Si Atoms Research Articles

First-principles study on the lithiation process of amorphous SiO anode for Li-ion batteries with Bayesian optimization.

Amorphous silicon monoxide (a-SiO), which contains Si atoms with various valence states, has attracted much attention as a high-performance anode material for lithium (Li) ion batteries (LIBs). Although current experiments have provided some information during charge/discharge cycles, further investigation of structural changes at the atomic scale is needed. To investigate the lithiation process of a-SiO using first-principles simulations and machine learning techniques, we developed a computational code employing Bayesian optimization to efficiently identify stable sites for Li insertion in the large search-space of amorphous models to reproduce the actual lithiation process and compared this approach to the conventional random scheme by applying it to an a-SiO model previously generated with neural network potentials. The lithiation process based on Bayesian optimization resulted in lower formation energies compared to the conventional random scheme, indicating a more stable structure. During lithiation, Li atoms tended to enter the silicon (Si) phase after the SiO2 phase, in agreement with experimental results. We analyzed the structural changes and observed significant differences in the structural evolution between the conventional and new schemes. Our study highlights the significant influence of the lithiation process on the structural transformation of a-SiO materials, which in turn affects the reversible capacity of the material. These findings will provide a framework for improving the performance and lifetime of a-SiO materials.

Comparative Study of Si−O−Al and Si−O−Si Bond Stability in HZSM‐5 Zeolite Under Steam and Hot Liquid Water Environments

AbstractUnderstanding the changes in the zeolite framework and catalytic active sites during zeolite‐based vapor‐phase and aqueous catalytic processes is crucial. Herein, the evolution of framework T atoms (Si and Al) in ammonium hexafluorosilicate (AHFS)‐treated HZSM‐5 zeolite under steam and hot liquid water (HLW) environments was inverstigated using various characterization techniques. In HLW, Si−O−Si bonds exhibit poorer hydrothermal stability than Si−O−Al bonds, in contrast to steam. Significant Si atom leaching occurs regardless of whether the framework tetrahedral Al atoms (Al(IV)‐1) are removed. Similar to steam, Al(IV)‐1 species in HLW sequentially evolve into partially coordinated framework Al species and then into extra‐framework Al (EFAL) species through partial and complete hydrolysis. The generated EFAL species act as Lewis acid sites, but their local structures or chemical environments may differ. These findings reveal the difference in the T−O−T bonds attacked by water molecules: the Si−O−Al bonds are primarily attacked in steam, whereas the Si−O−Si bond are primarily attacked in HLW.

Quantum Chemical Model Calculations of Adhesion and Dissociation between Epoxy Resin and Si-Containing Molecules

There is no doubt that when solid surfaces are modified, the functional groups and atoms directly bonded to solid atoms play a major role in adsorption interactions with molecules or resins. In this study, the adhesion and dissociation between epoxy resin and molecules containing Si atoms were analyzed. The analysis, conducted in contact with the solid surface of silicon, utilized quantum chemical calculations based on a molecular model. We compared some Si-containing molecular models to test quantum chemical calculations that contribute to the study of adhesion and dissociation between epoxy resins and solid surfaces somehow other than simple potential energy curve calculations. The AFIR (artificial force induced reaction) method, implemented in the GRRM (global reaction route mapping) program, was employed to separate an epoxy resin model molecule and three types of silicon compounds (Si(CH3)2(OH)2, Si(CH3)4, and (CH3)2SiF2) in three directions, determining their minimum dissociation energy when changing the applied energy by 2.5 kJ/mol. In systems with weak hydrogen bonds, such as Si(CH3)4 or (CH3)2SiF2, the energy required for dissociation was not large; however, in systems with strong hydrogen bonds, such as Si(CH3)2(OH)2, dissociation was more difficult in the vertical direction. Although anisotropy due to hydroxyl groups was calculated in the horizontal direction, dissociation remained relatively easy.

