly. We find that a film as thin as 2 unit cells, with LaO termination, is metallic above 150 K. An oxygen K-edge feature in the x-ray absorption spectra is clearly inked to the transition to the insulating phase as well as oxygen vacancies. We conclude that dimensionality and strain are not sufficient to induce the MIT without the contribution of oxygen vacancies in LNO ultrathin films. Dimensionality, strain, crystallinity, cation stoichiometry, and oxygen vacancies are all indispensable ingredients in a true control of the electronic properties of nanoscale strongly–correlated materials. / Physics

Des matériaux d'électrode positive pour batteries Li-ion de formule Li(Ni,M)O2 (M = Al, Co/Al et Mn) ont été synthétisés par coprécipitation, puis caractérisés par diffraction des rayons X et des neutrons, par des mesures magnétiques et des tests galvanostatiques. La dégradation thermique des phases désintercalées Lix(Ni,M)O2 (M = Al, Co/Al et Mn, x = 0.50 et 0.30) a ensuite été étudiée par analyses thermogravimétriques couplées à la spectrométrie de masse, corrélées à des expériences de diffraction des rayons X in situ, afin d'en déterminer le mécanisme et d'expliquer les différences de stabilité observées suivant la composition des matériaux. Pour tous les composés étudiés, la dégradation se déroule en deux étapes, correspondant à la transition de la phase lamellaire initiale de type α-NaFeO2 en une phase “ LiM2O4 ” de type pseudo-spinelle qui se transforme ensuite, à plus haute température, en une phase dérivant de NiO. L'influence de la nature de l'élément substituant a été discutée.