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.

The Effect of Carbon Doping on the Crystal Structure and Electrical Properties of Sb2Te3.

As a new generation of non-volatile memory, phase change random access memory (PCRAM) has the potential to fill the hierarchical gap between DRAM and NAND FLASH in computer storage. Sb2Te3, one of the candidate materials for high-speed PCRAM, has high crystallization speed and poor thermal stability. In this work, we investigated the effect of carbon doping on Sb2Te3. It was found that the FCC phase of C-doped Sb2Te3 appeared at 200 °C and began to transform into the HEX phase at 25 °C, which is different from the previous reports where no FCC phase was observed in C-Sb2Te3. Based on the experimental observation and first-principles density functional theory calculation, it is found that the formation energy of FCC-Sb2Te3 structure decreases gradually with the increase in C doping concentration. Moreover, doped C atoms tend to form C molecular clusters in sp2 hybridization at the grain boundary of Sb2Te3, which is similar to the layered structure of graphite. And after doping C atoms, the thermal stability of Sb2Te3 is improved. We have fabricated the PCRAM device cell array of a C-Sb2Te3 alloy, which has an operating speed of 5 ns, a high thermal stability (10-year data retention temperature 138.1 °C), a low device power consumption (0.57 pJ), a continuously adjustable resistance value, and a very low resistance drift coefficient.

Enhanced Hardness in Transition-Metal Monocarbides via Optimal Occupancy of Bonding Orbitals.

An efficient strategy that can guide the synthesis of materials with superior mechanical properties is important for advanced material/device design. Here, we report a feasible way to enhance hardness in transition-metal monocarbides (TMCs) by optimally filling the bonding orbitals of valence electrons. We demonstrate that the intrinsic hardness of the NaCl- and WC-type TMCs maximizes at valence electron concentrations of about 9 and 10.25 electrons per cell, respectively; any deviation from such optimal values will reduce the hardness. Using the spark plasma sintering technique, a number of W1-xRexC (x = 0-0.5) have been successfully synthesized, and powder X-ray diffractions show that they adopt the hexagonal WC-type structure. Subsequent nanoindentation and Vickers hardness measurements corroborate that the newly developed W1-xRexC samples (x = 0.1-0.3) are much harder than their parent phase (i.e., WC), marking them as the hardest TMCs for practical applications. Furthermore, the hardness enhancement can be well rationalized by the balanced occupancy of bonding and antibonding states. Our findings not only elucidate the unique hardening mechanism in a large class of TMCs but also offer a guide for the design of other hard and superhard compounds such as borides and nitrides.

High-entropy A2B2O7-type oxide ceramics: A potential immobilising matrix for high-level radioactive waste.

The sustainable development of civil nuclear energy requires the fabrication of the durable nuclear wasteforms, in particular for high-level radioactive waste, which involves the design of the composition and microstructure. Herein, we demonstrated that high-entropy ceramics (Eu1-xGdx)2(Ti0.2Zr0.2Hf0.2Nb0.2Ce0.2)2O7 are the potential candidate as immobilizing hosts for high-level radioactive waste. The static aqueous leaching test indicates that the normalized leaching rates for the simulated radionuclides Ce (LRCe) and Gd (LRGd) in as-prepared high-entropy ceramics are approximately 10-6~10-8 g·m-2·d-1 after 42 days testing, much lower than those reported values in doped-Gd2Zr2O7 (10-6~10-3 g·m-2·d-1). The excellent chemical durability is mainly due to the synergistic effects of the compositional complexity and severe lattice distortion. Compared to their ternary oxides, the low oxygen vacancy concentration slows down the migration and diffusion of cations. Moreover, the lattice distortion increases the lattice potential energy, also inhibiting the migration of cations. This study provides a strategy for the development and application of high-entropy ceramics as the wasteforms.

Pressure-driven switching of magnetism in layered CrCl3.

Layered transition-metal compounds with controllable magnetic behaviors provide many fascinating opportunities for the fabrication of high-performance magneto-electric and spintronic devices. The tuning of their electronic and magnetic properties is usually limited to the change of layer thickness, electrostatic doping, and the control of electric and magnetic fields. However, pressure has been rarely exploited as a control parameter for tailoring their magneto-electric properties. Here, we report a unique pressure-driven isostructural phase transition in layered CrCl3 accompanied by a simultaneous switching of magnetism from a ferromagnetic to an antiferromagnetic ordering. Our experiments, in combination with ab initio calculations, demonstrate that such a magnetic transition hinders the bandgap collapse under pressure, leading to an anomalous semiconductor-to-semiconductor transition. Our findings not only reveal the potential applications of this material in electronic and spintronic devices but also establish the basis for exploring unusual phase transitions in layered transition-metal compounds.

The mechanism of anomalous hardening in transition-metal monoborides.

