Order–disorder competition in equiatomic 3d–transition–metal quaternary alloys: phase stability and electronic structure

We use high-throughput first-principles sampling to investigate competitive factors that determine the crystal structure of high-entropy alloys (HEAs) and the energetics dependence of the stable phase on the atomic configuration of ‘semi-ordered’ L12, D022, and random solid solution (RSS) phases of equiatomic quaternary alloys comprising four of the six constituent elements (Cr, Mn, Fe, Co, Ni, and Cu). Note that, generally, an FCC lattice consists of four L12/D022 sublattices. In this study, we call ‘semi-ordered’ phase a FCC lattice where one of the L12/D022 sublattices is fully occupied by a certain element, whereas the others are randomly occupied by the other elements like RSS. Considering the configurational entropy, we demonstrate that valence electron concentration (VEC) and temperature are crucial to determine the phase stability of HEAs at finite temperatures, wherein the ‘rather enthalpy-driven’ ordered phases are energetically more favorable than ‘rather entropy-driven’ RSS phases. Some D022 phases with high VEC are energetically more stable than L12 phases, though both phases are metastable. Furthermore, we explore magnetic configurations to identify the origin of the enthalpy term. The calculations reveal that ordered phases comprising antiferromagnetic atoms surrounded by ferromagnetic atoms are energetically stable. Relationships between magnetic ordering and atomic arrangements are also discussed.</p

Mechanistic Insight into the Chemical Exfoliation and Functionalization of Ti₃C₂ MXene

MXene, a two-dimensional layer of transition metal carbides/nitrides, showed great promise for energy storage, sensing, and electronic applications. MXene are chemically exfoliated from the bulk MAX phase; however, mechanistic understanding of exfoliation and subsequent functionalization of these technologically important materials is still lacking. Here, using density-functional theory we show that exfoliation of Ti₃C₂ MXene proceeds via HF insertion through edges of Ti₃AlC₂ MAX phase. Spontaneous dissociation of HF and subsequent termination of edge Ti atoms by H/F weakens Al–MXene bonds. Consequent opening of the interlayer gap allows further insertion of HF that leads to the formation of AlF₃ and H₂, which eventually come out of the MAX, leaving fluorinated MXene behind. Density of state and electron localization function shows robust binding between F/OH and Ti, which makes it very difficult to obtain controlled functionalized or pristine MXene. Analysis of the calculated Gibbs free energy (Δ<i>G</i>) shows fully fluorinated MXene to be lowest in energy, whereas the formation of pristine MXene is thermodynamically least favorable. In the presence of water, mixed functionalized Ti₃C₂F_<i>x</i>(OH)_1–<i>x</i> (<i>x</i> ranges from 0 to 1) MXene can be obtained. The Δ<i>G</i> values for the mixed functionalized MXenes are very close in energy, indicating the random and nonuniform functionalization of MXene. The microscopic understanding gained here unveils the challenges in exfoliation and controlling the functionalization of MXene, which is essential for its practical application

Engineering of Band Gap in Metal–Organic Frameworks by Functionalizing Organic Linker: A Systematic Density Functional Theory Investigation

A systematic investigation on electronic band structure of a series of isoreticular metal–organic frameworks (IRMOFs) using density functional theory has been carried out. Our results show that halogen atoms can be used as functional groups to tune not only the band gap but also the valence band maximum (VBM) in MOFs. Among halogen atoms (F, Cl, Br, I), iodine is the best candidate to reduce the band gap and increase the VBM value. In addition, it has been found that for the antiaromatic linker DHPDC (1,4-dihydropentalene-2,5-dicarboxylic acid) the energy gap is 0.95 eV, which is even lower than those calculated for other aromatic linkers, i.e., FFDC (furo­[3,2-b]­furan-2,5-dicarboxylic acid<b>)</b> and TTDC (thieno­[3,2-b]­thiophene-2,5-dicarboxylic acid). By analyzing the lowest unoccupied molecular orbital–highest occupied molecular orbital gaps calculated at the molecular level, we have highlighted the important role of the corresponding organic linkers in the MOF band gap. In particular, the change of C–C–CO dihedral angle in the organic linker can be used to analyze the difference of band gaps in MOF crystals. It is shown that a deep understanding of chemical bonding within linker molecules from electronic structure calculations plays a crucial role in designing semiconductor properties of MOF materials for engineering applications

