First-principles study of multifunctional Mn2B3 materials with high hardness and ferromagnetism

Transition metal boride TM2B3 is widely studied in the field of physics and materials science. However, Mn2B3 has not been found in Mn-B systems so far. Mn2B3 undergoes phase transitions from Cmcm (0-28 GPa) to C2/m (28-80 GPa) and finally to C2/c (80-200 GPa) under pressure. Among these stable phases, Cmcm- and C2/m-Mn2B3s comprise six-membered boron rings and C2/c-Mn2B3 has wavy boron chains. They all have good mechanical properties and can become potential multifunctional materials. The strong B-B covalent bonding is mainly responsible for the structural stability and hardness. Comparison of the hardness of the five TM2B3s with different bonding strengths of TM-B and B-B bonds reveals a nonlinear change in the hardness. According to the Stoner model, these structures possess ferromagnetism, and the corresponding magnetic moments are almost the same as those of GGA and GGA + U (U = 3.9 eV, J = 1 eV).

The structure and multifunctionality of high-boron transition metal borides. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2023;35:173001. [PMID: 36758243 DOI: 10.1088/1361-648x/acbad6] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

High boron content transition metal (TM) borides (HB-TMBs) have recently been regarded as the promising candidate for superhard multifunctional materials. High hardness stems from the covalent bond skeleton formed by high content of boron (B) atoms to resist deformation. High valence electron density of TM and special electronic structure fromp-dhybridization of B and TM are the sources of multifunction. However, the reason of hardness variation in different HB-TMBs is still a puzzle because hardness is a complex property mainly associated with structures, chemical bonds, and mechanical anisotropy. Rich types of hybridization in B atoms (sp, sp², sp³) generate abundant structures in HB-TMBs. Studying the intrinsic interaction of structures and hardness or multifunction is significant to search new functional superhard materials. In this review, the stable structure, hardness, and multifunctionality of HB-TMBs are summarized. It is concluded that the structures of HB-TMBs are mainly composed by sandwiched stacking of B and TM layers. The hardness of HB-TMBs shows a increasing tendency with the decreasing atom radius. The polyhedron in strong B skeleton provides hardness support for HB-TMBs, among which C2/mis the most possible structure to meet the superhard standard. The shear modulus (G₀) generates a positive effect for hardness of HB-TMBs, but the effect from bulk modulus (G₀) is complex. Importantly, materials with a value ofB₀/G₀less than 1.1 are more possible to achieve the superhard standard. As for the electronic properties, almost all TMB₃and TMB₄structures exhibit metallic properties, and their density of states near the Fermi level are derived from the d electrons of TM. The excellent electrical property of HB-TMBs with higher B ratio such as ZrB₁₂comes from the channels between B-Bπ-bond and TM-d orbitals. Some HB-TMBs also indicate superconductivity from special structures, most of them have stronger hybridization of d electrons from TM atoms than p electrons from B atoms near the Fermi level. This work is meaningful to further understand and uncover new functional superhard materials in HB-TMBs.

Exploring the Magnetic and Electrocatalytic Properties of Amorphous MnB Nanoflakes

Two-dimensional (2D) metal borides are a class of ceramic materials with diverse structural and topological properties. These diverse material properties of metal borides are what forms the basis of their interdisciplinarity and their applicability in various research fields. In this study, we highlight which fundamental and practical parameters need to be taken into consideration when designing nanomaterials for specific applications. A simple one-pot chemical reduction method was applied for the synthesis of manganese mono-boride nanoflakes at room temperature. How the specific surface area and boron-content of the as-synthesized manganese mono-boride nanoflakes influence their magnetic and electrocatalytic properties is reported. The sample with the highest specific surface area and boron content demonstrated the best magnetic and electrocatalytic properties in the HER. Whereas the sample with the lowest specific surface area and boron content exhibited the best electric conductivity and electrocatalytic properties in the OER.

