Phase transitions and suppression of magnetoresistance in WTe2-xSexsystem.

X-ray diffraction, Raman spectroscopy, and electrical resistivity measurements on polycrystalline WTe2-xSex(0 ⩽ x ⩽ 0.8) reveal aTd-1T'structural phase transition and suppression of magnetoresistance atx = 0.2. These phenomena are consistent with the pressure phase diagram of WTe2. However, chemical pressure due to substitution of smaller Se ion cannot generate pressure required for the phase transition. Strain induced by sample inhomogeneity is believed to be a trigger to the behaviors. In agreement with previous predictions and reports, a mixed phase of1T'and 2Hstructures was also detected in Se-rich samples. Coincidentally atx = 0.2, electrical resistivity analysis suggests a phase transition from a metallic phase to a nonmetallic phase that is possibly a topological-insulating phase.

Effect of atomic configuration and spin-orbit coupling on thermodynamic stability and electronic bandgap of monolayer 2H-Mo1-xWxS2 solid solutions.

Through a combination of density functional theory calculations and cluster-expansion formalism, the effect of the configuration of the transition metal atoms and spin-orbit coupling on the thermodynamic stability and electronic bandgap of monolayer 2H-Mo1-xWxS2 is investigated. Our investigation reveals that, in spite of exhibiting a weak ordering tendency of Mo and W atoms at 0 K, monolayer 2H-Mo1-xWxS2 is thermodynamically stable as a single-phase random solid solution across the entire composition range at temperatures higher than 45 K. The spin-orbit coupling effect, induced mainly by W atoms, is found to have a minimal impact on the mixing thermodynamics of Mo and W atoms in monolayer 2H-Mo1-xWxS2; however, it significantly induces change in the electronic bandgap of the monolayer solid solution. We find that the band-gap energies of ordered and disordered solid solutions of monolayer 2H-Mo1-xWxS2 do not follow Vegard's law. In addition, the degree of the SOC-induced change in band-gap energy of monolayer 2H-Mo1-xWxS2 solid solutions not only depends on the Mo and W contents, but for a given alloy composition it is also affected by the configuration of the Mo and W atoms. This poses a challenge of fine-tuning the bandgap of monolayer 2H-Mo1-xWxS2 in practice just by varying the contents of Mo and W.

Phase stability of three-dimensional bulk and two-dimensional monolayer As1-x Sb x solid solutions from first principles.

The mixing thermodynamics of both three-dimensional bulk and two-dimensional mono-layered alloys of As1-x Sb x as a function of alloy composition and temperature are explored using a first-principles cluster-expansion method, combined with canonical Monte-Carlo simulations. We observe that, for the bulk phase, As1-x Sb x alloy can exhibit not only chemical ordering of As and Sb atoms at x = 0.5 to form an ordered compound of AsSb stable upon annealing up to [Formula: see text] K, but also a miscibility gap at 475 K [Formula: see text] T [Formula: see text] 550 K in which two disordered solid solutions of As1-x Sb x of different alloy compositions thermodynamically coexist. At T > 550 K, a single-phase solid solution of bulk As1-x Sb x is predicted to be stable across the entire composition range. These results clearly explain the existing uncertainties in the alloying behavior of bulk As1-x Sb x alloy, as previously reported in the literature, and also found to be in qualitative and quantitative agreement with the experimental observations. Interestingly, the alloying behavior of As1-x Sb x is considerably altered, as the dimensionality of the material reduces from the three-dimensional bulk state to the two-dimensional mono-layered state-for example, a single-phase solid solution of monolayer As1-x Sb x is predicted to be stable over the whole composition range at T > 250 K. This distinctly highlights an influence of the reduced dimensionality on the alloying behavior of As1-x Sb x .

Stability of SnSe1-x S x solid solutions revealed by first-principles cluster expansion.

The configurational thermodynamics of a pseudo-binary alloy SnSe1-x S x in the Pnma phase is studied using first-principles cluster-expansion method in combination with canonical Monte Carlo simulations. We find that, despite the alloy having a tendency toward a phase decomposition into SnSe and SnS at 0 K, the two constituent binaries readily mix with each other to form random SnSe1-x S x solid solutions even at a temperature below room temperature. The obtained isostructural phase diagram of SnSe1-x S x reveals that the alloy is thermodynamically stable as a single-phase random solid solution over a whole composition range above 200 K. These findings provide a fundamental understanding on the alloying behavior of SnSe1-x S x and bring clarity to the debated clustering tendency in this alloy system.

