First-principles study on structural stabilities, mechanical properties, and biaxial strain-induced superconductivity in Janus MoWC monolayer.

The unique attributes of hydrophilicity, expansive surface groups, remarkable flexibility, and superior conductivity converge in MXene, a pioneering 2D material. Owing to MXene's exceptional properties, diverse strategies have been explored to enhance its characteristics. Janus MXene and stress-strain response considerations represent the primary avenues of interest today. In this study, we investigated the Janus MXene structure under biaxial stress using first-principles calculations. The most stable configuration of Janus MoWC MXene identified in our analysis exhibits an atomic arrangement known as the hexagonal (2H) phase. Subsequently, we examined the mechanical and electronic properties of 2H-MoWC when subjected to biaxial strain. Our findings indicate that the 2H phase of Janus MoWC MXene demonstrates superior strength compared to the tetragonal (1T) phase. Analysis of the ELF of the 2H-MoWC structure unveiled that the robust C-C bond within the material is the underlying factor enabling the 2H phase to withstand a maximum of 9% tensile strain. Furthermore, we demonstrate that 2H-MoWC is a superconductor with the superconducting temperature (Tc) of 1.6 K, and the superconductivity of 2H phase can be enhanced by biaxial strain with the Tc reaching 7 K. This study offers comprehensive insights into the properties of Janus MoWC monolayer under biaxial stress, positioning it as a promising candidate for 2D straintronic applications.

First-principles study of 2H-Mo2C-based MXenes under biaxial strain as Li-battery anodes.

MXenes, a family of superior 2D materials, have been intensively investigated because they have many promising properties, particularly high-performance energy storage and high flexibility. To approach the expected critical benchmarks of such materials, the strain dependence of the atomic structure is widely considered for tuning the related properties. In this work, by means of density functional theory, we demonstrate the potential application of the strained 2H phase of Mo2C-based MXenes (Mo2C and Mo2CO2) as anode materials for lithium-ion batteries (LIBs). Adsorption and diffusion of Li on the surfaces of both materials and the impact of biaxial strain (εb) in the range of -4% to 4% are insightfully investigated. The lowest adsorption energy of Mo2C is -0.96 eV, and that of Mo2CO2 is -3.13 eV at εb = 0%. The diffusion of Li ions, considering the pathway between the first two most favorable adsorption sites, reveals that the biaxial strain refinement under compressive strain decreases the energy barrier, but the induction of tensile strain increases it in both MXenes. The ranges of the energy barriers of Li-ion adsorption on the surfaces of Mo2C and Mo2CO2 are 31-57 meV and 177-229 meV, respectively. Interestingly, the storage capacity of Li can reach three layers corresponding to a comparably high theoretical capacity of 788.61 mA h g-1 for Mo2C and 681.64 mA h g-1 for Mo2CO2. The atomic configurations are stable, as verified by the negative adsorption energy as well as the slightly distorted structures, by using ab initio molecular dynamics (AIMD) simulations at 400 K. Moreover, average open circuit voltages (OCVs) of 0.35 V and 0.63 V (at εb = 0%) are reported for Mo2C and Mo2CO2, respectively. Furthermore, the tensile strain results in an increase in the OCVs, while compression has the opposite effect. These computational results provide some basic information on the behaviors of Li-ion adsorption and diffusion on Mo2C-based MXenes upon tuning biaxial strain. They also give a guideline on what conditions are appropriate for practically implementing these MXenes as electrode materials in LIBs.

Biaxial stress and functional groups (T = O, F, and Cl) tuning the structural, mechanical, and electronic properties of monolayer molybdenum carbide.

MXenes are a family of novel two-dimensional (2D) materials attracting intensive interest because of the rich chemistry rooted from the highly diversified surface functional groups. This enables the chemical optimization suitable for versatile applications, including energy conversion and storage, sensors, and catalysis. This work reports the ab initio study of the crystal energetics, electronic properties, and mechanical properties, and the impacts of strain on the electronic properties of tetragonal (1T) and hexagonal (2H) phases of Mo2C as well as the surface-terminated Mo2CT2 (T = O, F, and Cl). Our findings indicate that 2H-Mo2C is energetically more stabilized than the 1T counterpart, and the 1T-to-2H transition requires a substantial energy of 210 meV per atom. The presence of surface termination T atoms on Mo2C intrinsically induces variations in the atomic structure. The calculated structures were selected based on the energetic and thermodynamic stabilities (400 K). The O atom prefers to be terminated on 2H-Mo2C, whereas the Cl atom energetically stabilizes on 1T-Mo2C. Meanwhile, with certain configurations, 2H-Mo2CF2 and 1T-Mo2CF2 with slightly different energies could exist simultaneously. The Mo2CO2 possesses the highest mechanical strength and elastic modulus (σmax = 52 GPa at εb = 20% and E = 507 GPa). The nature of the ordered centrosymmetric layer and the strong bonding between 4 d-Mo and 2 p-O of 2H-Mo2CO2 are responsible for its promising mechanical properties. Interestingly, the topological properties of 2H-Mo2CO2 at a wide range of strains (-10% to 12%) are reported. Moreover, 2H-Mo2CF2 is metallic through the range of calculation. Meanwhile, originally semiconducting 1T-Mo2CF2 and 1T-Mo2CCl2 preserve their features under the ranges of the strain of -2% to 10% and -1% to 5%, respectively, beyond which they undergo the semiconductor-to-metal transitions. These findings would guide the potential applications in modern 2D straintronic devices.

