The physics of doped transition metal perovskite has been an area of intense research in the last few decades due to their interesting magnetic and transport properties. Various exciting phenomena such as, colossal magneto resistance, high Tc superconductivity, multiferroicity, ferroelectricity, high temperature ferromagnetism, etc., have made these systems more fascinating in terms of fundamental study as well as technological applications. There are several intrinsic material characteristics in these perovskite oxides that can impact their magnetic properties. Lattice distortion and chemical in homogeneity are two important ones. Changes in valence and ionic radius in rare earth (A- site) and transition metal (B- site) directly result in structural modification through internal pressure. Consequently, atomic distances and bond angles between the transition metals vary. This, intern, influences the nearest neighbour exchange coupling energy and magnetic interaction. A detailed investigation has been carried out on two A-site doped perovskite namely, La0.85Sr0.15CoO3 & La0.5Sr0.5CoO3 and two B-site doped perovskite, LaMn0.5Co0.5O3 & LuMn0.5Ni0.5O3 with a view to study the impact of chemical in homogeneity and lattice distortion on their respective magnetic ground states. The thesis is organized in seven chapters. A brief summary of each is given below: Chapter 1: Provides a brief introduction about the perovskite structure. Origins of lattice distortions and its effect on the magnetic properties are discussed. It includes a discussion on different types of indirect magnetic interactions involved in perovskite oxide structure. The chapter concludes with a description of spin-glass, phase separation/ cluster-glass, memory effect in glassy magnetism, critical behaviour at phase transition and specific heat in magnetic systems. Chapter 2: This chapter outlines basic principles of the experimental techniques employed for the work presented in this thesis. Chapter 3: Details macroscopic as well as microscopic investigations carried out to understand the glassy magnetic anomalies in La0.85Sr0.15CoO3 samples. The origin of phase separation (PS) has been reinvestigated. Since the magnetic behavior of La0.85Sr0.15CoO3 (LSCO15) lies in the border of spin glass (SG) and ferromagnetic (FM) region in the x-T phase diagram, it is subject to controversial debate for the last several years. While some research groups favour PS, others regard SG behaviour as the dominant phenomenon. In-depth investigation carried out to elucidate these views is outlined in this chapter in two sections. The first section deals with the glassy magnetic anomalies in single crystals of LSCO15 grown by optical floating zone method. Since the sample crystallizes from melt, it possesses good compositional homogeneity and the phase purity is confirmed by XRD pattern. Many characteristics of canonical SG systems are discernible in the magnetic study, such as, kink in field-cooling curve below Tf, frequency-dependent peak shift and the time dependent memory effect. The relaxation time in sub-pico second range (~10-13 s) is very similar to that of the typical SG systems. Time dependent transport relaxation study exhibits memory effect and the time evolution of resistance scales with magnetization and strictly adheres to the stretched exponential behaviour as commonly expected for a SG-like disordered system. However, a detailed study on transport mechanism and temperature-dependent inverse susceptibility reveals the existence of nanoscopic PS in the sample. In the second section, the origin of PS has been examined through a comprehensive study on two sets of LSCO15 polycrystalline samples prepared from the same initial mixture but subjected to different heat treatment processes. This study depicts the dependence of PS on the preparation conditions. The contrasting magnetic behaviour of PS and SG was resolved by experiments of dc magnetization, linear & non-linear ac susceptibility, neutron depolarization and field-cooled magnetic relaxation. Both samples conform to the general characteristics of a glassy behaviour: a kink in FC magnetization, frequency-dependent peak shift (Vogel–Fulcher law), dc bias-dependent peak shift in accordance with de Almeida–Thouless relation, and characteristic relaxation time in the range of 10-13/10-14 s. This is despite their internal spin structure and interaction being much different at a microscopic level. It is found that the sample processed through a proper homogenization process mimics the SG behaviour, whereas the sample prepared by the conventional method behaves like the PS phase. It is confirmed from neutron depolarization experiments that no ferromagnetic correlation exists in the SG phase of La0.85Sr0.15CoO3, a result in contrast to that of PS phase. Higher harmonic ac susceptibility measurement complements the above observation by the evidence that of 2nd order harmonics are not present in the SG phase of La0.85Sr0.15CoO3. The field-cooled magnetic relaxation study makes a distinct reference to the relaxation process and the strength of interaction between PS and SG like phases. In essence, a concerted effect is made to identify and resolve the spin-glass phase from phase-separated/ cluster-glass. This work shows that chemical in homogeneity is a key factor responsible for phase separation in La0.85Sr0.15CoO3; also intrinsic differences between