The quest for novel nanomaterials with unusual functionalities continues to be a central challenge to modern nanoscience. Here we report a surprisingly anomalous hardening behavior exhibited by a class of transition-metal monoborides (TMB). Most remarkable is the occurrence of the intrinsic hardness maximum at the valence-electron concentration (VEC) of about 8 electrons per formula unit (e per f.u.); both lower and higher VECs lead to the decrease of hardness, contrasting with the expected positive correlation between hardness and VEC. Such an unexpected phenomenon originates from the presence of two sorts of bands near the Fermi level that respond oppositely to the movement of dislocations within the metal bilayer. Furthermore, we demonstrate that the hardness is closely related to the formation energy for TMB, which justifies the importance of the thermodynamic stability in designing superhard materials. Our findings not only elucidate the unique mechanism responsible for unusual atom-scale hardening but also open a new avenue towards designing novel multifunctional nanomaterials with the coexistence of high hardness and excellent electrical conductivity.

Discovery of elusive structures of multifunctional transition-metal borides.

A definitive determination of crystal structures is an important prerequisite for designing and exploiting new functional materials. Even though tungsten and molybdenum borides (TMBx) are the prototype for transition-metal light-element compounds with multiple functionalities, their elusive crystal structures have puzzled scientists for decades. Here, we discover that the long-assumed TMB2 phases with the simple hP3 structure (hP3-TMB2) are in fact a family of complex TMB3 polytypes with a nanoscale ordering along the axial direction. Compared with the energetically unfavorable and dynamically unstable hP3-TMB2 phase, the energetically more favorable and dynamically stable TMB3 polytypes explain the experimental structural parameters, mechanical properties, and X-ray diffraction (XRD) patterns better. We demonstrate that such a structural and compositional modification from the hP3-TMB2 phases to the TMB3 polytypes originates from the relief of the strong antibonding interaction between d electrons by removing one third of metal atoms systematically. These results resolve the longstanding structural mystery of this class of metal borides and uncover a hidden family of polytypic structures. Moreover, these polytypic structures provide an additional hardening mechanism by forming nanoscale interlocks that may strongly hinder the interlayer sliding movements, which promises to open a new avenue towards designing novel superhard nanocomposite materials by exploiting the coexistence of various polytypes.

Phonon-assisted crossover from a nonmagnetic Peierls insulator to a magnetic Stoner metal.

We report a unique temperature-induced insulator-metal transition in MnB4 that is accompanied by a simultaneous magnetostructural change from a nonmagnetic monoclinic mP20 phase to a magnetic orthorhombic oP10 phase. Such a concurring magnetostructural and insulator-metal transformation is a manifestation of a strong competition between Peierls and Stoner mechanisms that governs a crossover from an electron-pairing to an electron-localization scenario in this system. Therefore, the phase stability of MnB4 is controlled by a subtle interplay among the Peierls mechanism, Stoner mechanism, and phonon free energy. Our findings not only resolve the longstanding magnetostructural puzzle of MnB4 but also provide a realistic system for the Peierls-Hubbard model.

Polytypism in superhard transition-metal triborides.

The quest of novel compounds with special structures and unusual functionalities continues to be a central challenge to modern materials science. Even though their exact structures have puzzled scientists for decades, superhard transition-metal borides (TMBs) have long been believed to exist only in simple crystal structures. Here, we report on a polytypic phenomenon in superhard WB3 and MoB3 with a series of energetically degenerate structures due to the random stacking of metal layers amongst the interlocking boron layers. Such polytypism can create a multiphase solid-solution compound with a large number of interfaces amongst different polytypes, and these interfaces will strongly hinder the interlayer sliding movement within each polytype, thereby further increase the hardness of this particular material. Furthermore, in contrast to the conventional knowledge that intrinsically strong chemical bonds in superhard materials should lead to high lattice thermal conductivity, the polytypic TMB3 manifest anomalously low lattice thermal conductivity due to structural disorders and phonon folding. These findings promise to open a new avenue to searching for novel superhard materials with additional functionalities.

The band structure and Fermi surface of (Sr(3)Sc(2)O(5))Fe(2)As(2).

Inspired by the experience in CuO-based superconductor that a larger spacing distance between CuO planes induced a higher T(C), some researchers synthesized (Sr(3)Sc(2)O(5))Fe(2)As(2) and related compounds with spacing distances between FeAs planes as large as 15 Å and expected a higher T(C). Our density functional calculations indicate that the Fermi surface of (Sr(3)Sc(2)O(5))Fe(2)As(2) is very similar to that of LaOFeAs, while the projected band structure shows some differences. From Fermi surface nesting and the calculated Lindhard response function χ(q), we predict that a spin density wave (SDW) and stripe antiferromagnetism (AF) may exist in the undoped compound and that electron or hole doping will suppress this SDW state. Similar to LaFeAsO, both the stabilization energy and the moment are very sensitive to the As atom positions. Because of the considerable similarity to LaFeAsO, (Sr(3)Sc(2)O(5))Fe(2)As(2) is expected to become a superconductor with electron or hole doping.