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Multicomponent ceramics have attracted increasingly attention due to their potential applications. Employing density functional theory (DFT) and the special quasi-random structure (SQS), the structural stability and elastic characteristics of TiCNO and TiZrCNO are systematically explored in this study. The available experimental data and the optimized lattice parameters coincide very well. Both high entropy oxycarbonitrides are thermodynamically stable because the formation enthalpies are negative. According to the present results of elastic parameters, the slight expense of the strength and stiffness the ductility is improved. The increase of ductility presumably originates from the multicomponent mixing in the anion sublattice, the subsequent incorporation of Zr results in the stronger influence due to the more severe lattice distortion, as discussed in our previous study. The anisotropy of two ceramics are studied systematically, the role of the anisotropy is explored in wear/cutting applications. Then the thermodynamic properties of the two ceramics are also studied further applying Debye-Grüneisen model. Thermodynamic properties of two materials exhibit essentially similar trends as temperature rises. Because the larger lattice distortion by the subsequent addition of Zr, the bulk moduli and thermal expansion coefficient for TiZrCNO change slowly with temperature. Clearly the influences of the multianion mixing, especially the contribution from the following incorporation of Zr which leads to larger internal distortion should pay high attention. The current investigations provide valuable insight into the mechanical and thermodynamic behavior of these two multicomponent ceramics.

High-entropy perovskite oxides are novel high-entropy ceramics developing recently. In this work, the local lattice distortion, mechanical and thermodynamic properties of perovskites A(Zr0.2Sn0.2Ti0.2Hf0.2Nb0.2)O3 (A = Sr, Ba) are explored using the first principle investigation. The equilibrium lattice parameter and bulk moduli of high-entropy perovskites obtained by fitting approximately satisfy the rule of mixture of the components, while the entropy effect of component mixing is very small. The bond length distribution reveals the B-site disorder results in large local lattice distortion. The atomic displacement indicates larger average displacement of O atoms contributes mainly to the wider bond length distribution for most B-O bond. The distortion degree at B-site is naturally associated with A-site. The obtained elastic constants of high-entropy perovskites are also closed to the rule of mixing of the components. Compared with ternary SrTiO3 and BaTiO3 perovskites, B-site mixing for high-entropy perovskites enhances their toughness at expense of strength and stiffness, showing the elastic moduli can be modified and adjusted through multi-component design strategy. Relevant thermodynamic property reveals the B-site disorder is benefit to improve the resistance to softening and suppress volume expansion at high temperature. Electronic structures show the insulator–metal transition takes place, also uncover that the interaction of B-O bond is stronger.

Theoretical study of structural stability and elastic properties of α- and β-MgPd3 intermetallic compounds as well as their hydrides have been carried out based on density functional theory. The results indicate α-MgPd3 is more stable than β phase with increased stability in their hydrides. The calculated elastic constants of α-MgPd3 are overall larger than β phase. After hydrogenation, the elastic constants are enlarged. And the elastic moduli exhibit similar tendency. The anisotropy of α-MgPd3 is larger than β phase, and the hydrides demonstrate larger anisotropy. Their ductility follows the order of α-MgPd3H0.5

Based on density functional theory and thermodynamic model, we compile a phase diagram for the polar NiO(111) surface as a function of temperature and oxygen pressure. The electronic correlation between Ni-3d electrons has also been included in the form of GGA+U method. Consistent with recent experiments, present GGA+U calculation indicates that over a broad range of oxygen partial pressure, the most stable phases are the oxygen and Ni terminated octopolar structures, which are almost degenerate in energy. We also show that the stabilization of the NiO(111) surface goes together with remarkable changes in the geometrical and electronic structure. [ABSTRACT FROM AUTHOR]

Three-dimensional time-dependent quantum wave packet calculation for the O(1D)+HBr reaction has been carried out using an accurate ab initio global potential energy surface [K. A. Peterson, J. Chem. Phys. 113, 4598 (2000)]. The calculations show that the initial state-selected reaction probabilities are dominated by resonance structures, and the lifetime of the resonance is generally in the subpicosecond time scale. The energy dependence of the reaction cross section is computed, which manifests still resonance structures, and is a decreasing function of the translational energy. The thermal rate constants are also computed, which are nearly independent on the temperature. The calculation results are discussed and compared to similar reaction with deep well. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Density functional theory is used for studying the stability of three possible [Mg.sub.3]Nd(001) surface along with the structural and electronic properties of O adsorption on the most stable [Mg.sub.3]Nd(001) surface. The studies have shown that during the initial oxidation stage of the alloy, Nd atoms can get a priority of oxidation, followed by Nd rich oxide film.