PS and SG are identified that can serve as guiding tools for research in other similar magnetic oxide systems. It is concluded that the true ground state magnetic property of La0.85Sr0.15CoO3 is spin-glass in nature. Chapter 4: This chapter contains two sections. In the first part, the origin of the re-entrant spin-glass (RSG) behaviour in La0.5Sr0.5CoO3 has been investigated using the conventional magnetometer measurements. Polycrystalline samples prepared by the conventional solid-state synthesis exhibit RSG characteristics with a glassy transition at 190 K. The nature of frequency dependence of χ″(T), a pronounced memory effect and the sluggish response in dc magnetization measurement, all of which clearly indicate the re-entrant behaviour. But, once the sample is taken through a rigorous homogenization procedure of repeated grinding and annealing, its phase turns into pure ferromagnetic one. During the course of this homogenization process, the sample loses oxygen with concurrent degeneration of TC to a lower level. In order to regain the oxygen stoichiometry, it is necessary to anneal the sample in oxygen environment at 900 oC, which triggers deleterious ageing effect by which TC falls progressively with time. In the second part, the effect of oxygen stoichiometry on La0.5Sr0.5CoO3 (LSCO50) thin-films has been investigated. The highest TC reported so far for LSCO50 thin film is 250 K, which is significantly less compared to the bulk TC (262 K) of an oxygen stoichiometric compound. This work focuses on achieving the highest ferromagnetic transition temperature (TC) for LSCO50 films under optimized growth conditions. The analysis of experimental data suggests that the Curie temperature can be enhanced to 262 K, irrespective of whether or not, (a) the film on LAO or STO or (b) any induced strain occurs in the LSCO50 film. Apart from different thin-film growth parameters such as oxygen pressure and substrate temperature during the growth, and post-growth annealing temperature and oxygen pressure, the profile of the laser beam used for ablation of bulk material profile also plays an important role. The elevation of Curie temperature observed in thin-films to that close to the bulk value is believed to be a result of improved stoichiometric composition of oxygen facilitated during thin film growth. However, the strong ageing effect seen is quite close to that is observed in oxygen-annealed polycrystalline sample. Chapter 5: Of the three segments constituting this chapter, the first outlines different magnetic anomalies induced by lattice distortion in LaMn0.5Co0.5O3 (LMCO) single crystals. Single crystals of LMCO compound [(100) orientation] have been successfully grown using the optical floating zone method. Powder as well as single crystal x-ray diffraction analyses provides evidence of large strain dependent structural distortion in as-grown crystals. Spatially resolved 2-D Raman scan reveals that the strain generates a distribution of octahedral distortion in the lattice. While some are compressive in nature, others in the nearby territory relate to tensile distortion. The ac susceptibility measurement elucidates distinct changes in the ferromagnetic transition temperature (TC) in the as grown (strained) crystal. It is possible to release strain by rigorous annealing process. Which also results in a uniform TM-O octahedral deformation. Room temperature 2-D Raman spectra bears testimony to this. Upon annealing, the single crystalline order is diminuend by the atomic rearrangement. This causes tilting of the oxygen octahedra, by decreasing intra-octahedral angle θTM-O-TM, and lowering of exchange energy Jex between the magnetic ions. The transition temperature falls and the magnetic phase merges with that in the strain-free polycrystalline material. A detailed critical analysis performed in the vicinity of paramagnetic to ferromagnetic phase transition in both the samples establishes that the ground state magnetic behaviour, assigned to the strain-free LMCO crystal is of 3D Heisenberg type. But the local octahedral distortion present in the as-grown crystal causes mean field like magnetic interaction at few local sites. This serves as a key drive for the critical exponents to distance from the 3D Heisenberg model towards the mean-field type. The second part of this chapter concerns the anomalous re-entrant glassy magnetic behaviour observed in LMCO single crystals. The ac susceptibility study illustrates the low temperature anomalous glassy magnetic ordering in these crystals. The material behaves like a normal magnetic glass, (frequency-dependent peak-shift in ac susceptibility) in conformance with the phenomenological Vogel-Fulcher law, of spin flips time: ~10-4 s. However, the crystal does not respond to the external dc bias and just as well remains free from memory effect. Anomalous behaviour of this kind is rare in magnetic oxides. The magneto-dielectric effect in LMCO is discussed in the third section of this chapter. The real part of dielectric permittivity (ε′) has a colossal value of 1800 at 220 K and 10 kHz. However as the sample is cooled further, ε′ decreases slowly; followed by dielectric relaxation in the region, 120 - 150 K. Detailed analysis of the temperature dependence of the imaginary part of the dielectric permittivity (ε″) show that there is no relaxor-like phenomena in this compound. The frequency dependence of ε″ reveals