The stacking structure of Nb₂CSe₂, a newly synthesized layered metal carbo-selenide, is elucidated by scanning transmission electron microscopy. Nb₂CSe₂ features Se-Nb-C-Nb-Se quintuple atomic layers. These layers stacked in Bernal mode. According to the mode, Nb₂CSe₂ crystallizes in a trigonal symmetry (space group P3(_)m1, No. 164), with lattice parameters of a = 3.33 Å, and c = 18.20 Å. Electronic structure calculations indicate that the metal carbo-selenide has Fermi energy crossing the bands where they touch to give a zero gap, showing it is an electronic conductor. As evidenced experimentally, the electrical conductivity is as high as 6.6 ´ 10⁵ S m^-1, outperforming the counterparts in the MXene family. Due to the layered structure, the bonding in Nb₂CSe₂ with an ionic formula of (Nb^1.48+)₂(C^1.74-)(Se^0.61-)₂ is highly anisotropic with metallic–covalent–ionic bonding in intralayers while weak bonding between interlayers. The layered nature is further evidenced by elastic properties, interlayer energy, and friction coefficient determination. These characteristics recognize that Nb₂CSe₂ is exactly the analogue of MoS₂ that is the typical binary van der Waals solid (vdW). Moreover, vibrational properties are reported, which may offer an optical identification standard to the new ternary vdW solid in spectroscopic studies including Raman scattering and infra-red absorption.

In response to the development of the concepts of “carbon neutrality” and “carbon peak”, it is critical to developing materials with high near-infrared (NIR) solar reflectivity and high emissivity in the atmospheric transparency window (ATW; 8–13 μm) to advance zero energy consumption radiative cooling technology. To regulate emission and reflection properties, a series of high-entropy rare earth stannate ceramics (HE-RE₂Sn₂O₇: (Y_0.2La_0.2Nd_0.2Eu_0.2Gd_0.2)₂Sn₂O₇, (Y_0.2La_0.2Sm_0.2Eu_0.2Lu_0.2)₂Sn₂O₇, and (Y_0.2La_0.2Gd_0.2Yb_0.2Lu_0.2)₂Sn₂O₇) with severe lattice distortion were prepared using a solid phase reaction followed by a pressureless sintering method for the first time. Lattice distortion is accomplished by introducing rare earth elements with different cation radii and mass. The as-synthesized HE-RE₂Sn₂O₇ ceramics possess high ATW emissivity (91.38%–95.41%), high NIR solar reflectivity (92.74%–97.62%), low thermal conductivity (1.080–1.619 W·m⁻¹·K⁻¹), and excellent chemical stability. On the one hand, the lattice distortion intensifies the asymmetry of the structural unit to cause a notable alteration in the electric dipole moment, ultimately enlarging the ATW emissivity. On the other hand, by selecting difficult excitation elements, HE-RE₂Sn₂O₇, which has a wide band gap (E_g), exhibits high NIR solar reflectivity. Hence, the multi-component design can effectively enhance radiative cooling ability of HE-RE₂Sn₂O₇ and provide a novel strategy for developing radiative cooling materials.

CrTaO₄ (or Cr_0.5Ta_0.5O₂) has been unexpectedly found to play a decisive role in improving the oxidation resistance of Cr and Ta-containing refractory high-entropy alloys (RHEAs). This rarely encountered complex oxide can effectively prevent the outward diffusion of metal cations from the RHEAs. Moreover, the oxidation kinetics of CrTaO₄-forming RHEAs is comparable to that of the well-known oxidation resistant Cr₂O₃- and Al₂O₃-forming Ni-based superalloys. However, CrTaO₄ has been ignored and its mechanical and thermal properties have yet to be studied. To fill this research gap and explore the untapped potential for its applications, here we report for the first time the microstructure, mechanical and thermal properties of CrTaO₄ prepared by hot-press sintering of solid-state reaction synthesized powders. Using the HAADF and ABF-STEM techniques, rutile crystal structure was confirmed and short range ordering was directly observed. In addition, segregation of Ta and Cr was identified. Intriguingly, CrTaO₄ exhibits elastic/mechanical properties similar to those of yttria stabilized zirconia (YSZ) with Young’s modulus, shear modulus, and bulk modulus of 268, 107, and 181 GPa, respectively, and Vickers hardness, flexural strength, and fracture toughness of 12.2±0.44 GPa, 142±14 MPa, and 1.87±0.074 MPa·m^1/2. The analogous elastic/mechanical properties of CrTaO₄ to those of YSZ has spurred inquiries to lucrative leverage it as a new thermal barrier material. The measured melting point of CrTaO₄ is 2103±20 K. The anisotropic thermal expansion coefficients are α_a = (5.68±0.10)×10⁻⁶ K⁻¹, α_c = (7.81±0.11)×10⁻⁶ K⁻¹, with an average thermal expansion coefficient of (6.39±0.11)×10⁻⁶ K⁻¹. The room temperature thermal conductivity of CrTaO₄ is 1.31 W·m⁻¹·K⁻¹ and declines to 0.66 W·m⁻¹·K⁻¹ at 1473 K, which are lower than most of the currently well-known thermal barrier materials. From the perspective of matched thermal expansion coefficient, CrTaO₄ pertains to an eligible thermal barrier material for refractory metals such as Ta, Nb, and RHEAs, and ultrahigh temperature ceramics. As such, this work not only provides fundamental microstructure, elastic/mechanical and thermal properties that are instructive for understanding the protectiveness displayed by CrTaO₄ on top of RHEAs but also outreaches its untapped potential as a new thermal barrier material.

