21-Component compositionally complex ceramics: Discovery of ultrahigh-entropy weberite and fergusonite phases and a pyrochlore-weberite transition

Abstract Two new high-entropy ceramics (HECs) in the weberite and fergusonite structures, along with unexpected formation of ordered pyrochlore phases ultrahigh-entropy compositions an abrupt pyrochlore-weberite transition, are discovered a 21-component oxide system. While Gibbs phase rule allows 21 equilibrium phases, 9 out 13 examined possess single HEC (with ultrahigh ideal configurational entropies: ?2.7 k B per cation or higher on one sublattice most cases). Notably, (15RE 1/15 )(Nb 1/2 Ta )O 4 monoclinic (C2/ c ) phase, 3 (Nb 1 O 7 form orthorhombic (C222 where 15RE represents Sc Y La Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu . Moreover, series eight 2+ x (Ti 1/4 Zr Ce H 2?2 specimens all exhibit transition occurs within 0.75 < 0.8125. This cubic-to-orthorhombic does not change temperature-dependent thermal conductivity appreciably, as amorphous limit may have already been achieved oxides. These discoveries expand diversity complexity HECs, towards many-component compositionally complex (CCCs) ceramics.