that the low frequency region is dominated by Maxwell-Wagner relaxation, whereas, at high frequency, a Debye type relaxation persists. The temperature dependent full-width at half-maximum for this Debye relaxation, peaks at the corresponding TC. The temperature variation of the relaxation time has two domains of different slopes. At zero external field, ε″(ω) has a low activation energy (U = 46.4 meV) in the ferromagnetic region, compared to that in the paramagnetic (60.1 meV) phase. The boundary lies near the corresponding TC. In the presence of external applied field 5 T, U remains unchanged in the ferromagnetic region, but decreases ( U ~ 5 meV) in the paramagnetic phase. These results signify the existence of strong magneto-dielectric coupling in LMCO crystals. The field variation of ε′(ω) at fixed temperature and specific frequency highlights the rise in magnetodielectricity (MD) as well as magneto-loss (ML) with increasing magnetic field. It is perceived that this variation is not due to the magneto resistance of LMCO or caused by LMCO - electrode interfaces. The influence of extrinsic parasitic contributions cannot be ruled out entirely, but the presence of positive MD as well as ML at frequencies above the time constant suggests that the relaxation process and the magneto-dielectric coupling are intrinsic to the LaMn0.5Co0.5O3 system. Chapter 6: This chapter describes the successful synthesis of a new perovskite oxide compound, LuMn0.5Ni0.5O3. The structural characterization employs the Rietveld refinement of powder X-ray diffraction pattern. The compound crystallizes in orthorhombic Pbnm crystal structure. dc magnetization reveals ferromagnetic ordering in the sample. However the low temperature glassy phase spotted in the ac susceptibility measurement might classify it as a re-entrant spin-glass compound. But the display of memory effect until the ferromagnetic transition indicates that intrinsic ant ferromagnetic interaction prevails over the dominant ferromagnetic interaction. A critical behaviour study was carried out in the vicinity of the ferromagnetic to paramagnetic phase transition, which provided the critical exponents: α = 0.37, β = 0.241 ± 0.003, γ = 1.142 ± 0.003 and δ = 5.77 ± 0.03. Interestingly, this set of critical exponents does not match with any of the conventional theories of mean field, 3D Heisenberg, and 3D Ising. Rather it fits quite well with data calculated for the stacked triangular 3D version of the (Z2 × S1) model [α = 0.34 ± 0.06, β = 0.25 ± 0.01, γ = 1.13 ± 0.05 and δ = 5.47 ± 0.27]. This study indicates that the magnetic ground state of LuMn0.5Ni0.5O3 is canted ferromagnetic. Chapter 7: Various important results are summarized in this chapter. It also provides a broad outlook in this area of research.

Ces dernières années, les matériaux à transition de spin (TS) ont suscité un intérêt marqué en raison de leurs applications potentielles, notamment en tant que capteurs ou dispositifs électroniques; typiquement, ces complexes présentent - via un stimulus externe (ex. : température, rayonnement) - une transition de spin réversible "Haut Spin (HS)  Bas Spin (BS)". Dans ce contexte, le travail présenté a porté sur la conception et les études magnétostructuralesde monocristaux de nouveaux systèmes FeII à TS à base de ligands 4-R-1,2,4-triazole comprenant un espaceur alkyl, en combinaison avec soit (i) des entités tétracyanométallates inorganiques ([M(CN)4]2-, M = NiII, PtII, PdII) ou (ii) des cyanocarbanions organiques spécifiques ((tcnsR')-: anions 1,1,3,3-tétracyano-2 thioalkylpropénide) - et ce afin de comprendre in fine l'origine physico-chimique de la forte coopérativité dans ces matériaux remarquables, mais aussi de rationaliser le contrôle et l’optimisation des propriétés de TS. Dans un premier temps, un nouveau sel de polymère 1D [Fe(bntrz)3][Pt(CN)4].H2O (1) - présentant une TS abrupte et une résilience exceptionnelle lors de cycles répétés de commutation - a été synthétisé; les études structurales fines - réalisées sur monocristaux - des états HS et BS ont clairement révélé l'impact des interactions à longue distance sur les propriétés de la TS dans ce système. Ensuite, la substitution de l'anion « rigide » tétracyanométallate ([Pt(CN)4]2-) par l'entité plus « flexible » (tcnset)- nous a conduit au premier exemple de complexe neutre trinucléaire à base de triazole [Fe3(bntrz)6(tcnset)6] (2) présentant une TS complète en une seule étape au-dessus de la température ambiante. La variation systématique du substituant R' de l'anion fonctionnalisé(tcnsR')- a permis l’obtention de deux nouveaux systèmes à TS, à savoir le complexe trinucléaire neutre [Fe3 (bntrz)6(tcnspr)6] (3) et le dérivé 1D [Fe3(bntrz)8(tcnsme)4].