Ultra‐Thin Strain‐Relieving Si1−xGex Layers Enabling III‐V Epitaxy on Si

AbstractThe explosion of artificial intelligence, the possible end of Moore's law, dawn of quantum computing, and the continued exponential growth of data communications traffic have brought new urgency to the need for laser integration on the diversified Si platform. While diode lasers on group III‐V platforms have long‐powered internet data communications and other optoelectronic technologies, direct integration with Si remains problematic. A paradigm‐shifting solution requires exploring new and unconventional materials and integration approaches. In this work, it is shown that a sub‐10‐nm ultra‐thin Si1−xGex buffer layer fabricated by an oxidative solid‐phase epitaxy process can facilitate extraordinarily efficient strain relaxation. The Si1−xGex layer is formed by ion implanting Ge into Si(111) and selectively oxidizing Si atoms in the resulting ion‐damaged layer, precipitating a fully strain‐relaxed Ge‐rich layer between the Si substrate and surface oxide. The efficient strain relaxation results from the high oxidation temperature, producing a periodic network of dislocations at the substrate interface, coinciding with modulations of the Ge content in the Si1−xGex layer and indicating the presence of defect‐mediated diffusion of Si through the layer. The epitaxial growth of high‐quality GaAs is demonstrated on this ultra‐thin Si1−xGex layer, demonstrating a promising new pathway for integrating III‐V lasers directly on the Si platform.

Annulated 1,4-Disilabenzene-1,4-diide and Dihydrogen Splitting.

The isolation of silicon analogues of phenyl anions such as (C6H5)- and (C6H4)2- is challenging owing to their extremely high reactivity associated with their silylene character and weak C-Si π-interaction. Herein, we report the first annulated 1,4-disilabenzene-1,4-diide compound [(ADC)Si]2 (5) based on anionic dicarbene (ADC) scaffolds (ADC = PhC{N(Dipp)C}2; Dipp = 2,6-iPr2C6H3) as a green-yellow crystalline solid. Compound 5 is prepared by KC8 reduction of the Si(IV) chloride [(ADC)SiCl3]2 (3) or the cyclic bis-chlorosilylene [(ADC)SiCl]2 (4), which are also prepared for the first time. 5 is a neutral molecule, and each of the two-coordinated Si(I) atoms has a lone pair and an unpaired electron. Experimental and theoretical data indicate delocalization of the silicon unpaired electrons, resulting in a 6π-electron C4Si2 ring in 5. The diradical character (y) for 5 amounts to 15%. At room temperature, 5 readily reacts with dihydrogen (H2) to form the elusive bis-hydridosilylenes [(ADC)SiH]2 (Z)-6 and (E)-6. The [4 + 2]-cycloaddition of 5 and PhC≡CPh in yielding the barrelene-type bis-silylene [(ADC)SiCPh]2 (7) emphasizes the diradical reactivity of 5. With elemental sulfur, 5 results in the S2- and S3-bridged silathione derivatives [(ADC)Si(S)]2(μ-S2) (8a) and [(ADC)Si(S)]2(μ-S3) (8b). Moreover, the treatment of 5 with Fe2(CO)9 affords the Fe(0) complex [(ADC)Si(Fe(CO)4)]2(μ-CO) (9), in which each silicon atom serves as a two-electron σ-donor ligand and shares one electron with the bridging CO unit to form two Si-C bonds. The molecular structures of all compounds have been established by X-ray diffraction, and representative compounds have been analyzed by quantum chemical calculations.

