Mechanical Properties and Electronic Structures of Cotunnite TiO2

Based on the first-principles plane-wave basis pseudopotential calculations, we investigate mechanical properties and electronic structures of the hardest known oxide, cotunnite TiO2. The calculated results show that cotunnite TiO2 has the highest bulk modulus （348 GPa） and hardness （32 GPa） among the high-pressure phases of TiO2, but its mechanical properties are not superior to those of c-BN. Moreover, the high hardness of cotunnite TiO2 can be understood from both the dense crystal structure （high valence electron density and short bond lengths） and the unusual mixtures of covalent and ionic bonding of Ti-O.     

Ground-State Structure and Physical Properties of NB_2 Predicted from First Principles

Using the newly developed particle swarm optimization algorithm on crystal structural prediction,we predict a new class of boron nitride with stoichiometry of NB_2 at ambient pressure,which belongs to the tetragonal I4m2 space group.Then,its structure,elastic properties,electronic structure,and chemical bonding are investigated by first-principles calculations with the density functional theory.The phonon calculation and elastic constants confirm that the predicted NB_2 is dynamically and mechanically stable,respectively.The large bulk modulus,large shear modulus,large Young's modulus,and small Poisson's ratio show that the I4m2 NB_2 should be a new superhard material with a calculated theoretical Vickers hardness value of 66 GPa.Further analysis on density of states and eiectron localization function demonstrate that the strong B-B and B-N covalent bonds are the main reason for its high hardness in I4m2 NB_2.     

Electronic structure, phase stability, and hardness of the osmium borides, carbides, nitrides, and oxides: First-principles calculations

The chemical bonding, elastic behavior, phase stability, and hardness of OsB, OsB₂, OsC, OsO₂, OsN, and OsN₂ have been systematically studied using first-principles calculations. The calculation suggests that the chemical bonding in these compounds is a mixture of covalent and ionic components. The structural stability of OsB, OsC, and OsN can be understood in terms of the band filling of the bonding states, and the results indicate that the hexagonal tungsten carbide structure is more stable. The hardness of these osmium compounds is calculated using both ab initio and semiempirical model calculations. Analysis of the ab initio hardness suggested that the large occupations and high strength of the covalent bonds are crucial for a superhard material, and there is no clear connection between bulk modulus and hardness in these osmium compounds.     

Fluctuations of tensile strength and hardness of c-BC?N crystals induced by difference in atomic configuration

At the atomistic level, the physical properties of a material are determined by its structure such as atomic arrangements. Here first-principles calculations were performed to investigate the effect of atomic configuration on the tensile strength and Vickers hardness of cubic-BC?N (c-BC?N) crystals. Depending on the degree of mixture between diamond and c-BN, the tensile strength of c-BC2N crystals can vary drastically from 27 to 77 GPa. The magnitude of the Vickers hardness fluctuations (~10 GPa) is also comparable to the experimental difference (~14 GPa). Thus, atomic-scale characterization of c-BC?N crystal structures may unveil the discrepancy of the measured Vickers hardness in experiments, and uncover the obvious differences of tensile strength described in theoretical calculations.     

Superhard monoclinic BN allotrope in M-carbon structure

Superhard materials have always attracted people's interesting due to their extensive industrial applications. In this work, two reasonable superhard monoclinic allotropes of boron nitrides with space group of Cm have been designed based on previously proposed M-carbon structure using first-principles calculations. Our results show that Cm-BN-1 and Cm-BN-2 are dynamically stable, and they are direct semiconductors with bandgap of 2.69 and 3.90 eV, respectively. Moreover, they could be potential superhard materials with Vickers hardness of 58.0 and 60.4 GPa, respectively. This work provides insights for exploring new superhard boron nitrides materials.     

Defects in silicon: the role of vibrational entropy

Vibrational free energies are calculated from first-principles in the same Si periodic supercells routinely used to perform defect calculations. The specific heat, vibrational entropy, and zero-point energy obtained in defect-free cells are very close to the measured values. The importance of the vibrational part of the free energy is studied in the case of two defect problems: the relative energies of the H₂ and H₂^∗ dimers and the binding energy of a copper pair. In both cases, the vibrational entropy term causes total energy differences to change by about 0.2 eV between 0 and 800 K. We also comment on the rotational entropy in the case of H₂ and the configurational entropy in the case of the Cu pair. These examples illustrate the importance of extending first-principles calculations of defects in semiconductors to include free energy contributions.     

First-Principles Studies on Properties of Boron-Related Impurities in c-BN

We investigate, by first-principles calculations, the pressure dependence of formation enthalpies and defective geometry and bulk modulus of boron-related impurities （VB, Cs, NB, and OB） with different charged states in cubic boron nitride （c-BN） using a supercell approach. It is found that the nitrogen atoms surrounding the defect relax inward in the case of CB, while the nitrogen atoms relax outward in the other cases. These boron-related impurities become much more stable and have larger concentration with increasing pressure. The impurity CB^＋1 is found to have the lowest formation enthalpy, make the material exhibit semiconductor characters and have the bulk modulus higher than ideal c-BN and than those in the cases of other impurities. Our results suggest that the hardness of c-BN may be strengthened when a carbon atom substitutes at a B site.     