(tcnsme)2,4H2O (4), avec des caractéristiques structurales et des comportements de TS distincts. Enfin, dans le but d'évaluer précisément l'impact de différents substituants en 4ème position sur le ligand 1,2,4-triazole, deux nouveaux matériaux 3D basés sur le triazole 2-(3-(4H-l,2,4-triazol-4-yl)propyl)isoindoline-1,3-dione (phtptrz) - i.e. {Fe3(μ2-phtptrz)6[μ2-Pt(CN)4]3}.C2H5OH,5.5H2O (5) et {Fe(phtptrz)[Pt(CN)4].H2O} (6) - ont été synthétisés et caractérisés; une TS en deux étapes est observée pour le composé 5, tandis que le dérivé 6 exprime un comportement paramagnétique caractéristique de l’état de spin S = 2. / In recent years, the spin-crossover (SCO) materials have attracted much interest because of their potential applications such as sensors or electronic displays; these complexes typically exhibit - via an external disturbance (e.g. temperature, radiation) - a reversible “High Spin (HS) Low Spin (LS)” spin transition. In this context, the present work has focused on the design and the magneto-structural investigations of single crystals of novel FeII SCO systems based on 4-R-1,2,4-triazole ligands including an alkyl spacer - with either (i) inorganic tetracyanometallates ([M(CN)4]2-, M = NiII, PtII, PdII) or (ii) specific organic cyanocarbanions ((tcnsR’)-: 1,1,3,3-tetracyano-2-thioalkylpropenide anions) - in order to ultimately understand the physicochemical origin of the strong cooperativity in such striking materials, but also to rationalize the tuning of the SCO properties. At first, a new 1D polymer [Fe(bntrz)3][Pt(CN)4].H2O (1) salt - exhibiting an abrupt spin transition and an exceptional resilience upon repeated switching - has been synthesized; the accurate single crystal investigations of both HS and LS states of the latter clearly revealed the impact of the longrange interactions on SCO properties in this system. Then, the substitution of the ‘rigid’ [Pt(CN)4]2- tetracyanometallate anion by the more ‘flexible’ (tcnset)- entity lead us to the first example of triazole-based SCO FeII trinuclear neutral complex [Fe3(bntrz)6(tcnset)6] (2) exhibiting a complete one-step spin transition above room temperature. The systematic variation of the R’ substituent from the functionalized (tcnsR’)- anion resulted in two new FeII SCO systems, i.e. the neutral trinuclear complex [Fe3(bntrz)6(tcnspr)6] (3) and the 1D [Fe3(bntrz)8(tcnsme)4](tcnsme)2.4H2O (4) derivative, with distinct structural characteristics and SCO behaviours. Finally, with the purpose to further evaluate the impact of different substituents at the 4th position onto 1,2,4-triazole ligand, two new 3D materials based on the functionalised triazole 2-(3-(4H-1,2,4-triazol-4-yl)propyl)isoindoline-1,3-dione (phtptrz) - {Fe3(μ2-phtptrz)6[μ2-Pt(CN)4]3}.C2H5OH,5.5H2O (5) and {Fe(phtptrz)[Pt(CN)4].H2O} (6) - have been synthesized and characterized; a two-step SCO behaviour is observed only in 5, while 6 shows a characteristic paramagnetic behavior.

Les technologies de filtres, résonateurs, oscillateurs et capteurs sont des éléments essentiels dans lesindustries des télécommunications, automobile, militaire, médical, etc. Les monocristaux de LiTaO3 (LT) etLiNbO3 (LN) sont les matériaux les plus utilisés pour la fabrication de filtres de radiofréquence à ondesélastiques des téléphones portables, car ils possèdent un facteur de couplage électromécanique (K2) élevé.Cependant, ils présentent une problématique liée à la variation de la fréquence de fonctionnement avec latempérature (CTF), dont la valeur est environ de -40 à -95 ppm/°C. D’autre part, il a été démontré dans lalittérature que les propriétés de LT et LN changent avec la non-stoechiométrie du Li.L’objectif de cette thèse a été l’étude de l’effet de la concentration en Li2O sur la performance desdispositifs acoustiques à ondes élastiques de surface, utilisant comme substrat piézoélectrique desmonocristaux de LT coupe YXl/42 (42 RY-LT) et LN coupe YXl/128 (128 RY-LT). Cette étude vise àl’amélioration du CTF sans la dégradation d’autres propriétés (K2 et pertes d’insertion) dans le cas du 42 RYLTet la stabilité de dispositifs utilisés à haute densité de puissance dans le cas du 128 RY-LN. Tout d’abord,nous avons préparé des monocristaux de LT et LN avec différente concentration en Li2O :48.5-50 mol%, enutilisant la méthode d’équilibration par transport en phase vapeur (VTE). Ensuite, nous avons fabriqué etcaractérisé des dispositifs à ondes élastiques de volume et de surface à base de LT et LN, traités par VTE, afind’étudier l’effet de la non-stoechiométrie de Li et l’effet des domaines ferroélectriques sur leur performance. / The filter technologies, resonators, oscillators and sensors are essential elements fortelecommunications, automotive, military, medical industries. The most of radio frequency surface acousticwave (RF-SAW) filters, present in mobile phones, are based in LiNbO3 (LN) and LiTaO3 (LT) single crystalsbecause they have high electromechanical coupling factor (K2). However, these materials have a problemrelated to the variation of the operating frequency with temperature (TCF), whose value is about -40 to -95ppm / ° C. On the other hand, it has been previously shown in the literature that the physical and structuralproperties of LT and LN change with Li non-stoichiometry, including elastic properties.The aim of this work was the investigation of the impact of Li2O concentration on the performance ofSAW devices based on YXl/42 (42 RY-LT) and YXl/128 (128 RY-LN) single crystals. In the case of 42 RY-LT,we focused in the reduction of TCF without the degradation of other properties (K2 and insertion losses) andin the case of 128 RY-LN crystals we focused in the stability of devices at high power densities. First, singlecrystals of LT and LN with different Li2O concentration: 48.5-50 ml% were prepared, by using the VaporTransport Equilibration (VTE) method. Afterwards, SAW and bulk acoustic wave (BAW) devices based on LTand LN VTE treated crystals, were fabricated and characterized, in order to study the effect of Li nonstoichiometryand the effect of ferroelectric domains on the performance of devices.