Twin boundaries have been exploited to stabilize ultrafine grains and improve mechanical properties of nanomaterials. The production of the twin boundaries and nanotwins is however prohibitively challenging in carbide ceramics. Using a scanning transmission electron microscope as a unique platform for atomic-scale structure engineering, we demonstrate that twin platelets could be produced in carbides by engineering antisite defects. The antisite defects at metal sites in various layered ternary carbides are collectively and controllably generated, and the metal elements are homogenized by electron irradiation, which transforms a twin-like lamellae into nanotwin platelets. Accompanying chemical homogenization, α-Ti₃AlC₂ transforms to unconventional β-Ti₃AlC₂. The chemical homogeneity and the width of the twin platelets can be tuned by dose and energy of bombarding electrons. Chemically homogenized nanotwins can boost hardness by ~45%. Our results provide a new way to produce ultrathin (< 5 nm) nanotwin platelets in scientifically and technologically important carbide materials and showcase feasibility of defect engineering by an angstrom-sized electron probe.

As a new category of ultra-high-temperature ceramics (UHTCs), multi-anionic high-entropy (HE) carbonitride UHTCs are expected to have better comprehensive performance than conventional UHTCs. However, how to realize the green and low-cost synthesis of high-quality multi-anionic HE carbonitride UHTC powders and prepare bulk ceramics with excellent mechanical properties still faces great challenges. In this work, a green, low-cost, and controllable preparation process of (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C_xN_1−x powders is achieved by sol–gel combined with the carbothermal reduction/nitridation method for the first time. The as-synthesized (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C_xN_1−x powders possess high compositional uniformity and controllable particle size. In addition, the obtained bulk ceramics prepared at 1800 ℃ exhibit superior fracture toughness (K_IC) of 5.39± 0.16 MPa·m^1/2 and high nanohardness of 35.75±1.23 GPa, elastic modulus (E) of 566.70±8.68 GPa, and flexural strength of 487±41 MPa. This study provides a feasible strategy for preparing the high-performance HE carbonitride ceramics in a more environmentally friendly and economical manner.

Ti₃C₂T_x MXene shows great potential in the application as microwave absorbers due to its high attenuation ability. However, excessively high permittivity and self-stacking are the main obstacles that constrain its wide range of applications. To tackle these problems, herein, the microspheres of SiO₂@Ti₃C₂T_x@CoNi with the hydrangea-like core–shell structure were designed and prepared by a combinatorial electrostatic assembly and hydrothermal reaction method. These microspheres are constructed by an outside layer of CoNi nanosheets and intermediate Ti₃C₂T_x MXene nanosheets wrapping on the core of modified SiO₂, engendering both homogenous and heterogeneous interfaces. Such trilayer SiO₂@Ti₃C₂T_x@CoNi microspheres are "magnetic microsize supercapacitors" that can not only induce dielectric loss and magnetic loss but also provide multilayer interfaces to enhance the interfacial polarization. The optimized impedance matching and core–shell structure could boost the reflection loss (RL) by electromagnetic synergy. The synthesized SiO₂@Ti₃C₂T_x@CoNi microspheres demonstrate outstanding microwave absorption (MA) performance benefited from these advantages. The obtained RL value was −63.95 dB at an ultra-thin thickness of 1.2 mm, corresponding to an effective absorption bandwidth (EAB) of 4.56 GHz. This work demonstrates that the trilayer core–shell structure designing strategy is highly efficient for tuning the MA performance of MXene-based microspheres.

The ternary or quaternary layered compounds called MAB phases are frequently mentioned recently together with the well-known MAX phases. However, MAB phases are generally referred to layered transition metal borides, while MAX phases are layered transition metal carbides and nitrides with different types of crystal structure although they share the common nano-laminated structure characteristics. In order to prove that MAB phases can share the same type of crystal structure with MAX phases and extend the composition window of MAX phases from carbides and nitrides to borides, two new MAB phase compounds Zr₂SeB and Hf₂SeB with the Cr₂AlC-type MAX phase (211 phase) crystal structure were discovered by a combination of first-principles calculations and experimental verification in this work. First-principles calculations predicted the stability and lattice parameters of the two new MAB phase compounds Zr₂SeB and Hf₂SeB. Then they were successfully synthesized by using a thermal explosion method in a spark plasma sintering (SPS) furnace. The crystal structures of Zr₂SeB and Hf₂SeB were determined by a combination of the X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). The lattice parameters of Zr₂SeB and Hf₂SeB are a = 3.64398 Å, c = 12.63223 Å and a = 3.52280 Å, c = 12.47804 Å, respectively. And the atomic positions are M at 4f (1/3, 2/3, 0.60288 [Zr] or 0.59889 [Hf]), Se at 2c (1/3, 2/3, 1/4), and B at 2a (0, 0, 0). And the atomic stacking sequences follow those of the Cr₂AlC-type MAX phases. This work opens up the composition window for the MAB phases and MAX phases and will trigger the interests of material scientists and physicists to explore new compounds and properties in this new family of materials.