Recent perspective on polymeric Semimetal (Si, Ge and As) and nonmetal (N and P) doped C70-Fullerene system: Comparative electronic, dynamic behavior and chemotherapy docking with ADMET analysis

Fullerene system, based on carbon atoms, was studied and its structural modification was built with consideration of several elements such as Si, N, P, Ge, and As, where doped modified systems were constructed. Synthetic routes survey was considered to acquire the structures' availability in further applications related to structure properties investigation. The computational investigation of these designed systems was described using the most popular approach of density functional theory (DFT). Electronic behavior for all systems was studied especially through density of states (DOS) spectra, molecular electrostatic potential (MEP), and frontier molecular orbitals (FMOs) for best comparison towards stability with less energetic properties, where C-fullerene was predicted as a more stable candidate as its ∆E with value of 1.714 eV. Molecular dynamic simulation (MD) was considered to predict the stability of the atoms in the system during the physical movement at 100 ps. A polymeric model of triple Fullerene rings was linked for each Semimetal atom (Si, Ge, and As) to predict the radial distribution function (RDF) and dynamic behavior for stability conditions. The calculated energy parameters such as potential, kinetic, and non-bond energies mainly described the system movement stability during the simulation time. Molecular docking analysis of the modified doped systems was performed for chemotherapy prediction of the studied systems against breast cancer target protein (5NWH) to evaluate the inhibition strength through active sites binding affinity. The estimated binding affinity of Si-fullerene was found the most favorable result (-12.73 kcal/mol). ADMET properties were estimated for further drug-like prediction through comparative pharmacokinetic factors.

Study of the physical nature of Mn4Si7 crystals formed by the diffusion method using an X-ray diffraction

Mn4Si7 silicide crystals obtained by the diffusion method were studied using an X-ray diffractometer (XRD-6100) SHIMADZU. As a result of research, 14 peaks were identified in the Mn4Si7 crystal, corresponding to the database (COD-1530134).The size of Mn4Si7 silicide crystals (DDiff) ranged from 6.2 × 10−10 m to 9.1 × 10−8 m, the lattice tension between crystal atoms (εDiff) from 0.31 to 3.71, the dislocation density on the surface (δDiff) varied in the range from 1 × 1011 to 3.2 × 1014. It was found that the degree of crystallization of Mn4Si7 was 9.3 %, and the degree of amorphism reached 90.7 %. It has been established that the degree of crystallization of Mn4Si7 silicides is relatively low due to the fact that the Mn and Si atoms are non-stoichiometrically bonded to each other, and the degree of amorphism is high.

Bi-intercalated epitaxial graphene on SiC(0001)

Abstract The intercalation of graphene with suitable atomic species is one of the most frequently applied methods to decouple the graphene layer from the substrate in order to establish the classical electronic properties of graphene. In this context, we studied the bismuth (Bi) intercalation of the (6√3×6√3)R30° reconstructed so-called "zeroth layer graphene'' on SiC(0001). 
As reported earlier by Sohn et al. [J. Korean Phys. Soc. 78, 157 (2021)], two phases are formed depending on the amount of intercalated Bi, which in turn is controlled by the annealing temperature: The α phase, showing a 1×1 periodicity with respect to the substrate, and, at higher temperatures, the √3×√3 reconstructed β phase. We characterise both phases and the transformation from the α to the β phase by photoelectron spectroscopy, normal incidence x-ray standing waves, electron diffraction and electron microscopy. We clearly see an almost complete intercalation of the graphene layers in both phases, with strong (covalent) interaction between the topmost Si atoms of the substrate and the Bi intercalant, but only weak (van der Waals) interaction between Bi and the graphene layer. The n-doping of the graphene found for the α phase decreases continuously during the phase transformation, in agreement with a reduced density of the Bi intercalating layer. Missing core level shifts of the surface species as well as the normal incidence x-ray standing waves results indicate that all surface Si atoms remain saturated during the transition and no dangling bonds are formed. Low energy electron microscopy and diffraction reveal the coexistance of both phases after annealing to intermediate temperatures and allow a quantitative analysis of island sizes and numbers.