Theoretical study of phase stability, crystal and electronic structure of MeMgN2 (Me = Ti, Zr, Hf) compounds.

Scandium nitride has recently gained interest as a prospective compound for thermoelectric applications due to its high Seebeck coefficient. However, ScN also has a relatively high thermal conductivity, which limits its thermoelectric efficiency and figure of merit (zT). These properties motivate a search for other semiconductor materials that share the electronic structure features of ScN, but which have a lower thermal conductivity. Thus, the focus of our study is to predict the existence and stability of such materials among inherently layered equivalent ternaries that incorporate heavier atoms for enhanced phonon scattering and to calculate their thermoelectric properties. Using density functional theory calculations, the phase stability of TiMgN2, ZrMgN2 and HfMgN2 compounds has been calculated. From the computationally predicted phase diagrams for these materials, we conclude that all three compounds are stable in these stoichiometries. The stable compounds may have one of two competing crystal structures: a monoclinic structure (LiUN2 prototype) or a trigonal superstructure (NaCrS2 prototype; R 3 ¯ mH). The band structure for the two competing structures for each ternary is also calculated and predicts semiconducting behavior for all three compounds in the NaCrS2 crystal structure with an indirect band gap and semiconducting behavior for ZrMgN2 and HfMgN2 in the monoclinic crystal structure with a direct band gap. Seebeck coefficient and power factors are also predicted, showing that all three compounds in both the NaCrS2 and the LiUN2 structures have large Seebeck coefficients. The predicted stability of these compounds suggests that they can be synthesized by, e.g., physical vapor deposition.

Effects of configurational disorder on the elastic properties of icosahedral boron-rich alloys based on B6O, B13C2, and B4C, and their mixing thermodynamics.

The elastic properties of alloys between boron suboxide (B6O) and boron carbide (B13C2), denoted by (B6O)(1-x)(B13C2)(x), as well as boron carbide with variable carbon content, ranging from B13C2 to B4C are calculated from first-principles. Furthermore, the mixing thermodynamics of (B6O)(1-x)(B13C2)(x) is studied. A superatom-special quasirandom structure approach is used for modeling different atomic configurations, in which effects of configurational disorder between the carbide and suboxide structural units, as well as between boron and carbon atoms within the units, are taken into account. Elastic properties calculations demonstrate that configurational disorder in B13C2, where a part of the C atoms in the CBC chains substitute for B atoms in the B12 icosahedra, drastically increase the Young's and shear modulus, as compared to an atomically ordered state, B12(CBC). These calculated elastic moduli of the disordered state are in excellent agreement with experiments. Configurational disorder between boron and carbon can also explain the experimentally observed almost constant elastic moduli of boron carbide as the carbon content is changed from B4C to B13C2. The elastic moduli of the (B6O)(1-x)(B13C2)(x) system are also practically unchanged with composition if boron-carbon disorder is taken into account. By investigating the mixing thermodynamics of the alloys, in which the Gibbs free energy is determined within the mean-field approximation for the configurational entropy, we outline the pseudo-binary phase diagram of (B6O)(1-x)(B13C2)(x). The phase diagram reveals the existence of a miscibility gap at all temperatures up to the melting point. Also, the coexistence of B6O-rich as well as ordered or disordered B13C2-rich domains in the material prepared through equilibrium routes is predicted.

Phase stability of the nanolaminates V2Ga2C and (Mo1-xVx)2Ga2C from first-principles calculations.

We here use first-principles calculations to investigate the phase stability of the hypothetical laminated material V2Ga2C and the related alloy (Mo1-xVx)2Ga2C, the latter for a potential parent material for synthesis of (Mo1-xVx)2C, a new two-dimensional material in the family of so called MXenes. We predict that V2Ga2C is thermodynamically stable with respect to all identified competing phases in the ternary V-Ga-C phase diagram. We further calculate the stability of ordered and disordered configurations of Mo and V in (Mo1-xVx)2Ga2C and predict that ordered (Mo1-xVx)2Ga2C for x≤ 0.25 is stable, with an order-disorder transition temperature of ∼1000 K. Furthermore, (Mo1-xVx)2Ga2C for x = 0.5 and x≥ 0.75 is suggested to be stable, but only for disordered Mo-V configurations, and only at elevated temperatures. We have also investigated the electronic and elastic properties of V2Ga2C; the calculated bulk, shear, and Young's modulus are 141, 94, and 230 GPa, respectively.

