Ultra-incompressible High-Entropy Diborides

Mechanical, Thermal Properties, and Extreme Phase Stability of High-Entropy Diborides (V0.2Nb0.2Ta0.2Cr0.2W0.2)B2

High-entropy diborides (HEDBs) have gained significant attention in industrial applications due to their vast composition space and tunable properties. We propose a solid solution reaction at high temperatures and pressures that successfully synthesized and sintered a novel, dense, and phase-pure HEDB (V0.2Nb0.2Ta0.2Cr0.2W0.2)B2. A high asymptotic Vickers hardness of 26.3 ± 0.6 GPa and a bulk modulus of 320.5 ± 10.6 GPa were obtained. Additionally, we investigated the thermal oxidation process using TG-DSC from room temperature to 1500 °C and explored the phase stability of HEDBs under high-pressure conditions through in situ high-pressure synchrotron radiation X-ray diffraction. We analyzed the formation of lattice distortion, chemical bonding, and band structure in (V0.2Nb0.2Ta0.2Cr0.2W0.2)B2 using first-principles calculations. Surprisingly, we found that the predominant distortion in diborides occurs in the boron layer, supported by ELF. This may be due to uneven electron transfer rather than a straightforward correlation with the atomic radius. These results provide a novel synthesis process and additional experimental data on the mechanical and thermal properties and high-pressure phase stability of HEDBs. Our study offers further insights into the microscopic structure of lattice distortion in HEDBs, which could prove crucial for the selection and design of engineering advanced HEDBs.

Coexistence of Superhardness and Metal-Like Electrical Conductivity in High-Entropy Dodecaboride Composite with Atomic-Scale Interlocks

High electrical conductivity and super high hardness are two sought-after material properties, but both are contradictory because the effective suppression of dislocation movement generally increases the scattering of conducting electrons. Here we synthesized a high-entropy dodecaboride composite (HEDC) with a large number of atomic-scale interlocking layers. It shows a Vickers hardness of 51.2 ± 3.6 GPa under an applied load of 0.49 N and an electrical resistivity of 44.5 μΩ·cm at room temperature. Such HEDC achieves superhardness by inheriting the high intrinsic hardness of its constituent phases and restricting the dislocation motion to further enhance the extrinsic hardness through forming numerous atom-scale interlocks between different slip systems. Moreover, the HEDC maintains the excellent electrical conductivity of the constituent borides, and the competition between two correlating structures produces the special kind of coherent boundary that minimizes the scattering of conducting electrons and does not largely deteriorate the electrical conductivity.

Atomic stiffness for bulk modulus prediction and high-throughput screening of ultraincompressible crystals

Determining bulk moduli is central to high-throughput screening of ultraincompressible materials. However, existing approaches are either too inaccurate or too expensive for general applications, or they are limited to narrow chemistries. Here we define a microscopic quantity to measure the atomic stiffness for each element in the periodic table. Based on this quantity, we derive an analytic formula for bulk modulus prediction. By analyzing numerous crystals from first-principles calculations, this formula shows superior accuracy, efficiency, universality, and interpretability compared to previous empirical/semiempirical formulae and machine learning models. Directed by our formula predictions and verified by first-principles calculations, 47 ultraincompressible crystals rivaling diamond are identified from over one million material candidates, which extends the family of known ultraincompressible crystals. Finally, treasure maps of possible elemental combinations for ultraincompressible crystals are created from our theory. This theory and insights provide guidelines for designing and discovering ultraincompressible crystals of the future.

High Entropy Borides Synthesized by the Thermal Reduction of Metal Oxides in a Microwave Plasma

Metal oxide thermal reduction, enabled by microwave-induced plasma, was used to synthesize high entropy borides (HEBs). This approach capitalized on the ability of a microwave (MW) plasma source to efficiently transfer thermal energy to drive chemical reactions in an argon-rich plasma. A predominantly single-phase hexagonal AlB2-type structural characteristic of HEBs was obtained by boro/carbothermal reduction as well as by borothermal reduction. We compare the microstructural, mechanical, and oxidation resistance properties using the two different thermal reduction approaches (i.e., with and without carbon as a reducing agent). The plasma-annealed HEB (Hf_0.2, Zr_0.2, Ti_0.2, Ta_0.2, Mo_0.2)B₂ made via boro/carbothermal reduction resulted in a higher measured hardness (38 ± 4 GPa) compared to the same HEB made via borothermal reduction (28 ± 3 GPa). These hardness values were consistent with the theoretical value of ~33 GPa obtained by first-principles simulations using special quasi-random structures. Sample cross-sections were evaluated to examine the effects of the plasma on structural, compositional, and mechanical homogeneity throughout the HEB thickness. MW-plasma-produced HEBs synthesized with carbon exhibit a reduced porosity, higher density, and higher average hardness when compared to HEBs made without carbon.

Metallization and Superconductivity in the van der Waals Compound CuP2Se through Pressure-Tuning of the Interlayer Coupling

Emergent layered Cu-bearing van der Waals (vdW) compounds have great potentials for use in electrocatalysis, lithium batteries, and electronic and optoelectronic devices. However, many of their alluring properties such as potential superconductivity remain unknown. In this work, using CuP₂Se as a model compound, we explored its electrical transport and structural evolution at pressures up to ∼60 GPa using both experimental determinations and ab initio calculations. We found that CuP₂Se undergoes a semiconductor-to-metal transition at ∼20 GPa at room temperature and a metal-to-superconductor transition at 3.3-5.7 K in the pressure range from 27.0 to 61.4 GPa. At ∼10 and 20 GPa, there are two isostructural changes in the compound, corresponding to, respectively, the emergence of the interlayer coupling and start of interlayer atomic bonding. At a pressure between 35 and 40 GPa, the vdW layers start to slide and then merge, forming a new phase with high coordination numbers. We also found that the Bardeen-Cooper-Schrieffer (BCS) theory describes quite well the pressure dependence of the critical temperature despite occurrence of a possible medium-to-strong electron-phonon coupling, revealing the determinant roles of the enhanced bulk modulus and electron density of states at high pressure. Moreover, nanosizing of CuP₂Se at high pressure further increased the critical temperature even at sizes approaching the Anderson limit. These findings would have important implications for developing novel applications of layered vdW compounds through simple pressure tuning of the interlayer coupling.