Tuning the Electronic and Magnetic Properties of Phosphorene by Vacancies and Adatoms

We report a density functional theory (DFT) study regarding the effects of atomic defects, such as vacancies and adatom adsorption, on the electronic and magnetic properties of phosphorene (a two-dimensional monolayer of black phosphorus). A monovacancy in the phosphorene creates an in-gap state in the band gap of pristine phosphorene and induces a magnetic moment, even though pristine phosphorene is nonmagnetic. In contrast, both planar and staggered divacancies do not change the magnetic properties of phosphorene, although a staggered divacancy creates states in the gap. Our DFT calculations also show that adsorption of nonmetallic elements (C, N, and O) and transition metal elements (Fe, Co, and Ni) can change the magnetic properties of phosphorene with or without vacancies. For example, the nonmagnetic pristine phosphorene becomes magnetic after the adsorption of N, Fe, or Co adatoms, and the magnetic phosphorene with a monovacancy becomes nonmagnetic after the adsorption of C, N, or Co atoms. We also demonstrate that for O- or Fe-adsorbed monovacancy structure the electronic and magnetic properties are tunable via the control of charge on the phosphorene system. These results provide insight for achieving metal-free magnetism and a tunable band gap for various electronic and spintronic devices based on phosphorene

Atomistic Origin of Phase Stability in Oxygen-Functionalized MXene: A Comparative Study

Oxygen-functionalized MXene, M₂CO₂ (M = group III–V metals), are emergent formidable two-dimensional (2D) materials with a tantalizing possibility for device applications. Using first-principles calculations, we perform an intensive study on the stability of fully O-functionalized (M₂CO₂) MXenes. Depending on the position of O atoms, the M₂CO₂ can exist in two different structural phases. On one side of MXene, the O atom occupies a site which is exactly on the top of the metal atom from the opposite side. On the other side, the O atom can occupy either the site on the top of the metal atom of the opposite side (BB′ phase) or on the top of the C atom (CB phase). We find that for M = Sc and Y the CB phase is stable, whereas for M = Ti, Zr, Hf, V, Nb, and Ta the stable phase is BB′. The electron localization function, the atom-projected density of states, the charge transfer, and the Bader charge analyses provide a rational explanation for the relative stability of these two phases and justify the ground state structure by giving information about the preferential site of adsorption for the O atoms. We also calculate the phonon dispersion relations for both phases of M₂CO₂. The BB′-Sc₂CO₂ and the CB-Ti₂CO₂ are found to be dynamically unstable. Finally, we find that the instability of BB′-M₂CO₂ (M = Sc and Y) originates from the weakening of M–C interactions, which manifest as a phonon mode with imaginary frequency corresponding to the motion of C atom in the <i>a</i>–<i>b</i> plane. The insight into the stability of these competing structural phases of M₂CO₂ presented in this study is an important step in the direction of identifying the stable phases of these 2D materials

Stability and Composition of Helium Hydrates Based on Ices I_h and II at Low Temperatures

The recently developed approach describing host lattice relaxation, guest–guest interactions and the quantum nature of guest behavior (Belosudov, R. V.; Subbotin, O. S.; Mizuseki, H.; Kawazoe, Y.; Belosludov, V. R. J. Chem. Phys. 2009, 131, 244510) has been used to derive the thermodynamic properties of helium hydrates based on ices I_h and II. The<i> p</i>–<i>T</i> phase diagrams of the helium hydrates in different ices are presented for a wide range of pressures and temperatures, and the structural transitions between pure ice I_h and ice II as well as between ice I_h-based helium hydrate and ice II-based helium hydrate have been found to be in agreement with the available experimental data. The “ice II-based helium hydrate–ice I_h-based helium hydrate” equilibrium shifts toward the higher pressures in comparison with the line of “ice II–ice I_h” equilibrium. The degrees of interstitial space filling by helium in ice I_h-based and ice II-based hydrates decrease with increasing temperature and lowering of pressure. It is demonstrated that the helium filling in ice I_h proceeds more slowly than in ice II. However, the mole fraction of helium in the hydrate based on ice I_h is significantly higher than that in the ice II-based hydrate. We predict that during the phase transition from the ice I_h-based hydrate to the ice II-based one a discharge of gaseous helium should be observed. This may serve as an indicator of the phase transition in experiment