A theoretical investigation of mixing thermodynamics, age-hardening potential, and electronic structure of ternary M(1)1-x M(2)xB2 alloys with AlB2 type structure.

Transition metal diborides are ceramic materials with potential applications as hard protective thin films and electrical contact materials. We investigate the possibility to obtain age hardening through isostructural clustering, including spinodal decomposition, or ordering-induced precipitation in ternary diboride alloys. By means of first-principles mixing thermodynamics calculations, 45 ternary M(1)1-x M(2)xB2 alloys comprising M(i)B2 (M(i) = Mg, Al, Sc, Y, Ti, Zr, Hf, V, Nb, Ta) with AlB2 type structure are studied. In particular Al1-xTixB2 is found to be of interest for coherent isostructural decomposition with a strong driving force for phase separation, while having almost concentration independent a and c lattice parameters. The results are explained by revealing the nature of the electronic structure in these alloys, and in particular, the origin of the pseudogap at EF in TiB2, ZrB2, and HfB2.

A critical evaluation of GGA + U modeling for atomic, electronic and magnetic structure of Cr2AlC, Cr2GaC and Cr2GeC.

In this work we critically evaluate methods for treating electron correlation effects in multicomponent carbides using a GGA + U framework, addressing doubts from previous works on the usability of density functional theory in the design of magnetic MAX phases. We have studied the influence of the Hubbard U-parameter, applied to Cr 3d orbitals, on the calculated lattice parameters, magnetic moments, magnetic order, bulk modulus and electronic density of states of Cr2AlC, Cr2GaC and Cr2GeC. By considering non-, ferro-, and five different antiferromagnetic spin configurations, we show the importance of including a broad range of magnetic orders in the search for MAX phases with finite magnetic moments in the ground state. We show that when electron correlation is treated on the level of the generalized gradient approximation (U = 0 eV), the magnetic ground state of Cr2AC (A = Al, Ga, Ge) is in-plane antiferromagnetic with finite Cr local moments, and calculated lattice parameters and bulk modulus close to experimentally reported values. By comparing GGA and GGA + U results with experimental data we find that using a U-value larger than 1 eV results in structural parameters deviating strongly from experimentally observed values. Comparisons are also done with hybrid functional calculations (HSE06) resulting in an exchange splitting larger than what is obtained for a U-value of 2 eV. Our results suggest caution and that investigations need to involve several different magnetic orders before lack of magnetism in calculations are blamed on the exchange-correlation approximations in this class of magnetic MAX phases.

Magnetic self-organized atomic laminate from first principles and thin film synthesis.

The first experimental realization of a magnetic M(n+1)AX(n) (MAX) phase, (Cr(0.75)Mn(0.25))(2)GeC, is presented, synthesized as a heteroepitaxial single crystal thin film, exhibiting excellent structural quality. This self-organized atomic laminate is based on the well-known Cr(2)GeC, with Mn, a new element in MAX phase research, substituting Cr. The compound was predicted using first-principles calculations, from which a variety of magnetic behavior is envisaged, depending on the Mn concentration and Cr/Mn atomic configuration within the sublattice. The analyzed thin films display a magnetic signal at room temperature.

Effect of consecutive antibacterial therapy on bacteriuria in hospitalized geriatric patients.

The effects of treatment with a consecutive series of different antibacterial drugs, most of them commonly used in urinary tract infections, were studied in 36 hospitalized geriatric patients with bacteriuria. Most of the patients had advanced handicaps and all had urinary incontinence. Urinary acidifying drugs, penicillin V, sulfonamide, nitrofurantoin, nalidixic acid, and trimethoprim/sulfamethoxazole were used as antibacterial agents. The general conclusion was that even the administration of a series of potent antibacterial drugs only rather seldom eliminates bacteriuria in such patients, and that the original bacteria in most cases are replaced by strains highly resistant to antibacterial drugs. The treatment had obvious influence on the clinical state only in one patient.