  The exploitation of advanced materials for novel energy, health and computing
applications requires fundamental understanding of enabling physicochemical
mechanisms, including ionic and electronic conductivity, defect formation
processes and reaction kinetics. Therefore, access to underlying constants of
functional materials via advanced but straightforward experimental techniques
is key. We present a novel, cheap and widely applicable approach to analyze
oxygen-tracer-diffusion in thin films with unprecedented time resolution based
on the novel in situ isotope-exchange Raman spectroscopy (IERS) methodology.
Raman spectroscopy is sensitive to changes in the local isotopic composition,
manifested by a frequency shift of the oxygen Raman modes. Employing a Raman
transparent capping layer allows to establish an in-plane tracer gradient and
follow the isotope exchange and diffusion processes via consecutive spatial and
time resolved in situ Raman line scans. Mass-transport coefficients are
calculated from these isotopic gradients, similar to conventional techniques,
but with an additional time-component, not accessible by other techniques.
Here, we study gadolinium doped ceria (CGO) thin films, capped with Si3N4 or
Al2O3. We report diffusion coefficients within the temperature range of
interest for intermediate temperature emerging applications (300-500{\deg}C)
and confirm the validity of the measurement procedure and extracted parameters
by comparison with FEM simulations and literature results.