Theoretical aspects in structural distortion and the electronic properties of lithium peroxide under high pressure.

The structural phase transition and electronic properties of Li2O2 under pressures up to 500 GPa have been investigated using first-principles calculations. Two new structural phase transitions have been proposed at pressures around 75 GPa from the P63/mmc structure to the P21 structure and around 136 GPa from the P21 structure to the P21/c structure. The calculated phonon spectra have confirmed the dynamical stability of these structures. The pressure dependence of the lattice dynamics, O-O bond length, and band gaps in Li2O2 have also been reported. The band gaps of the P63/mmc, P21, and P21/c structures calculated by PBE and HSE06 have shown increasing trends with increasing pressure. Interestingly, the P63/mmc band gap and c/a ratio have significantly decreased with the increasing O-O bond length and ELF value around 11 and 40 GPa. At these pressures, the phonon frequency of the O-O stretching modes has softened. This finding reveals the effects of structural distortion in three phases of Li2O2. Our study provides structural understanding and the electronic properties of Li2O2 under high pressure, which might be useful for investigating the charge transport through Li2O2 in lithium-air batteries and CO2 capture.

Role of relativity in high-pressure phase transitions of thallium.

We demonstrate the relativistic effects in high-pressure phase transitions of heavy element thallium. The known first phase transition from h.c.p. to f.c.c. is initially investigated by various relativistic levels and exchange-correlation functionals as implemented in FPLO method, as well as scalar relativistic scheme within PAW formalism. The electronic structure calculations are interpreted from the perspective of energetic stability and electronic density of states. The full relativistic scheme (FR) within L(S)DA performs to be the scheme that resembles mostly with experimental results with a transition pressure of 3 GPa. The s-p hybridization and the valence-core overlapping of 6s and 5d states are the primary reasons behind the f.c.c. phase occurrence. A recent proposed phase, i.e., a body-centered tetragonal (b.c.t.) phase, is confirmed with a small distortion from the f.c.c. phase. We have also predicted a reversible b.c.t. → f.c.c. phase transition at 800 GPa. This finding has been suggested that almost all the III-A elements (Ga, In and Tl) exhibit the b.c.t. → f.c.c. phase transition at extremely high pressure.

High pressure-induced distortion in face-centered cubic phase of thallium.

The complex and unusual high-pressure phase transition of III-A (i.e. Al, Ga, and In) metals have been investigated in the last several decades because of their interesting periodic table position between the elements having metallic and covalent bonding. Our present first principles-based electronic structure calculations and experimental investigation have revealed the unusual distortion in face-centered cubic (f.c.c.) phase of the heavy element thallium (Tl) induced by the high pressure. We have predicted body-centered tetragonal (b.c.t) phase at 83 GPa using an evolutionary algorithm coupled with ab initio calculations, and this prediction has been confirmed with a slightly distorted parameter ([Formula: see text] × a - c)/c lowered by 1% using an angle-dispersive X-ray diffraction technique. The density functional theory (DFT)-based calculations suggest that s-p mixing states and the valence-core overlapping of 6s and 5d states play the most important roles for the phase transitions along the pathway h.c.p[Formula: see text]f.c.c.[Formula: see text]b.c.t.

Revealing an unusual transparent phase of superhard iron tetraboride under high pressure.

First principles-based electronic structure calculations of superhard iron tetraboride (FeB4) under high pressure have been undertaken in this study. Starting with a "conventional" superconducting phase of this material under high pressure leads to an unexpected phase transition toward a semiconducting one. This transition occurred at 53.7 GPa, and this pressure acts as a demarcation between two distinct crystal symmetries, metallic orthorhombic and semiconducting tetragonal phases, with Pnnm and I41/acd space groups, respectively. In this work, the electron-phonon coupling-derived superconducting T(c) has been determined up to 60 GPa and along with optical band gap variation with increasing pressure up to 300 GPa. The dynamic stability has been confirmed by phonon dispersion calculations throughout this study.