Transition Metal Silicides Research Articles

Enhancement of superconductivity in W5Si3 with Tc ∼ 6.2 K by P-doping

The effects of phosphorus doping on the prototypical transition metal silicide W5Si3, predicted to host a topologically nontrivial band structure and exhibit superconductivity below its critical temperature (Tc) of 2.7 K, have been investigated. Upon P-doping, the lattice parameter a decreases and c increases for W5Si3–xPx up to x = 0.7. Notably, Tc and the upper critical field of W5Si3–xPx are enhanced by over twofold as x increases, achieving 6.2 K and 5.5 T at x = 0.7, respectively. This enhancement in bulk superconductivity is attributed to a shift of the Fermi level toward a local maximum in the density of states upon P-doping, as indicated by the first-principles calculations. Our results suggest an effective doping strategy which significantly enhances the superconductivity in W5Si3. The emerging W5Si3-type superconductors offer a promising platform for realizing possible topological superconductivity.

Covalent Transition Metal Borosilicides: Reaction Pathways in Molten Salts for Water Oxidation Electrocatalysis.

The properties of transition metal borides and silicides are intimately linked to the covalent character of the chemical bonds within their crystal structures. Bringing boron and silicon together within metal borosilicides can then engender different competing covalent networks and complex charge distributions. This situation results in unique structures and atomic environments, which can impact charge transport and catalytic properties. Metal borosilicides, however, hold the status of unusual exotic species, difficult to synthesize and with poor knowledge of their properties. Our strategy consists of developing a redox pathway to synthesize transition metal borosilicides in inorganic molten salts as high-temperature solvents. By studying the formation of Ni6Si2B, Co4.75Si2B, Fe5SiB2, and Mn5SiB2 with in situ X-ray diffraction, we highlight how new reaction routes, maintaining covalent structural building blocks, draw a general scheme of their formation. This pathway is driven by the covalence of the chemical bonds within the boron coordination framework. Next, we demonstrate high efficiency for water oxidation electrocatalysis, especially for Ni6Si2B. We ascribe the strongly increased resistance to corrosion, high stability, and electrocatalytic activity of the Ni6Si2B-derived material to three factors: (1) the two entangled boron and silicon covalent networks; (2) the ability to codope with boron and silicon an in situ generated catalytic layer; and (3) a rare electron enrichment of the transition metal by back-donation from boron atoms, previously unknown within this compound family. With this work, we then unveil a new chemical dimension for Earth-abundant water oxidation electrocatalysts by bringing to light a new family of materials.

Temperature dependence mechanism of high-temperature oxidation of transition metal silicide MoSi2

Transition metal silicides represented by MoSi2 have excellent oxidation resistance and are widely used as high-temperature anti-oxidation coatings in hot end components of power equipment. However, the mechanism of temperature-dependent growth of MoSi2 oxidation products has not been revealed. Therefore, this study investigated the formation characteristics of oxide film and silicide-poor compound on MoSi2 at temperatures of 1000 °C–1550 °C through high-temperature oxidation experiments, combined with microscopic Raman spectroscopy, scanning electron microscope, and x-ray diffraction (XRD) characterizations. The result showed that MoSi2 underwent high-temperature selective oxidation reactions at 1000 °C–1200 °C, forming MoO2 and SiO2 oxide film on the substrate. As the oxidation temperature increased to 1550 °C, after 100 h of oxidation, along with the disappearance of MoO2 and the phase transformation of SiO2, a continuous Mo5Si3 layer with a thickness of approximately 47 μm was formed at the SiO2–MoSi2 interface. Thermodynamics and kinetic calculations further revealed the mechanism of temperature-dependent growth of oxidation products (MoO2 and Mo5Si3) during high-temperature oxidation process of MoSi2. As the temperature increased, the diffusion flux ratio of O and Si decreased, leading to a decrease in oxygen concentration at the interface and promoting the growth of the Mo5Si3 layer. Its thickness is an important indicator for evaluating the oxidation resistance of MoSi2 coatings during service. This study provides experimental and mechanistic insights into the temperature-dependent growth behavior of Mo5Si3 during the high-temperature oxidation of MoSi2 coating, and provides guidance for predicting the service life and improving the oxidation resistance of silicide coatings.