No description available.

In der vorliegenden Arbeit werden Ergebnisse von mechanischen, elektronenmikroskopischen und rasterkraft-mikroskopischen Untersuchungen an mittelorientierten Nickeleinkristallen dargestellt, die bei 77 K bis in den Bereich der Stabilisierung der mechanischen Eigenschaften zyklisch verformt worden sind. Im Zentrum des Interesses steht neben der Analyse der stabilisierten Versetzungsstruktur auf makroskopischer, mesoskopischer und mikroskopischer Maßstabsskala das Studium der Gleitlokalisationsphänomene bei tiefen Temperaturen. Nach Verformung bei 77 K ergibt sich sowohl hinsichtlich des mechanischen Verhaltens und der Oberflächengleiterscheinungen als auch bezüglich der Versetzungsanordnungen auf mesoskopischem Strukturniveau (Strukturlängen in der Größenordnung 1 µm) prinzipiell ein ähnliches Erscheinungsbild wie bei höheren Verformungstemperaturen: Die zyklische Spannungs-Dehnungs-Kurve ist in drei Bereiche gegliedert, sie weist ein Plateaugebiet auf; es treten Grobgleitspuren auf der Probenoberfläche auf, die eine Lokalisation der Gleitung anzeigen; es kommt zur Ausbildung einer mesoskopisch heterogenen Versetzungsstruktur. Die Tendenz, daß mit sinkender Verformungstemperatur die Strukturlängen auf den unterschiedlichen Maßstabsniveaus kleiner werden, setzt sich bei der Wechselplastizierung bei 77 K fort. Die detaillierten quantitativen Untersuchungen zur Gleitlokalisation bei 77 K haben zu neuen Erkenntnissen geführt, deren Konsequenzen nicht nur die zyklische Plastizität bei tiefen Temperaturen betreffen. Die Ergebnisse lassen sich in folgenden Punkten zusammenfassen: Nach Halbzyklusverformung existiert ein weites Spektrum von lokalen Abgleitamplituden, das von 0,5 % bis 30 % reicht; die mittlere plastische Dehnung der Gleitbänder ist intrinsisch; das aktive Gleitvolumen im Halbzyklus beträgt nur 30 % des Volumens, in dem Gleitlokalisation nach einer hinreichend großen Zyklenzahl beobachtet wird; die Abgleitung in den aktiven Gleitbändern liegt um ungefähr einen Faktor drei über der oberen Plateaugrenzdehnung der zyklischen Spannungs-Dehnungs-Kurve. Die Meßergebnisse zum aktiven Gleitvolumen führen zu dem Schluß, daß in den Gleitbändern zeitlich gestaffelt Phasen hoher und niedriger Gleitaktivität auftreten. Die Diskussion der experimentellen Befunde zeigt, daß die Kerngedanken des WINTER-Modells bezüglich der Eigenschaften der Gleitbänder gültig bleiben, wenn man die Aussagen des Modells auf das mittlere räumliche und mittlere zeitliche Verhalten der Gleitbänder bezieht. Anhaltspunkte für die physikalischen Ursachen der Existenz eines Abgleitungsspektrums ergeben sich zum einen aus einer Modifikation des phänomenologischen WINTER-Modells nach ZAISER und zum anderen durch die Annahme von Spektren des Verfestigungskoeffizienten und/oder der effektiven Spannung.