New 5-6-6-5 (fourfold) and 5-9-6 defect Configurations in g-SiC (graphene-like hexagonal monolayer silicon carbide)

Abstract Computational studies using the density functional theory (DFT) were employed to analyze defect configurations in g-SiC. The SiC supercells containing 72 atoms were used for simulation. We simulate C-vacancy (VC) and Si-vacancy (VSi). We relaxed all atoms so that the atomic force tolerance is 1.0 × 10−4 Ha/Bohr. The unrelaxed defective configurations have the same symmetry as the perfect configuration, which is a D3h symmetry. During relaxation, atoms neighboring the vacancy were displaced to reach a ground state condition. In the case of VC, a Si atom at the center of the defect has four bonds, resulting in a new 5-6-6-5 fourfold configuration with a C2v symmetry. In the case of VSi, a C atom forms bonds with two other C atoms, resulting in a new configuration, namely a 5-9-6 configuration. We identified that this configuration also has a C2v symmetry. Thus, symmetry breaking (lowering) occurs from D3h to C2v. We calculated the formation energy, which is 3.28 eV for the 5-6-6-5 fourfold and 3.92 eV for the 5-9-6 configuration. We also calculated the Density of States (DOS), and the results show that both configurations have semiconductor material properties suitable for promising optoelectronic devices with an infrared spectrum for future applications.

Characterization of L12-Al3Ce phase and its purification mechanism in the Al-Ce-TiCN alloy

Rare earth element Ce has a positive purifying effect on Fe and Si impurities in pure aluminum. In this study, we unveiled the mechanism of Ce purification of Fe and Si impurities in commercial pure aluminum at the atomic scale via utilizing the properties of TiCN nanoparticles, which exhibit distribution along grain boundaries and impede solute atom diffusion at specific cooling rates. The transition phase L12-Al3Ce was characterized in aluminum, which is considered to be an important transition phase to purified products. Furthermore, High Angle Dark Field Scanning Transmission Electron Microscopy (HADDF-STEM) and 3D Atom Probe Tomography (3D-APT) results showed that Ce could respectively enrich the Fe and Si impurities, resulting in the formation of Al-Ce-Fe and Al-Ce-Si clusters. Density functional theory (DFT) results indicated that Fe and Si atoms can incorporate into L12-Al3Ce crystal, forming more stable structures and therefore giving rise to the formation of Al-Ce-Si and Al-Ce-Fe nanoclusters. This study provides atomic-scale insights into the mechanism of Ce purifying Fe and Si impurities in aluminum.

The evolution of micro-photoluminescence spectra of PECVD diamond microcrystals along the vertical growth direction and their dependence on CH4 concentration

The changes in the shape of micro-photoluminescence spectra of PECVD diamond micro-crystal measured, depending on the position of the excitation laser spot along the crystallite height, were analyzed. It was ascertained that the processes of SiV defect formation non-monotonically depend on the distance from the Si substrate. At the distances of 2–20 μm the concentration of SiV defects increases, then at distances larger than 20 μm the number of SiV defects decreases. The concentration of NV− and NV0 defects monotonically increases with the distance from the Si substrate. The predomination of SiV defect formation at the beginning stages of the crystal growth is explained by the substantial concentration of carbon vacancies required for their formation. With the increase in the distance from the substrate, the crystalline perfection increases, the concentration of carbon vacancies decreases and the processes of NV− and NV0 defect formation dominate. The increase in CH4 fraction within 0.75–6 % leads to the increase in the volume fraction of graphite-like carbon, which is the good diffusion channel for Si atoms from the substrate into the plasma. Therefore, the concentration of SiV, NV−, and NV0 defects on the surface of the crystal depends on the volume fraction of graphite-like carbon defined by CH4 content.

Dynamics of charged and neutral silicon-vacancy color centers in electron-irradiated 28Si-doped single crystal chemical vapor deposition diamond upon annealing: Optical spectroscopy study