Eco-friendly transition metal silicide for high temperature thermoelectricity

Eco-friendly transition metal silicide for high temperature thermoelectricity

“Anomalous” diffusion in multi-layer silicide systems

The transition metal silicide coatings have excellent oxidation resistance. In the high-temperature oxidization environment, the “anomalous” diffusion phenomenon of the reverse concentration gradient occurs in the multilayer silicide coating structure, which has a significant impact on the coating degradation. This study is to explore the physical mechanism of “anomalous” diffusion phase transformation in the MoSi2–NbSi2 bilayer silicide coating on the Nb alloy substrate. Through vacuum annealing experiments, combined with micro-Raman spectroscopy and electron probe microanalysis measurements, the diffuisonal phase transformation of multilayer silicide coating in the oxygen-free environment at high temperatures was studied. By decoupling the oxidation and diffusion, the experiments indicate that high-temperature oxidation is not the dominant driving factor for the “anomalous” diffusion of atoms. The thermodynamic analysis reveals that the reduction in the nucleation barrier of the silicide-poor layer due to multicomponent solid solution and the non-uniform distribution of component chemical potential provide the driving force for the “anomalous” diffusion growth. Based on the diffusion kinetic modeling, the simulation of diffusion-controlled phase transformation in multilayer silicide coating was carried out, and the effect of tracer diffusion coefficients on the growth of the silicide-poor phase was analyzed. The research will have guiding significance for the recognition of failure mechanisms of silicide coating systems and performance improvement.

Monolithic-structured nickel silicide electrocatalyst for bifunctionally efficient overall water splitting

The rational design and synthesis of cost-effective and efficient bifunctional electrocatalysts are crucial for developing hydrogen energy yet challenging. Here we report porous monolith electrocatalysts (PMECs) comprising transition metal silicide (e.g., nickel silicide) with high activity and durability. These PMECs offer strong synergetic effects and high exposure of active sites, resulting in excellent kinetics in catalyzing both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), which surpass the benchmark catalysts RuO2 and Pt/C. The doping strategy is demonstrated to further enhance electrocatalytic performance by constructing Mo-doped Ni2Si PMEC, which requires only a cell voltage of 1.60 V at 100 mA cm−2. Density functional theory calculations display that the synergistic effect of Ni and Si can reduce the energy barriers of intermediate adsorption, and the introduction of Mo into Ni2Si can further decrease the energy barrier of determining step and optimize the H* adsorption energy, thus enhancing the electrochemical kinetics for OER and HER. Our work paves the way for designing high-efficiency and low-cost porous monolith catalysts through a facile and scalable method, showing great prospects for industrialization.

B20 Weyl Semimetal CoSi Film Fabricated by Flash-Lamp Annealing.

B20-CoSi is a newly discovered Weyl semimetal that crystallizes into a noncentrosymmetric crystal structure. However, the investigation of B20-CoSi has so far been focused on bulk materials, whereas the growth of thin films on technology-relevant substrates is a prerequisite for most practical applications. In this study, we have used millisecond-range flash-lamp annealing, a nonequilibrium solid-state reaction, to grow B20-CoSi thin films. By optimizing the annealing parameters, we were able to obtain thin films with a pure B20-CoSi phase. The magnetic and transport measurements indicate the appearance of the charge density wave and chiral anomaly. Our work presents a promising method for preparing thin films of most binary B20 transition-metal silicides, which are candidates for topological Weyl semimetals.