Mechanical properties and multifunctionality of AlB2-type transition metal diborides. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2022;35:074002. [PMID: 36541538 DOI: 10.1088/1361-648x/aca85f] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Transition metal diborides (TMdBs,P6/mmm, AlB₂-type) have attracted much attention for decades, due to TMdBs can be conductors, superconductors, magnetism materials, and catalysts. The layered structure caused by the borophene subunit is the source of functions and also makes TMdBs a potential bank of Mbene. However, TMdBs also exhibit high hardness which is not supposed to have in the layered structure. The high hardness of TMdBs arises from covalent bonds of boron-boron (B-B) and strongp-dorbit hybridization of B and TM. While strong B-TM bonds will eliminate the layered structure which may damage the functional properties. Understanding the basic mechanism of hardness and function is significant to achieve optimal TMdBs. This work summarizes the basic properties of TMdBs including hardness, superconductor, and catalytic properties. It can be found that Young's modulus (E) and Shear modulus (G) are beneficial for the hardness of TMdBs and the Poisson's ratio is the opposite. Increasing the atomic radius of TM brings an improvement in the hardness of TMdBs before it reaches the highest value of 1.47 Å, beyond which hardness decreases. Besides, TMdBs also have excellent activity comparable with some noble metals for hydrogen evolution reaction, which is closely related to the d-band center. More importantly, higher valence electron concentrations were found to be adverse to the hardness and superconductivity of TMdBs and greatly affect their catalytic properties. This review is of guiding significance for further exploring the relationship between structures and properties of TMdBs.

An electrically conductive and ferromagnetic nano-structure manganese mono-boride with high Vickers hardness

The combination of various desired physical properties greatly extends the applicability of materials. Magnetic materials are generally mechanically soft, yet the combination of high mechanical hardness and ferromagnetic properties is highly sought after. Here, we report the synthesis and characterization of nanocrystalline manganese boride, CrB-type MnB, using the high-pressure and high-temperature method in a large volume press. CrB-type MnB shares the specificity of large numbers of unpaired electrons of manganese ions and strong covalent boron zigzag chains. Thus, manganese mono-boride exhibits "strong" ferromagnetic, magnetocaloric behavior, and possesses high Vickers hardness. We demonstrate that zigzag boron chains in this structure not only play a pivotal role in strengthening mechanical properties but also tuning the exchange correlations between manganese atoms. Nontoxic and Earth-abundant CrB-type MnB is much more incompressible and tougher than traditional ferromagnetic materials. The unique combination of high mechanical hardness, magnetism, and electrical conductivity properties makes it a particularly promising candidate for a wide range of applications.

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 W_1-xRe_xC (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 W_1-xRe_xC 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.

Anomalous lattice stiffening in tungsten tetraboride solid solutions with manganese under compression

Tungsten tetraboride (WB₄)-based solid solutions represent one of the most promising superhard metal candidates; however, their underlying hardening mechanisms have not yet been fully understood. Here, we explore the lattice compressibility of WB₄ binary solid solutions with different manganese (Mn) concentrations using high-pressure x-ray diffraction (XRD) up to 52 GPa. Under initial compression, the lattices of low and high Mn-doped WB₄ alloys (i.e. W_0.96Mn_0.04B₄ and W_0.84Mn_0.16B₄) are shown to be more and less compressible than pure WB₄, respectively. Then, a c-axis softening is found to occur above 39 GPa in WB₄, consistent with previous results. However, an anomalous sudden a-axis stiffening is revealed at ~36 GPa in W_0.96Mn_0.04B₄, along with suppression of c-axis softening observed in WB₄. Furthermore, upon Mn addition, a simultaneous stiffening of a- and c-axes is demonstrated in W_0.84Mn_0.16B₄ at ~37 GPa. Speculation on the possible relationship between this anomalous stiffening and the combined effects of valence-electron concentration (VEC) and atomic size mismatch is also included to understand the origin of the nearly identical hardness enhancement in those two solid solutions compared to WB₄. Our findings emphasize the importance of accurate bonding and structure manipulation via solute atoms to best optimize the hardness of WB₄ solid solutions.

Crystal structure prediction: reflections on present status and challenges

In these Concluding Remarks, I try to summarize my personal view of the enormous progress made in the field of CSP and the open questions and challenges that keep this field more exciting than ever.

