Academic literature on the topic 'Cation and Anion'

The investigation in this thesis focuses on the synthesis of cation-ordered perovskite phases by introducing anion vacancies into the structure. Complex cation-ordered phases Ba2YMO5 and Ba3YM2O7.5 (M = Fe, Co) have been synthesized using ceramic or citrate gel methods under flowing argon. Close inspection reveals that the structures are constructed from Y2M2O102 basic units which consist of two YO6 octahedra and two MO4 tetrahedra in a rock-salt type arrangement. In the structure of Ba2YMO5 (M = Fe, Co), the neighbouring Y2M2O102 units are connected with an equivalent one in the yz-plane with YO6 octahedra sharing an apex. In the structure of Ba3YM2O7.5 (M = Fe, Co), the basic units are connected to each other by the M2O7 dimers via a chain of Y – O – M – O – M – O – Y bonds. Complex cation ordering can be achieved by carefully controlling the anion vacancies and selecting the cations with different ionic radii. The anion vacancies present in Ba2YMO5 (M = Fe, Co) (space group P21/n) allow the intercalation of anions like O2- and F- into the lattice. The fluorination of Ba2YCoO5 leads to the formation of a new orthorhombic phase Ba2YCoO5F0.42 (space group Pbnm) in which the inserted fluoride ions are distributed in a disordered manner. In contrast, the topochemical oxidation of Ba2YFeO5 leads to the formation of a new orthorhombic phase Ba2YFeO5.5 (space group Pb21m), in which Fe4+ centres are located in 4-coordinate tetrahedral sites and 5-coordinate pyramidal sites, respectively. The polar structure of Ba2YFeO5.5 is confirmed by the observation of second-harmonic generation activity and pyroelectric behaviour. Ba2YFeO5.5 also exhibits a combination of ferromagnetic and antiferromagnetic behaviours at low temperature. LaCa2Fe2GaO8 adopts a six-layer structure consisting of an OOTLOOTR stacking sequence of layers of (Fe/Ga)O6 octahedra (O) and (Fe/Ga)O4 tetrahedra (T), related to that of the four-layer brownmillerite structure (space group Pbma). The chains of tetrahedra in the structure of LaCa2Fe2GaO8 exhibit a cooperative twisting distortion in which the twisting direction of the chains of tetrahedra alternates in adjacent tetrahedral layers. LaxSr2-xCoGaO5+δ (0.5 < x < 1) adopts brownmillerite structures which consist of octahedral and tetrahedral layers with mixed valence of Co2+/Co3+. The members with x = 0.5, 0.6 and 0.7 adopt structures with I2mb space group symmetry, in which all the tetrahedra twist in the same direction. The members with x = 0.8, 0.9 and 1.0 adopt structures with Imma space group symmetry, in which the chains of the tetrahedra twist in a disordered manner. A change in the Co3+ spin state from high spin (HS) to low spin (LS) is observed as the La/Sr ratio increases. The change of the Co3+ spin state can be rationalized on the basis of internal chemical pressure.

Les polyoxométallates et plus particulièrement les hétéropolymolybdates sont des composés très recherchés en catalyse, homogène ou hétérogène. A l'état solide, leur comportement réactif dépend de nombreux paramètres dont la nature des contre-ions présents, le type de molécules de solvatation, le système cristallin et leur evolution thermique. Plusieurs sels (sodium, potassium, alkyl(aryl)ammonium) des ions dodécamolybdophosphate (3-), vanadoundécamolybdophosphate (4-) et de structures apparentées (octadécamolybdophosphate (6-)) ont été préparés. Ils ont été étudiés en solution par polarographie et à l'état solide par spectrométries de vibration (infra-rouge, Raman) et spectrométrie RMN de 31P. Leur comportement thermique a été suivi par analyse thermique et les différentes phases décelées étudiées par RPE. Ces mesures ont permis d'identifier plusieurs types d'interactions susceptibles de rendre compte de la réactivité en oxydation sélective. Deux d'entre elles, interactions anion-anion et anion-cation, sont fortement dépendantes des arrangements cristallographiques et de leur évolution thermique. L'interaction anion-anion est attendue augmenter la stabilité de l'anion, alors que l'interaction anion-cation, antagoniste, est attendue la diminuer.

The fundamental bulk and surface electronic properties of a novel class of semiconductors, characterised by a significant mismatch between the size and electro-negativity of the cation and anion (SCAMS), have been investigated. The characteristic examples of CdO, In2O3, and InN were studied using high-resolution x-ray photoemission spectroscopy, infrared reflectivity, optical absorption spectroscopy, and single-field Hall effect measurements. The behaviour of not only defects, dopants and impurities, which dominate the bulk electronic properties, but also surface states was shown to depend on the position of a single energy level, the charge neutrality level (CNL), unifying bulk and surface electronic properties of semiconductors. For the materials studied, the CNL was shown to be located within the conduction band (0.39 eV, »0:65 eV, and 1.19 eV above the conduction band minimum (CBM) in CdO, In2O3, and InN, respectively; see figure) in contrast to the vast majority of semiconductors where the CNL lies within the fundamental band gap (as, for example, in the classic case of GaAs). In CdO, this was shown to lead to native defects, hydrogen impurities and surface states all being donors, even in already n-type material. The donor surface states result in electron accumulation at the CdO surface. Such an electron accumulation is also present at InN surfaces, and this was shown to exhibit a remarkable independence on surface orientation, and to lead to inversion layers at the surface of p-type InN. The changes in surface space-charge regions were investigated across the In(Ga,Al)N composition range, for both undoped and Mg-doped alloys. The influence of the CNL position on interface properties and conductivity in InN was considered. Electron accumulation was observed in In2O3, in contrast to previous reports. Muonium, and by analogy hydrogen, was also shown to be a shallow donor in this material. The location of the CNL above the CBM in SCAMS was used to explain many of their striking bulk electronic properties, such as why materials like In2O3 are able to be conducting despite being optically transparent, two normally contradictory properties. The conclusions drawn from these studies are applicable to a wide variety of other materials, in particular other SCAMS such as ZnO or SnO2. Surface electron accumulation is treated here mainly within a one-electron semi-classical approximation. The final section of this work moves beyond this, using angle-resolved photoemission spectroscopy measurements and theoretical calculations to consider both the quantized nature of an electron accumulation layer, and the influence of many-body effects.

Layered double hydroxides (LDH) have been principally known as anion-exchanging, clay-like materials for several decades, and continues to be the main driving force for current and future research. The chemical interactions of LDH, with transition metallocyanides, have been a popular topic of investigation for many years, partly due to the use of powder x-ray diffraction and infrared spectroscopy as the main characterization tools. Each transition metallocyanide has a characteristic infrared stretching frequency that can be easily observed, and their respective sizes can be observed while intercalated within the interlayer of the LDH. The ability of LDH to incorporate metal cations or any ions/molecules/complexes, that have a postive charge, have not been previously investigated, mainly due to the chemical and physical nature of LDH. The possibility of cationic incorporation with LDH would most likely occur by surface adsorption, lattice metal replacement, or by intercalation into the LDH interlayers. Although infrared spectroscopy finds it main use through the identification of the anions incorporated with LDH, it can also be used to study and identify the various active and inactive bending and stretching modes that the metal hydroxide layers have.