  Improved thermomechanical properties have been reported for various
high-entropy oxides containing typically five metal cations. This study further
investigates a series of duodenary (11 metals + oxygen) high-entropy oxides by
mixing different fractions of a five-cation fluorite-structured niobate and a
seven-cation pyrochlore (both containing Yb) with matching lattice constants.
Nine compositions of duodenary high-entropy oxides have been examined. All of
them exhibit single high-entropy phases of either disordered fluorite or
ordered pyrochlore structure. An order-disorder transition (ODT) is evident
with changing composition, accompanied by a reduction in thermal conductivity
(k). In comparison with the ODT criteria developed from ternary oxides, these
duodenary oxides are more prone to disorder, but the ODT is still controlled by
similar factors (but at different thresholds). Interestingly, there are abrupt
increases in Young's modulus (E) at low mixing concentrations near both
endmembers. The E/k ratios are increased, in comparison with both endmembers.
This study suggests a new route to tailor high-entropy ceramics via controlling
cation ordering vs. disordering.