Structural search for stable Mg–Ca alloys accelerated with a neural network interatomic model

We have combined a neural network formalism with metaheuristic structural global search algorithms to systematically screen the Mg–Ca binary system for new (meta)stable alloys.

Manganese mono-boride, an inexpensive room temperature ferromagnetic hard material

We synthesized orthorhombic FeB-type MnB (space group: Pnma) with high pressure and high temperature method. MnB is a promising soft magnetic material, which is ferromagnetic with Curie temperature as high as 546.3 K, and high magnetization value up to 155.5 emu/g, and comparatively low coercive field. The strong room temperature ferromagnetic properties stem from the positive exchange-correlation between manganese atoms and the large number of unpaired Mn 3d electrons. The asymptotic Vickers hardness (AVH) is 15.7 GPa which is far higher than that of traditional ferromagnetic materials. The high hardness is ascribed to the zigzag boron chains running through manganese lattice, as unraveled by X-ray photoelectron spectroscopy result and first principle calculations. This exploration opens a new class of materials with the integration of superior mechanical properties, lower cost, electrical conductivity, and fantastic soft magnetic properties which will be significant for scientific research and industrial application as advanced structural and functional materials.

A first-principles investigation of a new hard multi-layered MnB2 structure

A novel high-pressure phase of multi-layers Immm-MnB₂ is confirmed on the convex hull diagram of the Mn-B systems at 50 GPa (b).

Novel magnesium borides and their superconductivity

With the motivation of searching for new superconductors in the Mg–B system, we performed ab initio evolutionary searches for all the stable compounds in this binary system in the pressure range of 0–200 GPa.

Crystal Field Splitting is Limiting the Stability and Strength of Ultra-incompressible Orthorhombic Transition Metal Tetraborides

The lattice stability and mechanical strengths of the supposedly superhard transition metal tetraborides (TmB4, Tm = Cr, Mn and Fe) evoked recently much attention from the scientific community due to the potential applications of these materials, as well as because of general scientific interests. In the present study, we show that the surprising stabilization of these compounds from a high symmetry to a low symmetry structure is accomplished by an in-plane rotation of the boron network, which maximizes the in-plane hybridization by crystal field splitting between d orbitals of Tm and p orbitals of B. Studies of mechanical and electronic properties of TmB4 suggest that these tetraborides cannot be intrinsically superhard. The mechanical instability is facilitated by a unique in-plane or out-of-plane weakening of the three-dimensional covalent bond network of boron along different shear deformation paths. These results shed a novel view on the origin of the stability and strength of orthorhombic TmB4, highlighting the importance of combinational analysis of a variety of parameters related to plastic deformation of the crystalline materials when attempting to design new ultra-incompressible, and potentially strong and hard solids.

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.

Prediction of new thermodynamically stable aluminum oxides

Recently, it has been shown that under pressure, unexpected and counterintuitive chemical compounds become stable. Laser shock experiments (A. Rode, unpublished) on alumina (Al₂O₃) have shown non-equilibrium decomposition of alumina with the formation of free Al and a mysterious transparent phase. Inspired by these observations, we have explored the possibility of the formation of new chemical compounds in the system Al-O. Using the variable-composition structure prediction algorithm USPEX, in addition to the well-known Al₂O₃, we have found two extraordinary compounds Al₄O₇ and AlO₂ to be thermodynamically stable in the pressure ranges 330-443 GPa and above 332 GPa, respectively. Both of these compounds at the same time contain oxide O²⁻ and peroxide O₂²⁻ ions, and both are insulating. Peroxo-groups are responsible for gap states, which significantly reduce the electronic band gap of both Al₄O₇ and AlO₂.

Stable compositions and structures in the Na–Bi system

Predicted phase diversity and interesting properties of the Na–Bi system in the range of 0–320 GPa from first-principles calculations.

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.

First-principles study of ground-state properties of U2Mo

By means of first-principles calculations, we have systematically investigated the structural, elastic, vibrational, thermal and electronic properties of the ground-state phase for the intermetallic compound U₂Mo.