Face à la nécessité d’une réduction drastique des émissions de gaz à effet de serre, le déploiement des piles à combustibles est présenté comme une solution d’avenir. La production d’hydrogène décarbonée est un des enjeux futurs pour permettre une transition énergétique efficace. Dans cette optique, l’électrolyseur à membrane échangeuse de proton (PEMWE), combiné aux sources énergétiques renouvelables, est une technologie intéressante. De nombreux défis sont encore à relever pour permettre une commercialisation de cette technologie, en particulier côté anodique. L’oxyde d’iridium, matériau coûteux et très rare, est utilisé à l’anode pour sa capacité à catalyser le dégagement d’oxygène tout en résistant aux conditions acide et oxydante. Il subit néanmoins des dégradations au cours de son utilisation.Dans ce travail, différentes surfaces modèles d’iridium pour le dégagement d’oxygène ont été étudiées pour comprendre les mécanismes mis en jeu lors des premières étapes d’oxydation de la surface et du dégagement d’oxygène. Après caractérisations par spectroscopie d’impédance électrochimique dynamique (DEIS), technique innovante permettant d’analyser les systèmes dynamiques, les relations structure-activité-stabilité lors du dégagement d’oxygène ont été étudiées en comparant des surfaces modèles d’iridium ((111), (210) et (210) nanostructurée). Les résultats obtenus mettent en évidence qu’après quelques heures à haut potentiel (> 1,6 V vs. Electrode Réversible à Hydrogène), ces surfaces, de structures et compositions chimiques initiales différentes, tendent vers le même état. Enfin, l’étude de films minces d’iridium et de nickel@iridium, modélisant des particules cœur@coquille, a montré qu’après dissolution du nickel initialement présent, une couche poreuse active pour le dégagement d’oxygène est formée. Ces résultats sont prometteurs pour la synthèse de catalyseurs à base d’iridium pour le dégagement de dioxygène. / With the need for a drastic reduction of greenhouse gases, the deployment of fuel cells is one of the considered solutions. Decarbonated hydrogen production is subsequently a major challenge to enable an efficient energetic transition. From this perspective, Proton Exchange Membrane Water Electrolyser (PEMWE) is a technology of interest, especially if coupled with renewable energy sources. Key challenges are still to be addressed before commercializing this technology, in particular at the anode. Iridium oxide, a costly and rare material, is implemented in anodic catalytic layers to catalyse the Oxygen Evolution Reaction (OER) while being resistant to harsh acidic and oxidative conditions. It nonetheless undergoes some degradations.In this work, different iridium model surfaces for the OER where studied to understand mechanisms involved during the first oxidations step and oxygen evolution. After characterisations by Dynamic Electrochemical Impedance Spectroscopy (DEIS), an innovative technique used to study dynamic systems, structure-activity-stability relationships towards the OER were studied by comparing iridium model surfaces ((111), (210) and nanostructured (210)). Results showed that after few hours at high potential (> 1.6 V vs. Reversible Hydrogen Electrode)), these surfaces, with different initial chemical compositions and structures, tend to the same state. Finally, iridium and nickel@iridium thin films were studied, to model core@shell particles. Results indicate that the nickel dissolution lead to the formation of a porous layer more active towards the OER. These findings could help to design active iridium catalysts for the OER.

Das Ziel dieser Arbeit ist die optische Untersuchung von ferroelektrischen Domänen und Domänenwänden auf lokaler Skala. Dafür wurden neuartige nichtinvasive Ansätze entwickelt, die auf der Anwendung optischer Rastersondenmikroskopie basieren. Die untersuchten Schlüsseleigenschaften umfassen den elektrooptischen Effekt für verschiedene Domänenorientierungen und die Brechungindexänderungen an Domänenwänden an Bariumtitanat-Einkristallen. Die lokale Messung der elektrooptischen Eigenschaften wurde mit räumlich stark begrenzten elektrischen Feldern durchgeführt, die mittels elektrisch leitfähigen Spitzen angelegt wurden. Dieser experimentelle Ansatz erlaubt nicht nur die Messung verschiedener elektrooptischer Koeffzienten, sondern auch die Unterscheidung von allen auftretenden, auch antiparallelen, Domänenausrichtungen. Durch Anlegen eines zusätzlichen elektrischen Feldes mittels der gleichen Spitze konnte auch das ferroelektrische Schalten mit dieser optischen Methode untersucht werden. Die Experimente wurden durch eine numerische Modellierung der elektrischen Feldverteilung und der resultierenden elektrooptischen Antwort begleitet. Die Ergebnisse der Modellierung sind dabei in sehr guter Übereinstimmung mit den experimentellen Ergebnissen. Dies erlaubt auch die Trennung von Beiträgen verschiedener elektrooptischer Koeffzienten und den entsprechenden Feldkomponenten. ür die experimentelle Untersuchung von den theoretisch vorhergesagten Brechungsindexprofilen einzelner Domänenwände, wurde die Sensitivität der optischen Sonde auf lokale Änderungen des Brechungsindex mittels Polarisations- und Positionsmodulation erhöht. Obwohl die Abbildung einer einzelnen Domänenwand nicht gelang, konnte damit zumindest eine obere Grenze für den optischen Effekt einer Domänenwand experimentell gewonnen werden, welche verträglich mit den theoretischen Vorhersagen ist. / The goal