  Two new high-entropy ceramics (HECs) in the weberite and fergusonite
structures, along with unexpected formation of ordered pyrochlore phases with
ultrahigh-entropy compositions and an abrupt pyrochlore-weberite transition,
are discovered in a 21-component oxide system. While the Gibbs phase rule
allows 21 equilibrium phases, nine out of the 13 compositions examined possess
single HEC phases (with ultrahigh ideal configurational entropies: ~2.7kB per
cation or higher on one sublattice in most cases). Notably,
(15RE1/15)(Nb1/2Ta1/2)O4 possess a single monoclinic fergusonite (C2/c) phase
and (15RE1/15)3(Nb1/2Ta1/2)1O7 form a single orthorhombic (C2221) weberite
phase, where 15RE1/15 represents
Sc1/15Y1/15La1/15Pr1/15Nd1/15Sm1/15Eu1/15Gd1/15Tb1/15Dy1/15Ho1/15Er1/15Tm1/15Yb1/15Lu1/15.
Moreover, a series of eight
(15RE1/15)2+x(Ti1/4Zr1/4Ce1/4Hf1/4)2-2x(Nb1/2Ta1/2)xO7 specimens all exhibit
single phases, where a pyrochlore-weberite transition occurs within 0.75 < x <
0.8125. This cubic-to-orthorhombic transition does not change the
temperature-dependent thermal conductivity appreciably, as the amorphous limit
may have already been achieved in the ultrahigh-entropy 21-component oxides.
These discoveries expand the diversity and complexity of HECs, towards
many-component compositionally complex ceramics (CCCs) and ultrahigh-entropy
ceramics.