Adsorption and Reaction of CO on （100） Surface of SrTiO3 by Density Function Theory Calculation

Adsorption and reaction of CO on two possible terminations of SrTiO3 （100） surface are investigated by the first-principles calculation of plane wave ultrasoft pseudopotentiai based on the density function theory. The adsorption energy, Mulliken population analysis, density of states （DOS） and electronic density difference of CO on SrTiO3 （100） surface, which have never been investigated before as far as we know are performed. The calculated results reveal that the Ti-CO orientation is the most stable configuration and the adsorption energy （0.449eV） is quite small. CO molecules adsorb weakly on the SrTiO3 （100） surface, there is predominantly electrostatic attraction between CO and the surface rather than a chemical bonding mechanism.     

First-principles calculations on crystal structure and physical properties of rhenium dicarbide

Structural stability, elastic behavior, hardness, and chemical bonding of ideal stoichiometric rhenium dicarbide (ReC₂) in the ReB₂, ReSi₂, Hex-I, Hex-II, and Tet-I structures have been systematically studied using first-principles calculations. The results suggest that all these structures are mechanically stable and ultra-incompressible characterized by large bulk moduli. Formation enthalpy calculations demonstrated that they are metastable under ambient conditions, and the relative stability of the examined candidates decreases in the following sequence: Hex-I>Hex-II>ReB₂>Tet-I>ReSi₂. The hardness calculations showed that these structures are all hard materials, among which the Hex-I exhibits the largest Vickers hardness of 32.2 GPa, exceeding the hardness of α-SiO₂ (30.6 GPa) and β-Si₃N₄ (30.3 GPa). Density of states and electronic localization function analysis revealed that the strong C–C and C–Re covalent bonds are major driving forces for their high bulk and shear moduli as well as small Poisson's ratio.     

Structural, Elastic and Electronic Properties of ReO2

Structural, elastic and electronic properties of ReO₂ are investigated by first-principles calculations based on density functional theory. The ground stateof ReO₂ has an orthorhombic symmetry which belongs to space group Pbcn with a=4.7868Å b=5.5736Å, and c=4.5322Å. The calculated bulk moduli are 322GPa, 353GPa, and 345GPa for orthorhombic, tetragonal, and monoclinic ReO₂, respectively, indicating that ReO₂ has a strong incompressibility. ReO₂ is a metal ductile solid and presents large elastic anisotropy. The obtained Debye temperatures are 850K for orthorhombic, 785K for tetragonal, and 791K for monoclinic ReO₂.     

First-Principles Calculations of Elastic Properties of LaNi5 Compound

The equilibrium lattice constants, temperature dependence of bulk modulus, the pressure dependence of the normalized volume V/V0, elastic constants Cij and bulk modulus of LaNi5 crystal are obtained using the firstprincipies piane-wave pseudopotential method in the GGA-PBE generalized gradient approximation as well as the quasi-harmonic Debye model. We analyse the relationship between bulk modulus and temperature up to 2000 K and obtain the relationship between bulk modulus B and pressure at diFFerent temperatures. It is found that the bulk modulus B increases monotonously with increasing pressure. Moreover, the pressure dependences of Debye temperatures and the pressure derivatives of lattice constants are also successfully obtained. The calculated results are in agreement with the experimental data and the other theoretical results.     

Site Preference and Alloying Effect of Excess Ni in Ni-Mn-Ga Shape Memory Alloys

The formation energies and electronic structures of Ni-rich Ni-Mn-Ga alloys have been investigated by firstprinciples calculations using the pseudopotential plane wave method based on density functional theory. The results show that the alloying Ni prefers to occupy the Mn site directly in Ni9Mn3Ga4 and to occupy the Mn site and drive the displaced Mn atom to the Ga site in NigMn4Ga3, which is in accordance with the experimental result. According to the lattice constants and the density of states analyses, these site preference behaviours are closely related to the smaller lattice distortion and the lower-energy electronic structure when the excess Ni occupies the Mn site. The effect of Ni alloying on martensitic transformation is discussed and the enhancement of martensitic transformation temperature by Ni alloying is estimated by the calculated formation energy difference between austenite and martensite phases.     

Effect of S Substitution for P Point Defects in KDP Crystals： First-Principles Study

The electronic structure and geometric distribution of phosphor replaced by sulfur in potassium dihydrogen phosphate （KDP） are investigated by first-principles calculations. The point defect narrows down the energy gap to about 4.9eV, corresponding to a two-photon absorption of 355nm after correction. This can explain the decrease of the laser damage resistance in KDP crystals. Moreover, the defects twist the crystal structure and weaken bonds, especially the O-H bonds, so these bonds may be the first sites to crack under laser irradiation.     