of this thesis is the optical investigation of ferroelectric domains and domain walls at the very local scale. For that, novel noninvasive approaches based on optical scanning probe microscopy are developed. The key properties investigated are the electrooptic effect for different domain orientations and refractive-index changes at single domain walls of barium titanate single crystals. The local probing of the electro-optic response is performed with strongly confined electric fields, applied via a conductive tip. With this approach we can not only probe different electro-optic coeffcients, but also identify all occurring domain orientations, even antiparallel ones. The application of additional bias fields by the same tip is used to investigate ferroelectric switching and domain growth by optical means. The experiments are supported by numerical modelling of the electric-field distribution and the resulting electro-optic response. The modelling shows excellent agreement with the measurements, and allows us to separate the contributions of different electro-optic coeffcients and their associated electric-field components. For the experimental observation of the theoretically predicted refractive-index profiles at single ferroelectric domain walls, polarization and position modulation of the optical probe is used to obtain high sensitivity to local modifications of the refractive index. An upper limit to the optical effect to the optical effect of a single domain wall is deduced from the experiment, which is compatible with the effect predicted by theory.

<p>  </p> <p>A multidisciplinary study that evaluated Quality Control (QC) laboratory (lab) accreditation, and a salt screen for bedaquiline. Medicines testing facilities always seek to ensure the accuracy of data from their QC labs by attaining accreditation. This research proposed that an understanding of the cross-linkages in the requirements for implementing the 2 most widely used lab standards will facilitate testing efficiencies, and reduce the risks of accreditation failures. For the salt project, the study proposed that new salts of bedaquiline will be formed from acid-base reactions following the pKa rule. Characterizing the salts will provide specifications for the new molecular entities, and form a selection-criteria for a lead candidate.</p> <p>The research reviewed 2 lab standards: the ISO/IEC17025:2017 and the WHO Good Practices for Pharmaceutical QC labs, and identified the areas of overlap in their requirements. It then developed and tested affordable models that mitigate the 3 identified areas of high risks to lab accreditation. Additionally, it mixed<em> equimolar amounts of bedaquiline base with select counterions that have ≥ 2 pKa units in organic solvents, to yield salts</em>. ICHQ6 guidance was used to characterize the new salts.</p> <p>The highest risks to laboratory accreditation were linked to 3 quality system metrics, namely: ‘Proficiency Testing’, ‘Validation’, and ‘Measurement Traceability’. Using the identified areas of overlap in the 2 laboratory standards, this research provided tutorial videos, a competency matrix, and some instrument validation data, to optimize the requirements for lab accreditation. For the salt screen, five new candidates were synthesized as alternatives to the existing fumarate salt of bedaquiline. The results of their physicochemical properties were used for selecting a lead moiety.</p> <p>The research provided evidence that the multipronged models developed will improve efficiencies in QC labs, and increase their chances of attaining international accreditations. It also discovered the best modes for synthesizing the new salts of bedaquiline, and provided critical data to help Pharma make an informed choice for a lead candidate.</p>

Ce travail s’inscrit dans le cadre d’une étude systématique des propriétés physico-chimiques de polyphosphates à l’argent permettant d’avancer sur de nouvelles voies pour les dispositifs tels que l’éclairage, les lasers accordables dans le visible, la radiophotoluminescence. L'objectif de ce travail vise la compréhension des mécanismes de luminescence de l’ion Ag+ dans les composés Na1-xAgxLn(PO3)4 (Ln = La et Y) dont la structure a été parfaitement déterminée. La luminescence des monocristaux provient des ions Ag+, dans des sites isolés et proches de lacunes positives (Ag2+) résultant de la photosensibilité aux UV pour les cristaux au lanthane. La luminescence des verres Na1-xAgxLa(PO3)4 (seuls vitrifiables) a été élucidée grâce à une étude après irradiation et à celle des verres Na2-xAgxZnP2O7. Elle est due aussi aux ions Ag+ dans des sites isolés. / This work focuses on keeping with the general pattern of investigations of physical-chemical properties of silver-polyphosphates to find new ways for devices such as lighting, visible range laser applications and radiophotoluminescence. The goal of this study aims at understanding the processes of Ag+ luminescence in Na1-xAgxLn(PO3)4 (Ln = La et Y) compounds which structure has been completely determinated. The luminescence of the single crystals is induced by Ag+ ions in isolated sites and close to hole centers (Ag2+) resulting in UV photosensitivity for the lanthanum crystals. The photoluminescence of the Na1-xAgxLa(PO3)4 glasses (glass-forming only) has been resolved owing to UV irradiation investigations and comparison with the luminescence of Na2-xAgxZnP2O7 glass. Both are also due to Ag+ ions in isolated sites.