Diamond synthesis with silicon-vacancy (SiV) color centers is of high interest to nanophotonics and optical quantum technologies. The methods to control and/or maximize concentration of Si atoms incorporated in diamond lattice, and coupled to vacancies, together with improving crystallinity of diamond structure, are in demand. Here, we studied the effect of heat treatment of electron-irradiated single crystal CVD diamond doped with 28S isotope, on evolution of the neutral SiV0 and negatively charged SiV− centers. The crystal subjected to stepwise annealing at temperatures Tann from 200 °C to 1640 °C has been analyzed with photoluminescence (PL) and optical absorption spectroscopies at room and low-temperature (T = 5 K). We found a complex non-monotonous behavior of SiV0 and SiV− intensity both in absorption and PL, with sharp rise at Tann > 600 °C, reaching a maximum concentration peak, decline at 850 °C, and a repeated elevation to higher Tann, reflecting creation and annihilation processes of these defects. Moreover, we detected a group of seven lines of a Si-related defect in the range 828–871 nm, which strongly correlate with annealing dynamics of the SiV− and, especially, of SiV0 centers, and estimated the isotope spectral shift. In parallel, dynamics of other optical centers such as NV, R11 and GR1, in course of the annealing is traced. The controlled annealing opens the way to preparation of the SiV color centers with optimized optical properties, promising for quantum technologies.

Microstructure evolution and oxidation resistance of plasma sprayed AlSi-doped AlCoCrFeNi coatings with post-annealing

High entropy alloy (HEA) coatings of equimolar AlCoCrFeNi typically exhibit a lower oxidation rate at high temperatures by forming a protective passivation film. However, the metal elements consumption during long-term oxidation limitted the application. In this work, AlCoCrFeNi HEA coatings doped by AlSi as a supplement to passivation elements were prepared by atmospheric plasma spraying (APS), and AlSi capsules were diffused uniformly into the coating through annealing treatment to offset the element consumption during high-temperature oxidation. Results showed that annealing promoted Al and Si atoms diffusing into the solid solution, which stabilized BCC and inhibited FCC formation. During the oxidation at 900 °C, a protective Al2O3 film was formed on the coating surface, and AlSi capsules continuously transported Al ions to the consumption zone and reduced oxidation rate to 0.0015 g/cm2. The HEA coating doped by passivation element capsules provided a new approach for the design of novel antioxidant coatings.

Silicon Phthalocyanines Functionalized with Axial Substituents Targeting PSMA: Synthesis and Preliminary Assessment of Their Potential for PhotoDynamic Therapy of Prostate Cancer.

Photodynamic therapy (PDT) is a clinical modality based on the irradiation of different diseases, mostly tumours, with light following the selective uptake of a photosensitiser by the pathological tissue. In this study, two new silicon(IV)phtalocyanines (SiPcs) functionalized at both axial positions with a PSMA inhibitor are reported as candidate photosensitizers for PDT of prostate cancer, namely compounds SiPc-PQ(PSMAi)2 and SiPc-OSi(PSMAi)2. These compounds share the same PSMA-binding motif, but differ in the linker that connects the inhibitor moiety to the Si(IV) atom: an alkoxy (Si-O-C) bond for SiPc-PQ(PSMAi)2, and a silyloxy (Si-O-Si) bond for SiPc-OSi(PSMAi)2. Both compounds were synthesized by a facile synthetic route and fully characterized by 2D NMR, mass spectrometry and absorption/fluorescence spectrophotometry. The PDT agents showed a suitable solubility in water, where they essentially exist in monomeric form. SiPc-PQ(PSMAi)2 showed a higher singlet oxygen quantum yield ΦΔ, higher fluorescence quantum yields ΦF and better photostability than SiPc-OSi(PSMAi)2. Both compounds were efficiently taken up by PSMA(+) PC3-PIP cells, but not by PSMA(-) PC3-FLU cells. However, SiPc-PQ(PSMAi)2 showed a more specific photoinduced cytotoxicity in vitro, which is likely attributable to a better stability of its water solutions.

Unconventional Insertions of Internal Alkynes into a Mixed-Valent Silaiminyl-Silylene.