Nonlinear photoconductivities and quantum geometry of chiral multifold fermions

Chiral multifold fermions are quasiparticles that appear only in chiral crystals such as transition metal silicides in the cubic B20 structure (i.e., the CoSi family), and they may show exotic physical properties. Here we study the injection and shift photoconductivities and also the related geometrical quantities for several types of chiral multifold fermions, including spin-1/2 as well as pseudospin-1 and -3/2 fermions, dubbed as Kramers Weyl, triple-point, and Rarita-Schwinger-Weyl (RSW) fermions, respectively. We utilize the minimal symmorphic model to describe the triple-point fermions (TPF). We also consider the more realistic model Hamiltonian for the CoSi family including both linear and quadratic terms. We find that injection currents due to circularly polarized light are quantized as a result of the Chern numbers carried by the multifold fermions within the linear models. Surprisingly, we discover that in the TPF model, the linear shift conductivities, responsible for the shift current generation by linearly polarized light, are proportional to the pseudo spin-orbit coupling and independent of photon frequency. In contrast, for the RSW and Kramer Weyl fermions, the linear shift conductivity is linearly proportional to photon frequency. The numerical results agree with the power-counting analysis for quadratic Hamiltonians. The frequency independence of the linear shift conductivity could be attributed to the strong resonant symplectic Christoffel symbols of the flat bands. Moreover, the calculated symplectic Christoffel symbols show significant peaks at the nodes, suggesting that the shift currents are due to the strong geometrical response near the topological nodes.

Theoretical prediction of the influence of defects and atomic occupation on the elastic and electronic properties of NbSi2 form the first-principles calculations

In now days, the transition metal silicides are one of the promising materials for making high-temperature materials. To improve the inherent brittleness of the transition metal silicides, the elastic and electronic properties of the C40-type NbSi2 disilicides have been predicted using first-principles calculations. Six defect models (V-Va, Si-Va, N-V, N-Si, O-V, O-Si) were chosen to evaluate the elastic properties of transition metal silicides. According to the data of thermodynamic parameters, NbSi2 with the different defect and doping models shows the phase stability. The introduction of different defects and doping models changes the mechanical behavior. The defect and doping models lead to the decrease in the deformation resistance and hardness of the perfect NbSi2. Additionally, the defect and doping models result in the perfect NbSi2 to actualize the brittle-to-ductile transition. The Nb defects resulted in NbSi2 having the greatest degree of anisotropy. The electronic structures are calculated to discuss the brittle-to- ductile transition for NbSi2 with defects.

An integrated approach to solving the technological problem of obtaining target cathodes from transition metal silicides for vacuum ion‑plasma sources

The article discusses the technological principles of improving the manufacturing process of consumable cathodes-targets of vacuum electric arc evaporation plants from complex silicides. Which are based on: the selection of chemical composition based on the analysis of state diagrams of double and triple systems, the production of fast-cooled ingots using high-speed induction melting, subsequent grinding and grinding to a powder of the required fraction, static or dynamic pressing of the cathode blank, its sintering and soldering of the current guide. The results of laboratory studies of the process are presented and prototypes of products are obtained.

Identifying silicides via plasmon loss satellites in photoemission of the Ru-Si system

The phase and composition of several transition metal silicides are challenging to identify with common surface analysis techniques such as X-ray photoelectron spectroscopy (XPS). While silicide formation is concomitant with a distinct change in electronic structure, only minute changes in the main spectral features are observed for example for the family of Ru silicides. Here, the authors combine XPS, grazing-incidence X-ray diffraction, and density functional theory calculations to demonstrate that the characteristic excitation energies of plasmons in Ru and its silicides are a sensitive and easily accessible descriptor that reflects the change in electronic structure upon the formation of specific silicides in the XPS spectra. Electron energy loss satellites are reported to shift by more than 4 eV upon the formation of Ru silicide, by 1.1 eV between RuSi and Ru2Si3 and by 1.9 eV across the measured range of silicide layers, making these changes accessible even for basic experimental equipment. In the context of literature on metal silicides and electron energy loss spectroscopy, this approach is considered promising as a general pathway to enhance the chemical sensitivity of surface spectroscopy methods.

Ultrafine molybdenum silicide nanoparticles as efficient hydrogen evolution electrocatalyst in acidic medium

Molybdenum silicides are promising electrocatalysts for hydrogen evolution in acidic environment due to their dual characteristics of metal and ceramics as well as high electrical conductivity and acid resistance. At present, most of the transition metal silicides were synthesized at high temperature, resulting in large particle size and small specific surface area, which seriously limits their electrocatalytic applications. Herein, we report a low temperature strategy for the synthesis of ultrafine Mo5Si3 and MoSi2 nanoparticles with diameter of ∼5 nm by molten salt method. Results show that both of them demonstrated excellent electrocatalytic hydrogen evolution activity and stability in 0.5 M H2SO4 solution, in which the overpotentials of Mo5Si3 and MoSi2 nanoparticles at 10 mA cm−2 are 80 mV and 94 mV, respectively. This general strategy may light up the preparation of ultrafine transition metal silicides nanoparticles and facilitate their applications in electrocatalytic areas.