MAB phases are layered ternary compounds with alternative stacking of transition metal boride layers and group A element layers. Until now, most of the investigated MAB phases are concentrated on compounds with Al as the A element layers. In this work, the family of M₅SiB₂ (M = IVB–VIB transition metals) compounds with silicon as interlayers were investigated by density functional theory (DFT) methods as potential MAB phases for high-temperature applications. Starting from the known Mo₅SiB₂, the electronic structure, bonding characteristics, and mechanical behaviors were systematically investigated and discussed. Although the composition of M₅SiB₂ does not follow the general formula of experimentally reported (MB)_2zA_x(MB₂)_y (z = 1, 2; x = 1, 2; y = 0, 1, 2), their layered structure and anisotropic bonding characteristics are similar to other known MAB phases, which justifies their classification as new members of this material class. As a result of the higher bulk modulus and lower shear modulus, Mo₅SiB₂ has a Pugh’s ratio of 0.53, which is much lower than the common MAB phases. It was found that the stability and mechanical properties of M₅SiB₂ compounds depend on their valence electron concentrations (VECs), and an optimum VEC exists as the criteria for stability. The hypothesized Zr and Hf containing compounds, i.e., Zr₅SiB₂ and Hf₅SiB_2, which are more interesting in terms of high-temperature oxidation/ablation resistance, were found to be unfortunately unstable. To cope with this problem, a new stable solid solution (Zr_0.6Mo_0.4)₅SiB₂ was designed based on VEC tuning to demonstrate a promising approach for developing new MAB phases with desirable compositions.

Willemite Zn₂SiO₄ crystallizes in such a way that Zn and Si are tetrahedrally coordinated with O in an ionic-covalent manner to form ZnO₄ and SiO₄ tetrahedra as the building units. The tetrahedra are corner-sharing, of which one SiO₄ tetrahedron connects eight ZnO₄ tetrahedra, and one ZnO₄ tetrahedron links four ZnO₄ tetrahedra and four SiO₄ tetrahedra. The unique crystallographic configuration gives rise to parallel tunnels with a diameter of 5.7 Å along the c-axis direction. The tunnel structure of Zn₂SiO₄ definitely correlates with its interesting elastic and thermal properties. On the one hand, the elastic modulus, coefficient of thermal expansion (CTE), and thermal conductivity are low. Zn₂SiO₄ has low Vickers hardness of 6.6 GPa at 10 N and low thermal conductivity of 2.34 W/(m·K) at 1073 K. On the other hand, the elastic modulus and CTE along the c-axis are significantly larger than those along the a- and b-axes, showing obvious elastic and thermal expansion anisotropy. Specifically, the Young’s modulus along the z direction (E_z = 179 GPa) is almost twice those in the x and y directions (E_x = E_y = 93 GPa). The high thermal expansion anisotropy is ascribed to the empty tunnels along the c-axis, which are capable of more accommodating the thermal expansion along the a- and b-axes. The striking properties of Zn₂SiO₄ in elastic modulus, hardness, CTE, and thermal conductivity make it much useful in various fields of ceramics, such as low thermal expansion, thermal insulation, and machining tools.

Ferrites are the most widely used microwave absorbing materials to deal with the threat of electromagnetic (EM) pollution. However, the lack of sufficient dielectric loss capacity is the main challenge that limits their applications. To cope with this challenge, three high-entropy (HE) spinel-type ferrite ceramics including (Mg_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)Fe₂O₄, (Mg_0.2Fe_0.2Co_0.2Ni_0.2Cu_0.2)Fe₂O₄, and (Mg_0.2Fe_0.2Co_0.2Ni_0.2Zn_0.2)Fe₂O₄ were designed and successfully prepared through solid state synthesis. The results show that all three HE MFe₂O₄ samples exhibit synergetic dielectric loss and magnetic loss. The good magnetic loss ability is due to the presence of magnetic components; while the enhanced dielectric properties are attributed to nano-domain, hopping mechanism of resonance effect and HE effect. Among three HE spinels, (Mg_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)Fe₂O₄ shows the best EM wave absorption performance, e.g., its minimum reflection loss (RL_min) reaches −35.10 dB at 6.78 GHz with a thickness of 3.5 mm, and the optimized effective absorption bandwidth (EAB) is 7.48 GHz from 8.48 to 15.96 GHz at the thickness of 2.4 mm. Due to the easy preparation and strong EM dissipation ability, HE MFe₂O₄ are promising as a new type of EM absorption materials.