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.

A high-entropy metal disilicide, (Mo_0.2Nb_0.2Ta_0.2Ti_0.2W_0.2)Si₂, has been successfully synthesized. X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), and electron backscatter diffraction (EBSD) collectively show the formation of a single high-entropy silicide phase. This high-entropy (Mo_0.2Nb_0.2Ta_0.2Ti_0.2W_0.2)Si₂ possesses a hexagonal C40 crystal structure with ABC stacking sequence and a space group of P6₂22. This discovery expands the known families of high-entropy materials from metals, oxides, borides, carbides, and nitrides to a silicide, for the first time to our knowledge, as well as demonstrating that a new, non-cubic, crystal structure (with lower symmetry) can be made into high-entropy phase. This (Mo_0.2Nb_0.2Ta_0.2Ti_0.2W_0.2)Si₂ exhibits high nanohardness of 16.7 ± 1.9 GPa and Vickers hardness of 11.6 ± 0.5 GPa. Moreover, it has a low thermal conductivity of 6.9 ± 1.1 W m⁻¹ K⁻¹, which is approximately one order of magnitude lower than that of the widely-used tetragonal MoSi₂ and ~1/3 of those reported values for the hexagonal NbSi₂ and TaSi₂ with the same crystal structure.

The cold sintering process (CSP) and Bi₂O₃-activated liquid-phase sintering (LPS) are combined to densify Mg-doped NASICON (Na_3.256Mg_0.128Zr_1.872Si₂PO₁₂) to achieve high densities and conductivities at reduced temperatures. As an example, a cold-sintered specimen with the addition of 1.1 wt % Bi₂O₃ sintering additive achieved a high conductivity of 0.91 mS/cm (with ~96% relative density) after annealing at 1000 ℃; this conductivity is > 70% higher than that of a cold-sintered specimen without adding the Bi₂O₃ sintering additive, and it is > 700% of the conductivity of a dry-pressed counterpart with the same amount of Bi₂O₃ added, all of which are subjected to the same heating profile. The highest conductivity achieved in this study via combining CSP and Bi₂O₃-activated LSP is > 1.5 mS/cm. This study suggests an opportunity to combine the new CSP with the traditional LPS to sinter solid electrolytes to achieve high densities and conductivities at reduced temperatures. This combined CSP-LPS approach can be extended to a broad range of other materials to fabricate the "thermally fragile" solid electrolytes or solid-state battery systems, where reducing the processing temperature is often desirable.