Alloying Driven Multifold Fermion‐to‐Weyl Semimetal Transition in CoSi1−xAx (A = Ge, Sn)

Multifold chiral fermions with high‐fold degeneracy beyond the twofold Weyl fermions have recently been reported in the transition metal silicide CoSi (space group P213). Here, by alloying parent compound CoSi, an alloy engineering approach to realize different topological phases in CoSi1−x A x (A = Ge, Sn) is demonstrated. By using first‐principles calculations and symmetry analysis, it is shown that the gradual decrease of crystal symmetry due to alloying makes Weyl‐type low‐energy excitation phases appear in CoSi1−x A x . For the alloy structures considered, surface states (SS) and Fermi arcs (FAs) are investigated, and the dynamic stability of the alloy structures is also confirmed by the phonon spectrum. These findings provide a feasible method for customizing the fermion behavior of multifold fermions semimetal and expect future experimental verification.

Insights into the anisotropic, electronic and magnetism properties of ternary TM2Si2Ys (TM = Cr, Fe, Co, Ni) silicides from the first-principles calculations

In recent years, the transition metal silicides are promising advanced functional materials. However, the relevant physical properties of TM2Si2Ys are not well understood. In this work, the electronic, anisotropic and magnetic properties of SiY, Si2Y, Si3Y5 and TM2Si2Ys (TM = Cr, Fe, Co, Ni) were investigated using the first-principles calculations. The results show that these ternary TM2Si2Y silicides are thermodynamically stable. The elastic properties confirm that these compounds exhibit mechanically stable. According to the elastic constants and elastic modulus results, Co2Si2Y shows the stronger deformation resistance and the brittle behavior. The elastic anisotropy was characterized by the three-dimensional (3D) surface constructions and 2D projections. The band structure, density of states and electron density difference were systematically studied. In addition, the electronic structures explain the magnetic properties for TM2Si2Ys (TM = Cr, Fe, Co, Ni).

Plastic deformation of bulk and micropillar single crystals of Mo5Si3 with the tetragonal D8m structure

The plastic deformation behavior of single crystals of Mo5Si3 with the tetragonal D8m structure has been investigated in compression as a function of crystal orientation and temperature (1200–1500°C) in the bulk form and as a function of crystal orientation and specimen size at room temperature in the micropillar form. The slip system of {112¯}<111> is identified to be the only one that operates at high temperatures above 1200°C in bulk crystals, while any plastic flow is not detected at room temperature in micropillar crystals. The critical resolved shear stress (CRSS) for {112¯}<111> slip at room temperature estimated from the extrapolation of the temperature dependence of CRSS obtained for bulk crystals is considerably higher than fracture stresses obtained for micropillar crystals, indicating that the room-temperature brittleness is due in principle to the difficulty in dislocation motion arising from the very high CRSS value. The value of fracture toughness evaluated with a chevron-notched micro-beam specimen with a notch plane parallel to (001) is 1.54 MPa•m1/2, which is comparable to those reported for other transition-metal (TM) silicides of the TM5Si3-type. The selection of {112¯} slip plane and the dissociation of the 1/2<111> dislocation on the slip plane are discussed based on generalized stacking fault energy (GSFE) curves theoretically calculated by first-principles calculations.

