Abstract. We studied grain boundary diffusion and segregation of
La, Fe, Mg, and Ti in a crystallographically defined grain boundary in
yttrium aluminum garnet (YAG). Bi-crystals were synthesized by wafer
bonding. Perpendicular to the grain boundary, a thin-film diffusion source
of a La3.60Al4.40O12 was deposited by pulsed laser
deposition. Diffusion anneals were performed at 1000 and 1450 ∘C. Via a gas phase small amounts of elements were added during
the experiment. The element concentration distributions in our bi-crystals
were mapped using analytical transmission electron microscopy (ATEM). Our
results show strong segregation of La and Ti at the grain boundary. However,
in the presence of Ti, the La concentrations dropped below the detection
limit. Quantitative element distribution profiles along and across the grain
boundary were fitted by a numerical diffusion model for our bi-crystal
geometry that considers the segregation of elements into the grain boundary.
The shape of the diffusion profiles of Fe requires the presence of two
diffusion modes, e.g., the co-diffusion of Fe2+ as well as Fe3+.
The absence of a detectable concentration gradient along the grain boundary
in many experiments allows a minimum value to be determined for the product of
sDgb. The resulting sDgb are a minimum of 7 orders of magnitude
larger than their respective volume diffusion coefficient, specifically for La = 10−14 m2 s−1, Fe = 10−11 m2 s−1, Mg = Si = 10−12 m2 s−1, and Ti = 10−14 m2 s−1 at 1450 ∘C. Additionally, we model the effect
of convolution arising from the given spatial resolution of our analysis
with the resolution of our modeled system. Such convolution effects result
in a non-unique solution for the segregation coefficient, e.g., for example
for Mg between 2–3. Based on our data we predict that bulk diffusion of
impurities in a mono-phase polycrystalline aggregate of YAG is effectively
always dominated by grain boundary diffusion.
Grain boundary diffusion and its relation to segregation of multiple elements in yttrium aluminum garnet