Two new high-entropy ceramics (HECs) in the weberite and fergusonite structures, along with the 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, 9 out of the 13 compositions examined possess single HEC phases (with ultrahigh ideal configurational entropies: ~2.7k_B per cation or higher on one sublattice in most cases). Notably, (15RE_1/15)(Nb_1/2Ta_1/2)O₄ possess a single monoclinic fergusonite (C2/c) phase, and (15RE_1/15)₃(Nb_1/2Ta_1/2)₁O₇ form a single orthorhombic (C222₁) weberite phase, where 15RE_1/15 represents Sc_1/15Y_1/15La_1/15Pr_1/15Nd_1/15Sm_1/15Eu_1/15Gd_1/15Tb_1/15Dy_1/15Ho_1/15Er_1/15Tm_1/15 Yb_1/15Lu_1/15. Moreover, a series of eight (15RE_1/15)_2+x(Ti_1/4Zr_1/4Ce_1/4Hf_1/4)_2-2x(Nb_1/2Ta_1/2)_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.

A new class of high-entropy M₃B₄ borides of the Ta₃B₄-prototyped orthorhombic structure has been synthesized in the bulk form for the first time. Specimens with compositions of (V_0.2Cr_0.2Nb_0.2Mo_0.2Ta_0.2)₃B₄ and (V_0.2Cr_0.2Nb_0.2Ta_0.2W_0.2)₃B₄ were fabricated via reactive spark plasma sintering of high-energy-ball-milled elemental boron and metal precursors. The sintered specimens were ~98.7% in relative densities with virtually no oxide contamination, albeit the presence of minor (4-5 vol%) secondary high-entropy M₅B₆ phases. Despite that Mo₃B₄ or W₃B₄ are not stable phase, 20% of Mo₃B₄ and W₃B₄ can be stabilized into the high-entropy M₃B₄ borides. Vickers hardness was measured to be 18.6 and 19.8 GPa at a standard load of 9.8 N. This work has further expanded the family of different structures of high-entropy ceramics reported to date.