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Different SrTiO3 thin films are investigated to unravel the nature of ultra-low conductivities recently found in SrTiO3 films prepared by pulsed laser deposition. Impedance spectroscopy reveals electronically pseudo-intrinsic conductivities for a broad range of different dopants (Fe, Al, Ni) and partly high dopant concentrations up to several percent. Using inductively-coupled plasma optical emission spectroscopy and reciprocal space mapping, a severe Sr deficiency is found and positron annihilation lifetime spectroscopy revealed Sr vacancies as predominant point defects. From synchrotron-based X-ray standing wave and X-ray absorption spectroscopy measurements, a change in site occupation is deduced for Fe-doped SrTiO3 films, accompanied by a change in the dopant type. Based on these experiments, a model is deduced, which explains the almost ubiquitous pseudo-intrinsic conductivity of these films. Sr deficiency is suggested as key driver by introducing Sr vacancies and causing site changes (Fe-Sr and Ti-Sr) to accommodate nonstoichiometry. Sr vacancies act as mid-gap acceptor states, pinning the Fermi level, provided that additional donor states (most probably TiSr center dot center dot\[{\rm{Ti}}_{{\rm{Sr}}}{ \bullet \bullet }\]) are present. Defect chemical modeling revealed that such a Fermi level pinning also causes a self-limitation of the Ti site change and leads to a very robust pseudo-intrinsic situation, irrespective of Sr/Ti ratios and doping., Advanced Functional Materials, 32 (38), ISSN:1616-3028, ISSN:1616-301X

Phase transitions are a very important topic in materials science to understand the physics and chemistry involved in a class of materials. Often, these phase transitions are related to a change in the charge carrier density and can be achieved by various means e.g. chemical doping, like for this thesis. The material chosen here is La1-xSrxMnO3, a mixed valance manganite at half doping. This doping level corresponds to the border between ferro- and paramagnetism at room temperature. As a way to study the magnetic phase transitions in La1-xSrxMnO3 thin films, lithium doping is implemented. This can help to disentangle the mechanisms and their mutual connections leading to the magnetic phase transition. The focus lies therefore on the experimental investigation of the manganese oxidation state changes by Li-doping La1-xSrxMnO3 thin films as a model system and the correlation to the induced magnetic properties. For the doping experiments a chemical and electrochemical doping protocol for lithiated La1-xSrxMnO3 at x ~ 0.5 was established. To elucidate the electronic and magnetic properties and their changes upon de-/lithiation of La1-xSrxMnO3/ lithiated La1-xSrxMnO3 post-mortem (Mn L2,3-edge, O K-edge) X-Ray absorption spectroscopy (XAS) and vibrating sample magnetometry measurements were conducted. For those measurements the samples were cycled in Li-ion battery-like cells filled with liquid electrolyte and stopped at different potentials (lithiation steps). To implement the principle of electrochemical doping into microdevice multilayers, all-solid-state thin film lithium ion batteries were developed and grown. These devices were then used for operando XAS (Mn K-edge) measurements to follow the Mn oxidation state upon de-/lithiation as well as operando polarised neutron reflectivity measurements to measure the density and magnetisation changes within the layers of interest. An introductory overview is given within the first chapter. In the second chapter the sample preparation is described. This includes the material synthesis, compositional and structural characterisation as well as establishing a thin film growth protocol for various lithiated La1-xSrxMnO3 using pulsed laser deposition. In addition, an all-solid-state device was designed and grown with pulsed laser deposition and sputtering. X-ray absorption spectroscopy on the Mn L2,3-edge and O K-edge as well as vibrating sample magnetometry data after electrochemical cycling in a liquid electrolyte based electrochemical cell, are discussed in the third chapter. These data reveal phase transformations at the sample surfaces leading to self-reversed magnetic hysteresis. Operando X-ray absorption spectroscopy on the Mn K-edge and polarised neutron reflectometry data using a self-designed electrochemical cell and the all-solid-state devices are exhibited in the fourth chapter. The operando studies show a change in the Mn oxidation state as well as structural changes. In addition, these measurements indicate changes in density and magnetism upon electrochemical de-lithiation. Finally, an outlook and conclusion are given to finish this work.