Stephanie Mildner

Electronic structure of Pr1−xCaxMnO3

The electronic structure of

ETEM Studies of Electrodes and Electro-catalysts

{\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}

Strain driven phase decomposition in ion-beam sputtered Pr 1− x Ca x MnO 3 films

has been investigated using a combination of first-principles calculations, x-ray photoelectron spectroscopy (XPS), x-ray absorption spectroscopy (XAS), electron-energy loss spectroscopy (EELS), and optical absorption. The full range of compositions,

In Situ XANES/XPS Investigation of Doped Manganese Perovskite Catalysts

x=0,1/2,1

In situ TEM analysis of resistive switching in manganite based thin-film heterostructures

, and a variety of magnetic orders have been covered. Jahn-Teller as well as Zener polaron orders are considered. The free parameters of the local hybrid density functionals used in this study have been determined by comparison with measured XPS spectra. A model Hamiltonian, valid for the entire doping range, has been extracted. A simple local-orbital picture of the electronic structure for the interpretation of experimental spectra is provided. The comparison of theoretical calculations and different experimental spectra provide a detailed and consistent picture of the electronic structure. The large variations of measured optical absorption spectra are traced back to the co-existence of magnetic orders (respectively, to the occupation of local orbitals). A consistent treatment of the Coulomb interaction indicates a partial cancellation of Coulomb parameters and supports the dominance of the electron-phonon coupling.

Environmental TEM Study of Electron Beam Induced Electrochemistry of Pr0.64Ca0.36MnO3 Catalysts for Oxygen Evolution

Environmental TEM is an excellent tool for gaining insight into the atomic and electronic structure of electro-catalysts under operating conditions. Several electrochemical reactions such as oxidation/reduction processes of electrodes, heterogeneous gas phase catalysis of water splitting/oxygen evolution and electrochemical corrosion processes of materials have been studied in some pioneering experiments which will be summarized in this chapter. These experiments often reveal a strong change of the electrode due to the adsorption of gas species from the environment as well as due to the impact of the electron beam. We show that inelastic scattering of the high-energy electrons can induce electric potentials in the studied samples influencing the observed state of the catalyst. After an introduction to electrochemistry and ETEM investigations, we address, experimentally and theoretically, beam-induced potentials, their dependence on several parameters such as electron flux, electric conductivity, and geometry of samples, aiming at learning how to disentangle them from radiation damage. Our second focus is to control the electric potential distribution within and around samples by dedicated electrical TEM sample holders. To illustrate how this can be achieved, we present the results of a bias-controlled electro-corrosion experiment. We will discuss some of the main experimental and theoretical challenges for the development of controlled electrochemistry studies in transmission electron microscopes.

Temperature- and doping-dependent optical absorption in the small-polaron system Pr1−xCaxMnO3.

The deposition of heteroepitaxial thin films on single crystalline substrates by means of physical deposition methods is commonly accompanied by mechanical strain due to lattice mismatch and defect generation. Here we present a detailed analysis of the influence of strain on the Mn solubility of Pr1-XCaXMnO3 thin films prepared by ion-beam sputtering. Combining results from X-ray diffraction, transmission electron microscopy and in situ hot-stage stress measurements, we give strong evidence that large tensile strain during deposition limits the Mn solubility range of the Perovskite phase to near-stoichiometric composition. Mn excess gives rise to MnOz precipitates and the precipitation seems to represent a stress relaxation path. With respect to size and density of the precipitates, the relaxation process can be affected by the choice of substrate and the deposition parameters, that is, the deposition temperature and the used sputter gas.