This thesis explores an amazing family of oxide compounds - the nickelates - known for their metal-to-insulator transition and, in the case of LaNiO3, to be a possible building block for designing a synthetic high Tc superconductor. Competition between various fascinating phases makes these materials very sensitive to external parameters and it is thus possible to dramatically tune their properties. This work on ultrathin LaNiO3 and the solid solution Nd1-xLaxNiO3 has important implications for the search for superconductivity in this class of materials.

The electronic and structural properties of thin LaNiO3 films grown by using molecular beam epitaxy are studied as a function of the net ionic charge of the surface terminating layer. We demonstrate that electronic transport in nickelate heterostructures can be manipulated through changes in the surface termination due to a strong coupling of the surface electrostatic properties to the structural properties of the Ni--O bonds that govern electronic conduction. We observe experimentally and from first-principles theory an asymmetric response of the structural properties of the films to the sign of the surface charge, which results from a strong interplay between electrostatic and mechanical boundary conditions governing the system. The structural response results in ionic buckling in the near-surface NiO2 planes for films terminated with negatively charged NiO2 and bulklike NiO2 planes for films terminated with positively charged LaO planes. The ability to modify transport properties by the deposition of a single atomic layer can be used as a guiding principle for nanoscale device fabrication.

This thesis presents the results of resonant and non-resonant x-ray scattering experiments demonstrating the control of collective ordering phenomena in epitaxial nickel-oxide and copper-oxide based superlattices. Three outstanding results are reported: (1) LaNiO3-LaAlO3 superlattices with fewer than three consecutive NiO2 layers exhibit a novel spiral spin density wave, whereas superlattices with thicker nickel-oxide layer stacks remain paramagnetic. The magnetic transition is thus determined by the dimensionality of the electron system. The polarization plane of the spin density wave can be tuned by epitaxial strain and spatial confinement of the conduction electrons. (2) Further experiments on the same system revealed an unusual structural phase transition controlled by the overall thickness of the superlattices. The transition between uniform and twin-domain states is confined to the nickelate layers and leaves the aluminate layers unaffected. (3) Superlattices based on the high-temperature superconductor YBa2Cu3O7 exhibit an incommensurate charge density wave order that is stabilized by heterointerfaces. These results suggest that interfaces can serve as a powerful tool to manipulate the interplay between spin order, charge order, and superconductivity in cuprates and other transition metal oxides.

Perovskite-based ceramics are a significant class of innovative materials with fascinating physical properties, which are now receiving intensive research attention in condensed matter physics and in the area of practical device applications. Perovskite Ceramics provides a state-of-the-art review on the latest advances in perovskite-based ceramic materials, as well as the development of devices from these materials for different applications. Perovskite Ceramics: Recent Advances and Emerging Applications is divided into two main parts. The first part focuses on the basics of perovskite-based ceramic materials and includes chapters on the fundamentals, synthesis and processing, characterization, and properties of these materials. Chapters are also included on bulk and thin materials, phase transitions, polaronic effects and the compensation and screening of ferroelectricity. This section will allow the reader to familiarize themselves with the standard traditional approach, but it will also introduce new concepts that are fast evolving in this field. The second part presents an extensive review of up-to-date research on new and innovative advances in perovskite-based ceramic materials. Chapters cover multiferroic applications, lead-free perovskites, energy storage applications, perovskite-based memories, light manipulation and spectral modifications, and solar cells and fuel cells. All these fields of research are rapidly evolving, so the book acts a platform to showcase latest results on optical strategies and materials for light manipulation, and spectral up- and down-conversion too (mainly rare earth doped oxides and complexes). The book will be an essential reference resource for academic and industrial researchers working in materials research and development particularly in functional and oxide ceramics and perovskites. - A comprehensive and systematic review of advanced research in perovskite-based ceramics - Covers both oxide and halide perovskites, their synthesis, processing, properties and applications - Presents advanced methods of synthesis as well as latest applications - Discusses all aspects from theory to production - Covers the most important advances both in terms of new materials and application strategies

In this thesis chemical and epitaxial degrees of freedom are used to manipulate charge and spin ordering phenomena in two families of transition metal oxides, while taking advantage of state-of-the-art resonant x-ray scattering (RXS) methods to characterize their microscopic origin in a comprehensive manner. First, the relationship of charge density wave order to both magnetism and the "pseudogap" phenomenon is systematically examined as a function of charge-carrier doping and isovalent chemical substitution in single crystals of a copper oxide high-temperature superconductor. Then, in copper oxide thin films, an unusual three-dimensionally long-range-ordered charge density wave state is discovered, which persists to much higher temperatures than charge-ordered states in other high-temperature superconductors. By combining crystallographic and spectroscopic measurements, the origin of this phenomenon is traced to the epitaxial relationship with the underlying substrate. This discovery opens new perspectives for the investigation of charge order and its influence on the electronic properties of the cuprates. In a separate set of RXS experiments on superlattices with alternating nickel and dysprosium oxides, several temperature- and magnetic-field-induced magnetic phase transitions are discovered. These observations are explained in a model based on transfer of magnetic order and magneto-crystalline anisotropy between the Ni and Dy subsystems, thus establishing a novel model system for the interplay between transition-metal and rare-earth magnetism.

This thesis presents an experimental study of ordering phenomena in rare-earth nickelate-based heterostructures by means of inelastic Raman light scattering and elastic resonant x-ray scattering (RXS). Further, it demonstrates that the amplitude ratio of magnetic moments at neighboring nickel sites can be accurately determined by RXS in combination with a correlated double cluster model, and controlled experimentally through structural pinning of the oxygen positions in the crystal lattice. The two key outcomes of the thesis are: (a) demonstrating full control over the charge/bond and spin order parameters in specifically designed praseodymium nickelate heterostructures and observation of a novel spin density wave phase in absence of the charge/bond order parameter, which confirms theoretical predictions of a spin density wave phase driven by spatial confinement of the conduction electrons; and (b) assessing the thickness-induced crossover between collinear and non-collinear spin structures in neodymium nickelate slabs, which is correctly predicted by drawing on density functional theory.

In this book we explore new approaches to understanding the physical and chemical properties of emergent complex functional materials, revealing a close relationship between their structures and properties at the molecular level. The primary focus of this book is on the ability to synthesize materials with a controlled chemical composition, a crystallographic structure, and a well-defined morphology. Special attention is also given to the interplay of theory, simulation and experimental results, in order to interconnect theoretical knowledge and experimental approaches, which can reveal new scientific and technological directions in several fields, expanding the versatility to yield a variety of new complex materials with desirable applications and functions. Some of the challenges and opportunities in this field are also discussed, targeting the development of new emergent complex functional materials with tailored properties to solve problems related to renewable energy, health, and environmental sustainability. A more fundamental understanding of the physical and chemical properties of new emergent complex functional materials is essential to achieving more substantial progress in a number of technological fields. With this goal in mind, the editors invited acknowledged specialists to contribute chapters covering a broad range of disciplines.