A detailed theoretical study of structural, elastic and electronic properties of tI26-type Mg12RE (RE = Ce, Pr and Nd) phases has been carried out by means of first-principles calculations based on density functional theory. The optimized lattice parameters at T = 0K are in excellent agreement with the available experimental value at room temperature, and the obtained formation enthalpies show that with increasing atomic number of RE, the stability of Mg12RE alloys becomes lower. The elastic constants of tI26 Mg12RE are further calculated, then the bulk modulusB, shear modulusG,Young's modulus E and Poisson's ratio ν of polycrystalline aggregates are derived, and the relevant mechanical properties ofMg12RE alloys are also discussed. The elastic anisotropy of the three alloys is studied in detail using several methods, and the three-dimensional directional representation reveals the variation of Young's modulus along the crystallographic direction visually and completely. Finally, electronic density of states, charge density distribution and Bader charges are also calculated to reveal the underlying mechanism of structural stability and mechanical properties. [ABSTRACT FROM AUTHOR]

Based on ab initio atomistic thermodynamics model, the surface adsorption diagram of O/Ni(110) system over a wide range of temperature and pressure conditions has been determined. Different surface phases including oxygen adsorption on reconstructed and unreconstructed substrates are considered. Clean nickel surface, p(2×1)-O and p(3×1)f-O, and bulk oxide phase are found to be the most stable phases under different oxygen chemical potential regions. Our calculations suggest that the unreconstructed (n×1)(n=2,3)-O overlayer and p(3×1)i-O phases are just metastable at the equilibrium condition. These results could effectively solve the debate in different experiments and give a consistent description for the reproducibility of overlayer and p(3×1)i-O reconstructed phases. [ABSTRACT FROM AUTHOR]

Full three-dimensional time-dependent quantum wave-pack calculations have been carried out for the F+HCl and F+DCl reactions on a many-body expansion of the ground 2A[sup ′]HClF potential energy surface. The calculated energy-dependence of reaction probability exhibits oscillating structure in the F+HCl reaction but not in the F+DCl system. The effects of initial state excitation on the total reaction probabilities as a function of collision energy are investigated for reactions from various initial vibrational and rotational states of HCl and DCl. Our results show that reagent vibrational and/or rotational excitation can generally lead to an increase in reaction probability at low collision energy and a slight decrease at relatively high collision energy. Thermal rate constants for the title reactions are calculated and they are in generally good agreement with experimental measurement. Investigation of steric effects for the reactions indicates that the H (or D) side of HCl (or DCl) molecule is only slightly favored for reactive attack and reaction proceeds from almost all attack angles. The present results indicate that the H/D kinetic isotope effect should not be totally neglected. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A computer simulation methodology with which to study the single-particle dynamics in complex molecular liquids is presented. Molecular dynamics simulations of liquid water are performed in the temperature range of 238-473 K using the polarizable point charge (PPC) potential. The self part of the van Hove density-density correlation function is calculated. Using the Gaussian approximation of the van Hove function the mean self-diffusion coefficient for the PPC potential is calculated. The singularity temperature for supercooled PPC water, T[sub s]=218 K, estimated from the self-diffusion data appears to agree well with most estimates for real water. In order to elucidate the spatial picture of the single-particle molecular density in this complex liquid and its time evolution, we explicitly resolve the self van Hove function in the local frame of water molecules. The self-diffusion tensor is introduced and numerically evaluated from this spatial (separation and direction dependent) self van Hove function. The fluctuations of the single-particle molecular density in liquid water appear to be spatially anisotropic (nonspherical). At low temperatures these dynamical heterogeneities in liquid water tend to increase. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Due to the presence of a sizeable direct band gap, three-atom-thick transition metal dichalcogenide (TMDC) monolayers have been suggested as important candidates for flexible electronic and optoelectronic devices recently. The in-plane elasticity of TMDC monolayers has been investigated extensively, however, little is known about their bending rigidity. Here, we have determined bending rigidities of single-layer MX2 (M  =  Mo, W; X  =  S, Se) by fitting the energetics of single wall nanotubes from first-principles to the Helfrich Hamiltonian for the configurational energy of membranes. This parameter-free approach can avoid the controversy induced by ambiguous definition of the thickness of monolayers, which are required in the empirical determination of bending rigidity using classical shell theory. The obtained direction-dependent bending rigidities of single-layer MoS2 are 9.10 and 9.61 eV along the armchair and zigzag directions, which are larger than that estimated using shell theory but similar to the previous analytic formula based on an empirical potential. Moreover, the relative magnitude of bending rigidities for different TMDCs are found to be MoS2  <  MoSe2  <  WS2  <  WSe2, which is in conflict with shell theory. Further analysis indicates that this inconsistence can be understood by a competition mechanism between two-dimensional elastic modulus of monolayers and the structural relaxation of nanotubes. [ABSTRACT FROM AUTHOR]