X-RAY investigation of oxides interfaces of interest for energy applications

This thesis reports about studies of the structure of the interfaces between different type of oxides grown on NdGaO3 (110). Oxide heterostructures have been shown to exhibit unusual physics. In particular, at interfaces between complex oxides the interplay between the different degrees of freedom (charge, spin,orbital and lattice) hold to novel funcionalities promising for electronic applications. In this framework, measurements performed at synchrotron radiation facilities are used to provide insights into the structure of interface and to explore the relation with physical properties, such as high Tc superconductivity and proton conductivity. The connection between the occurrence of high Tc superconductivity in the bilayer CaCuO2/SrTiO3 and the increase of the interface thickness is investigated. Measurements of x-ray reflectivity combined with measurements of surface x-ray diffraction have been perfomed on superconducting and non-superconducting samples. The analysis showed evidence of a larger distance between the topmost Ca plane of CaCuO2 and first layer of TiO2 of SrTiO2 in the superconducting sample in respect with the non-superconducting one. The relation between enhanced proton conductivity and high density of defects in BaZr0.8Y0.2O3-x/NdGaO3 interface has been examined. In this mismatched system, the misfit strain is accomodated by a self-assemblig regular network of misift dislocations at the interface between film and substrate. The reported data indicate a strong involvement of the interface misfit dislocations array in the transport properties of perovskite oxides interfaces. In particular, the strong increase of the strain field associated with the dislocation network, after annealing in wet atmosphere, points toward an accumulation of hydroxyl groups in the cores of the dislocations rather than in the bulk. The consequent increase of in-plane electrical conductivity in the same atmosphere conditions suggests that the inteface dislocations act as preferential channels for charge carriers. Moreover, the interface between insulating undoped BaZrO3 and NdGaO3 is studied. The emerging proton conductivity is connected to the structural properties of the heterostructure. High resolution transmission electron microscopy and surface x-ray diffraction show that a large number of misfit dislocations is present at the interface to fully relax the epitaxial strain. The defective layer is found to have a thickness of about 3 nm and it is considered as the place where the major contribution to proton conduction occurs. Values of interface conductivity of about 0.5 S/cm at 650 ◦C are estimated with an activation energy of about 0.86 eV.