The reactivity of a mixed-valent silaiminyl-silylene [LSi-Si(NDipp)L] (L = PhC(NtBu)2, Dipp = 2,6-iPr-C6H3) toward various substituted internal alkynes was investigated. In contrast to previous reports that primarily yield [Si(μ-C2)Si]-modified rings via 1,2-addition of two silylenes in the center of the molecule, our study reveals a novel reaction pathway. The introduction of [R1-C≡C-R2] (R1 = Ph or SiMe3, R2 = Ph or C≡CSiMe3) gave unconventional insertion into one of the amidinate ligands, followed by migration of the {NtBu} group to bridge two Si atoms. This results in the formation of diverse expanded silacycles.

Thermal stability of MDM and oxidative decomposition mechanism under the condition of air infiltration: A combined experimental, ReaxFF-MD and DFT study

When siloxanes are used as the working fluids of organic Rankine cycle systems, there is a possibility of air infiltration due to the large vacuum present in the condenser. To address this issue, experimental and simulation studies were conducted on the thermal stability and pyrolysis product characteristics of octamethyltrisiloxane (MDM) in the presence of air. The results indicated that air significantly promotes the decomposition of MDM at the temperatures above 250 °C. Air was helpful in increasing the formation of CH2O, CO, and CO2, and also enhanced polymerization of decomposition product of siloxane, leading to higher yields of MD2M, MD3M, MD4M, D3, D4, and D5. By combining reactive force field molecular dynamics (ReaxFF-MD) simulations with density functional theory (DFT) calculations, the microscopic mechanisms of MDM's oxidative decomposition by O2 in air and H2O under humid conditions were elucidated. O2 can easily combine with unsaturated Si atoms to form Si-O bonds, combine with CH3 radicals to form C-O bonds, and undergoes hydrogen extraction reactions to initiate decomposition. H2O mainly undergoes initial decomposition through CH3 radical hydrogenation and binding with unsaturated Si atoms. The generated O2H, O, and OH radicals tend to bind with Si atoms in molecules and radicals, facilitating polymerization reactions.

Effect of О<sub>2</sub><sup>+</sup> Ion Implantation on the Elemental and Chemical Composition of the Si(111) Surface

Using the methods of secondary ion mass spectrometry, elastic peak electron spectroscopy and Auger electron spectroscopy, the elemental and chemical composition of the surface, concentration profiles of the distribution of atoms over the depth of silicon implanted with O2+ ions with energy E0 = 1 keV at a dose of D = 6 × 1016 cm–2 were studied. It was found that oxides and suboxides of Si (SiO2, Si2O and SiO0.5) were formed in the ion-doped layer, and it also contained unbound O and Si atoms. Post-implantation annealing at 850–900 K led to the formation of a stoichiometric SiO2 layer ~25–30 Å thick.

Zeolites as Solid Solvents: Explaining the Chloromethane Hydrolysis over Metal-Exchanged Zeolite Y by DFT Calculations.

We report a theoretical DFT study of the reaction pathways for chloromethane hydrolysis over metal-exchanged zeolites (Li+, Na+, K+, and Mg2+). A cluster of 78 T atoms (T referring to Si or Al atoms), comprising the zeolite Y super cavity coupled with the sodalite cage and three hexagonal prism units (Si(78-x)Al x O129H54; x = 1 or 2), was used in the calculations. The study was carried out using the ONIOM method. The high layer was computed at M062X/6-31++G(d,p), whereas the low layer was computed at the PM6 level of theory. The energy profile was obtained by single-point calculations at the M062X/6-31++G(d,p) for the entire structure. The first step studied was the adsorption of chloromethane on the zeolite, which involves an ion-dipole interaction between the metal cation and the chlorine atom. Then, two mechanistic pathways were investigated: one via formation of an adsorbed methoxide and the other involving a direct, one-step, nucleophilic attack of the water molecule to the adsorbed chloromethane. In all systems studied, the direct mechanism showed a lower energy of activation than the route involving the methoxide intermediate. The same behavior was also observed for chloromethane methanolysis to afford dimethyl ether on the metal-exchanged zeolites. The theoretical results are in agreement with the experiments of chloromethane hydrolysis over metal-exchanged zeolite Y, explaining the formation of dimethyl ether upon chloromethane methanolysis on zeolites of low or no acidity. The transition state for the direct route resembles a SN2 process assisted by the surface, reinforcing the concept of zeolites as solid solvents, providing a polar nanoenvironment that favors and assists the formation of ionic transition states and intermediates.