Structural Stabilities of Ordered Arrays of Nb4 Clusters on NaCl(100) Surface

Adsorption of ordered (2×2) arrays of Nb₄ clusters on the insulating surface of NaCl(100) is studied by the first-principles calculations within the density functional theory. The calculations on the relaxed geometries and cohesive energies show that both the tetrahedron and quadrangle-Nb₄ can be stably adsorbed on this substrate, which may have important applications. The adsorption of quadrangle-Nb₄ on the NaCl(100) surface is more stable than that of tetrahedron-Nb₄. Both the Nb₄ clusters studied and a single Nb atom prefer the top site of the Cl atom in the NaCl(100) surface. Electronic structure analysis suggests that the interactions between the Nb₄ clusters and the substrate are weak     

First-Principles Calculations for Thermodynamic Properties of Perovskite-Type Superconductor MgCNi3

The ground state properties and equation of state of the non-oxide perovstdte-type superconductor MgCNi3 are investigated by first-principles calculations based on the plane-wave basis set with the local density approximation （LDA） as well as the generalized gradient approximation （GGA） for exchange and correlation, which agree well with both theoretical calculations and experiments. Some thermodynamic properties including the heat capacity, the thermal expansion coefficient and the Griineisen parameter for perovskite structure MgCNi3 are obtained. The dependences of these thermodynamic properties on pressure and temperature are given for the first time.     

Effect of In-Doping on Electronic Structure and Optical Properties of Sr2TiO4

The effect of In doping on the electronic structure and optical properties of Sr2 TiO4 is investigated by a firstprinciples calculation of plane wave ultrasoft pseudopotentials based on density functional theory. The calculated results reveal that corner-shared TiO6 octahedra dominate the main electronic properties of Sr2TiO4 and the covalency of the Ti-O（1） bond in the ab plane is stronger than that of the Ti-O（2） bond along the c-axis. After In doping, there is a little lattice expansion in Sr2In0.125 Ti0.875 O4 and the interaction between the Ti-O bond near the impurity In atom is weakened. The binding energies of Sr2TiO4 and Sr2In0.125Ti0.875O4 estimated from the electronic structure calculations indicate that the crystal structure of Sr2In0.125 Ti0.875 O4 is still stable after doping, but its stability is lower than that of undoped Sr2TiO4. Moreover, the valence bands （VBs） of the Sr2In0.125Ti0.875O4 system consist of O 2p and In 4d states, and the mixing of O 2p and In 4d states makes the top VBs shift significantly to high energies, resulting in visible light absorption. The adsorption of visible light is of practical importance for the application of St2 TiO4 as a photocatalyst.     

First-Principles Study of Li Doping in a Double-Wall Carbon Nanotube

By performing first-principles calculations, we study Li doping in a double-wall carbon nanotube where a （5,0） tube is confined inside a （14,0） tube. There are three possible sites for Li doping and two of them are energetically favorable. The change of energy band structure is closely related to the doping sites and the charge transfer is investigated. Bader charge analysis indicates that Li prefers to donate its electron to the inner （5,0） tube. Moreover, the Li capacity of the system can reach LIC4.75 which makes it a promising candidate for Li-ion battery materials.     

Six-body van der Waals interactions

We report on the six-body van der Waals interactions within Rydberg atoms. Specifically, we focus on the octahedron case. The results are compared with previous calculations for two to five bodies' interactions. This research is useful for crystal structure in condensed matter physics, such as p-type doping in Silicon or other types of semiconductors. This research is also useful for studying big molecules in chemistry, chemical engineering, and other fields.     

First-Principles Calculations of Electronic Structures of New Ⅲ-Ⅴ Semiconductors： BxGa1-xAs and TlxGa1-xAs alloys

We investigate the electronic structures of new semiconductor alloys BxGa1-xAs and TlxGa1-xAs, employing first-principles calculations within the density-functional theory and the generalized gradient approximation. The calculation results indicate that alloying a small TI content with GaAs will produce larger modifications of the band structures compared to B. A careful investigation of the internal lattice structure relaxation shows that significant bond-length relaxations takes place in both the alloys, and it turns out that difference between the band-gap bowing behaviours for B and TI stems from the different impact of atomic relaxation on the electronic structure. The relaxed structure yields electronic-structure results, which are in good agreement with the experimental data. Finally, a comparison of formation enthalpies indicates that the production Tlx Ga1-xAs with TI concentration of at least 8% is possible.     

Theoretical Study of Compressibility and Thermoelasticity of LaNi5-xAlx （x= 0.3, 0.5 and 1.0）

The compressibility, the temperature dependence of bulk modulus, the pressure dependence of normalized volume V/V0, thermal expansion coefficient and Debye temperature of LaNi5-xAlx compounds are successfully obtained using the first-principles plane-wave pseudopotential （PW-PP） method, the EOSFIT6.0 software and the quasiharmonic Debye model. The rapid decrease of relative lattice constant a/a0 shows that the deformation is easier in directions normal to the c-axis than that along it. The relationships between bulk modulus B and pressure at different temperatures are also analysed. It is found that the bulk modulus B increases monotonically with increasing pressure. Moreover, the pressure dependences of thermal expansion and Debye temperature are also successfully obtained. The calculated results are in agreement with the experimental data.