La spectroscopie EPES (Elastic Peak Electron Spectroscopy) permet de mesurer le pourcentage he d’électrons rétrodiffusés élastiquement par la surface d’un échantillon soumis à un bombardement électronique. C’est une méthode non destructive et extrêmement sensible à la surface. L'objectif de ce travail est de modéliser le cheminement des électrons élastiques dans la matière grâce à une simulation informatique basée sur la méthode Monte Carlo. Cette simulation contribue de manière essentielle à la connaissance et à l'interprétation des résultats expérimentaux obtenus par spectroscopie EPES. Nous avons, de plus, adapté cette simulation à différentes surfaces transformées à l’échelle micrométrique et nanométrique. A l’aide d’une méthode originale, basée sur une description couche par couche du matériau, j’ai réalisé un programme informatique (MC1) rendant compte du cheminement des électrons élastiques dans les différentes couches du matériau. Le nombre d’électrons ressortant de la surface dépend de nombreux paramètres comme : la nature du matériau à étudier, l’énergie des électrons incidents, l’angle d’incidence, les angles de collection des analyseurs. De plus, je me suis intéressé à l’effet de la rugosité de la surface et j’ai démontré qu’elle joue un rôle déterminant sur l’intensité du pic élastique. Ensuite, grâce à l’association de la spectroscopie EPES et de la simulation Monte Carlo, j’ai déduit les modes de croissance de l’or sur substrat d’argent et de cuivre. Les effets de l’arrangement atomique et des pertes énergétiques de surfaces ont ensuite été étudiés. Pour cela, une deuxième simulation MC2 tenant compte de ces deux paramètres a été réalisée permettant d’étudier les surfaces à l’échelle nanométriques. Ces paramètres jusqu’alors non pris en compte dans notre simulation MC1, joue un rôle essentiel sur l’intensité élastique. Ensuite, j’ai obtenu une formulation simple et exploitable pour l’interprétation des résultats obtenus par la simulation MC2 pour un analyseur RFA. Afin de valider, les différents résultats de la simulationMC2, j’ai réalisé des surfaces de silicium nanostructurées, à l’aide de masques d’oxyde d’alumine réalisés par voie électrochimique. J’ai pu créer des nano-pores par bombardement ionique sous ultravide sur des surfaces de silicium. Afin de contrôler la morphologie de la surface, j’ai effectué de l’imagerie MEB ex-situ. La simulation Monte Carlo développée associée aux résultats EPES expérimentaux permet d’estimer la profondeur, le diamètre et la morphologie des pores sans avoir recours à d’autres techniques ex-situ.Cette simulation MC2 permet de connaître la surface étudiée à l’échelle nanométrique. / EPES (Elastic Peak Electron Spectroscopy) allows measuring the percentage he of elastically backscattered electrons from the surface excited by an electron beam. This is a non destructive method which is very sensitive to the surface region. The aim of this work is to model the trajectory of elastic electrons in the matter with a computer simulation based on Monte Carlo method. This simulation allows interpreting experimental results of the EPES spectroscopy. We have moreover adapted this simulation for different surfaces transformed to micrometer and nanometer scales. Using an original method, based on a description of material layer by layer, I realized a computer program (MC1) that takes into account the path of elastic electrons in different layers of material. The number of electrons emerging from the surface depends on many parameters such as: the electron primary energy, the nature of the material, the incidence angle and the collection angles of the analyzer. In addition, I was interested in the effect of surface roughness and I showed that it plays an important role in the intensity of the elastic peak. Then, through an association of the EPES and the Monte Carlo simulation results, I deduced the growth patterns of gold on silver and copper substrates. The effects of the atomic arrangement and the surface excitations were then studied. For this, a new simulation MC2 that takes into account these two parameters has been developed to study nanoscale surfaces. These parameters not previously included in our MC1simulation play a important role in the elastic intensity. Then I have got a simple formula for interpreting the results obtained by the simulation for a RFA analyzer. To validate the different results of the simulation MC2, I realized nano-structured silicon surfaces, using aluminium oxide masks. Nano-pores have been created by Ar+ ions bombardment in UHV chamber on silicon surfaces.To control the morphology of the surfaces, I realized SEM images (Techinauv Casimir) ex-situ. The Monte Carlo simulations, developed here, associated with the EPES experimental results can estimate the depth, the diameter, the morphology of pores without the help of other ex-situ techniques.