Multisite-assisted design of asymmetric silicon-bridged diphosphine ligands for selective ethylene tri-/tetramerization

A series of novel silicon-bridged asymmetric diphosphine ligands (PNSiP-type L1-L3 and PCSiP-type L4) and their corresponding Cr(Ⅲ) catalysts have been synthesized and evaluated for ethylene tri-/tetramerization. When DMAO/AlEt3 was used as a co-catalyst, catalyst 1 based on ligand L1, with a cyclopentyl group on the N atom and two methyl groups on the Si atom, showed the highest catalytic activity of 8.21 kg g-1 Cr/h with 90.45 % combined (60.53 % 1-C6= + 29.92 % 1-C8=) selectivity. Moreover, the catalytic system based on catalyst 1 achieved a remarkable selectivity for 1-C8= of 62.58 % at 15 °C, even though polymers were the main products. Catalyst 2 based on ligand L2, bearing a cyclopentyl group at N and one methyl and one phenyl group on the Si moiety, exhibited a catalytic activity of 8.09 kg g-1 Cr/h accompanied with 71.61 % 1-C6= and 14.12 % 1-C8=product selectivity. Catalyst 3 based on ligand L3 having an isopropyl group on the N atom and two methyl groups on the Si atom, displayed 6.95 kg g-1 Cr/h catalytic activity with 69.06 % 1-C6= and 17.88 % 1-C8=product selectivity. Catalyst 4 based on PCSiP-type ligand L4, showed poor catalytic activity and 79.15 % of high 1-C6= selectivity. Compared with the previously reported long-chain Cr(Ⅲ) catalysts based on silicon-bridged diphosphine ligands (PNSiCP-/PCSiCP-type), Cr(Ⅲ) catalysts based on short-chain PNSiP-type and PCSiP-type ligands designed in this work exhibited higher ethylene oligomerization performance. Among them, PNSiP-type ligands are more inclined to generate 1-C8= than PCSiP-type ligand, and reducing the steric bulk of substituents on the Si atom in PNSiP-type ligands is also beneficial for 1-C8= generation. Density functional theory (DFT) calculations also revealed that the PNSiP-type catalysts could exhibit excellent ethylene trimerization performance. The DFT calculations thus provide a theoretical foundation for a deeper understanding of the ethylene tri-/tetramerization mechanism and designing of new ligand structures.

A deep insight of re-entrant martensitic transformation mechanism in Co2Cr(Ga,Si) shape memory alloys

Co2Cr(Ga,Si) shape memory alloys have a wide application temperature range due to the unique re-entrant martensite phase transformation (RMT) behavior. Nevertheless, the microscopic mechanism of RMT remains elusive and unsystematic. The heart of this investigation lies the comprehensive exploration of phase stability, phase transformation path, and martensite slip direction during RMT from martensite (D022-PM) to austenite (L21-FM). In this work, we systematically investigate the re-entrant martensitic transformation of Co2Cr(Ga,Si) SMAs using first-principles methods for the first time. Firstly, the density of states (DOS) calculation indicates that the stability of L21-FM is higher than D022-PM. The charge density calculation further indicates that the bonding strength between Co atoms and Cr (Si) atoms in the L21-FM phase is the highest. In addition, Fermi surface calculation further reveals that phase instability is due to the Fermi surface nesting caused by electron-phonon coupling. Moreover, the minimum energy path was calculated by the G-SSNEB method for the first time, which indicates the RMT can occur. Finally, the calculation of the elastic constants indicates that RMT is caused by the crystal plane slip of the D022-PM phase along the <110> direction. Our calculations provide the electronic-level and thermodynamic mechanism for RMT of Co2Cr(Ga,Si) alloy and shed some light on the revelation of the phase transformation mechanism.