  Stagnation of grain growth is often attributed to impurity segregation.
Yttria-stabilized cubic zirconia does not evidence any segregation-induced
slowdown, as its grain growth obeys the parabolic law when the grain size
increases by more than one order of magnitude. However, lowering the
temperature below 1300 oC triggers an abrupt slowdown, constraining the average
grains to grow by less than 0.5 ${\mu}$m in 1000 h despite a relatively large
driving force imparted in the fine grains of ~0.5 ${\mu}$m. Yet isolated
pockets of abnormally large grains, along with pockets of abnormally small
grains, emerge in the same latter sample. Such microstructure bifurcation has
never been observed before, and can only be explained by an inhomogeneous
distribution of immobile four-grain junctions. The implications of these
findings for two-step sintering are discussed.