The topological nodal lines and drum-head-like surface states in semimetals CrSi2, MoSi2 and WSi2

The structural and electronic properties of I4/mmm XSi2 (X = Cr, Mo and W) are systematically studied by first-principles calculation. First, the dynamical stability of XSi2 (X = Cr, Mo and W) is verified by the phonon spectrums, the thermal stability is confirmed by the ab initio molecular dynamics simulations (AIMD) at 300 K and the mechanical stability is affirmed by the calculated elastic constants. Electronic structure computations uncover the clear drum-head-like surface states and the topological nodal lines (TNLs) which are close to the Fermi level EF and Γ centered in the Brillouin zone (BZ). Further investigations indicate the band structures and the TNLs are robust against the spin-orbit coupling (SOC), the biaxial and uniform strains, which is beneficial to experimental observation. Our results suggest tetragonal CrSi2, MoSi2 and WSi2 are ideal transition metal silicides to study the TNLs and drum-head-like surface states for topological physics.

First-principles prediction of structure and mechanical properties of TM5SiC2 ternary silicides

Although transition metal silicides are promising advanced functional materials, the adjustment of strength and ductility behavior is a great challenge. It is believed that the introduction of C element maybe improved the overall properties of transition metal silicides. Similar to the TM5SiB2, the structure, elastic properties and ductility of TM5SiC2(TM = Cr, Mo, Nb and W) ternary silicides are studied by the first-principles calculations. The results show that four novel TM5SiC2 ternary silicides are firstly predicted. Compared to TM5Si3, these TM5SiC2 ternary silicides have strong elastic deformation resistance due to the role of C element. In particular, these TM5SiC2 ternary silicides also exhibit better ductility compared to the brittle TM5Si3 phases. The calculated electronic structure shows that these TM5SiC2 ternary silicides have electronic properties because of the band overlap of C-2p state and TM-d state near the Fermi level. Therefore, this work can provide a new path to improve the mechanical properties of TM-Si-C ternary silicides high-temperature materials.

Strong enhancement of superconductivity in the topological transition metal silicide W5Si3 by Re doping

Chemical doping leads to a strong enhancement of superconductivity in the topological transition metal silicide W5Si3.

Nb-based double transition metal silicides MAX-phase: A first-principle study

To explore the structural rationality and potential applications of the hypothetical chemically ordered double transition metal silicides MAX-phase Nb 2 ZrSiC 2 and Nb 2 TiSiC 2 , we used density functional theory to systematically study the crystal structure and related properties of them and compared with the case of Nb 3 SiC 2 . Through the calculations of elastic constant and phonon dispersive curves, the stability of these three ceramics were investigated. Results show they are stable in mechanics and lattice dynamics. Then, normalized bond lengths as function of pressure for these compounds were computed to discuss their bond stiffness. In addition, the calculations of the densities of states for them indicate that they are all metallic-like conductors. And then, the calculations of dielectric function and reflectivity for these ceramics were performed to explore their optical properties. The good reflectivity performance in the infrared and close infrared region means they are potential coating materials on spacecraft in the future. At the end of this work, the Debye temperatures of these three ceramics compounds were obtained by calculating their longitudinal, transverse and mean and velocities, and results indicate they all exhibit a relatively good high temperature resistance.

Tailoring the strengthening-toughening behavior of the MoSi2 film by doping trace solute Au

Although the high hardness and wear resistance of transition metal silicides (TMSi) are widely used in protective films, overcoming the contradiction between hardness and toughness still faces huge challenges. Here the magnetron sputtering was used to introduce trace solute Au atoms into the MoSi2 to form a solid solution structure. X-ray diffraction and high-resolution transmission electron microscopy were used to characterize the microstructure of Mo-Si-Au film. In addition, hardness and toughness enhancement by solute Au was investigated the film through the combination of experimental and theoretical studies. The results show that the hardness and fracture toughness of Mo-Si-Au (Au/Mo atom ratio, R = 0.10) film is 15.4 GPa and 2.52 MPa m1/2, respectively. Compared with MoSi2, the hardness and toughness are increased by 41.3% and 68.0%, respectively. Density Function Theory calculations and electronic structure analyses further indicate that the increase in the hardness of the solid solution Mo-Si-Au film may be related to the increase in bulk modulus, C33, and C11. The increase in toughness is mainly due to the strengthening of the metal bonds in MoSi2 after doping a small amount of solute Au. The introduction of trace solute Au atoms in TMSi provides a new reliable scheme for overcoming the contradiction between hardness and toughness. As part of this research, a protective coating based on TMSi will be expected to be applied to aviation components.