  Solution derived La2Zr2O7 films have drawn much attention for potential
applications as thermal barriers or low-cost buffer layers for coated conductor
technology. Annealing and coating parameters strongly affect the microstructure
of La2Zr2O7, but different film processing methods can yield similar
microstructural features such as nanovoids and nanometer-sized La2Zr2O7 grains.
Nanoporosity is a typical feature found in such films and the implications for
the functionality of the films is investigated by a combination of scanning
transmission electron microscopy, electron energy-loss spectroscopy and
quantitative electron tomography. Chemical solution based La2Zr2O7 films
deposited on flexible Ni-5at.%W substrates with a {100}<001> biaxial texture
were prepared for an in-depth characterization. A sponge-like structure
composed of nanometer sized voids is revealed by high-angle annular dark-field
scanning transmission electron microscopy in combination with electron
tomography. A three-dimensional quantification of nanovoids in the La2Zr2O7
film is obtained on a local scale. Mostly non-interconnected highly facetted
nanovoids compromise more than one-fifth of the investigated sample volume. The
diffusion barrier efficiency of a 170 nm thick La2Zr2O7 film is investigated by
STEM-EELS yielding a 1.8 \pm 0.2 nm oxide layer beyond which no significant
nickel diffusion can be detected and intermixing is observed. This is of
particular significance for the functionality of YBa2Cu3O7-{\delta} coated
conductor architectures based on solution derived La2Zr2O7 films as diffusion
barriers.
