Structure,properties, and possible mechanisms of formation of diamond-like phases

An analysis was performed for relations between the structural parameters and the properties of 36 carbon diamond-like phases consisting of atoms occupying crystallographically equivalent positions. It was found that the crystal lattices of these phases were in stressed states with respect to the cubic diamond lattice. The density of diamond-like phases, their sublimation energies, bulk moduli, hardnesses, and band gaps depend on the deformation parameters Def and Str. The most stable phases must be phases with minimal parameters Def and Str and also with ring parameter Rng that is most close to the corresponding parameter of cubic diamond. The structures and energy characteristics of fullerites, nanotube bundles, and graphene layers of which diamond-like phases can be obtained as a result of polymerization at high pressures have been calculated.     

Modeling of Phase Transitions of Graphites to Diamond-Like Phases

The method of the density functional theory is used to study structural transformations between graphites and diamond-like phases. The calculations have been carried out in two approximations: a local density approximation and a generalized gradient approximation. It is found that the phase transitions of hexagonal graphene layers to a cubic diamond and diamond-like phases must occur at uniaxial compressions of ~57–71 GPa, whereas some diamond-like phases can be obtained from tetragonal graphene layers at significantly lower pressures of 32–52 GPa. The X-ray diffraction patterns have been calculated for the phase transition of graphite I4₁/amd to tetragonal LA10 phase that takes place at the minimum pressure that can be used for experimental identification of these compounds.     

Grounds for the application of fractional-order diffusion equations in channeling theory using the Langevin approach

Transverse-energy distribution densities in the case of ultrarelativistic proton channeling in the (100) planes of diamond-like crystals have been numerically calculated within the framework of the Langevin approach. Based on the self-similarity principle, the subdiffusion character of the motion under consideration has been established, which can be regarded as grounds for using fractional-order diffusion equations to adequately describe channeling and dechanneling processes mathematically.     

Structures of diamond-like phases

The diamond-like phases containing carbon atoms with the same degree of hybridization, which is close to sp ³, are classified. It is found that twenty such phases can exist, and ten of them are described for the first time. Molecular mechanics and semi-empirical quantum-mechanical methods are used to calculate the geometrically optimized structures of diamond-like phase clusters and to determine their structural parameters and properties, such as the density, the bulk modulus, and the sublimation energy. The difference between the properties of the diamond-like phases and those of diamond is found to be determined by the difference between the structures of these phases and diamond.     

First-principles study of structural, mechanical, electronic and optical properties of 3R- and 2H-CuGaO2

We calculated the structural parameters, elastic, mechanical, electronic and optical properties of 3R- and 2H-CuGaO₂ using the first-principles density-functional theory. The results show that the structural parameters of two phases are in good agreement with previous theoretical and experimental data. Two phases are mechanically stable, behave in ductile manner and have indirect band gap. The analyses of electronic structures and charge densities of two phases show mainly covalent nature in Cu-O bonds and coexistence of both ionic and covalent nature in Ga-O bonds. The optical properties are obtained and discussed, including the complex dielectric function, refractive index, extinction coefficient, optical reflectivity, absorption coefficient, energy-loss spectrum and complex conductivity function, which provide useful information for the future applications of CuGaO₂.     

New structural modifications of diamond: LA9, LA10, and CA12

The structural characteristics and properties of three new carbon phases (LA9, LA10, CA12), which have a diamond-like structure and atoms located in crystallographically equivalent positions, are described. The model mechanism of LA9 and LA10 formation is the linking of L₆ and L_4–8 graphene layers, respectively, and phase CA12 can be formed by linking C₄ tetrahedral clusters. Phases LA9, LA10, and CA12 can also be formed as a result of the polymorphic transformations of three-dimensional graphite phases, when all atoms transform from a three-coordinated into a four-coordinated state. LDA-DFT calculations of the LA9, LA10, and CA12 phases are used to find their geometrically optimized structures and properties (density, total energy, density of states). In addition, powder X-ray diffraction patterns are calculated for these phases and possible methods of their synthesis are analyzed.     

Classification and structure of silicon carbide phases

Molecular-mechanical and semiempirical quantum-mechanical methods have been applied to simulate and calculate a geometrically optimized structure of clusters of polymorphic types of silicon carbide, and their structural parameters and some properties (densities, sublimation energies) have been determined. A classification of silicon carbide phases has been proposed, which shows the possible existence of twenty one SiC phases whose atoms are at crystallographically equivalent sites. The structures of seventeen proposed silicon carbide phases have been described and studied for silicon carbide for the first time.     

用模拟退火模型研究非晶硅的结构和振动性质

采用Monte Carlo技术模拟高温退火物理过程,用计算机建立了非晶硅无序网络模型,它具有十分接近真实样品的结构特性。还用分子动力学方法得到该模型的振动态密度,研究了它的几何缺陷与局域振动模的关系。 关键词：     

Electron energy characteristics and one-electron state spectra of surfaces of molybdenum oxide and solid solutions thereof with niobium and technetium oxides with fluoropolymer coatings

The energy spectra of electrons belonging to the surface centers of molybdenum oxide and its solid solutions (interstitial phases) with niobium and technetium oxides have been analyzed. The densities of one-electron states and their atomic orbitals have been calculated for hydroxyl, dehydroxyl, and fluoropolymercoated surfaces. The calculation method is based on the density functional theory using the three-parameter hybrid Becke-Lee-Yang-Parr exchange-correlation potential in the basis 3-21G of linear combinations of Gaussian orbitals.     

Effects of Monopolar Resistive Switching in Thin Diamond-Like Carbon Layers

JETP Letters - The current-voltage characteristics have been studied for Pt/diamond-like C/Pt structures based on thin (20 nm) diamond-like carbon layers, in which the ratio between carbon phases...     

The electronic structure,electronic charge density and optical properties of the diamond-like semiconductor Ag2ZnSiS4

The electronic structure, electronic charge density and optical properties of the diamond-like semiconductor Ag₂ZnSiS₄ compound with the monoclinic structure have been investigated using a full-relativistic version of the full-potential augmented plane-wave method based on the density functional theory, within local density approximation (LDA), generalized gradient approximation (GGA), Engel–Vosko GGA (EVGGA) and modified Becke Johnson (mBJ) potential. Band structures divulge that this compound is a direct energy band gap semiconductor. The obtained energy band gap value using mBJ is larger than those obtained within LDA, GGA and EVGGA. There is a strong hybridization between Si-s and S-s/p, Si-p and Zn-s, Ag-s/p and Zn-s, and Ag-s and Ag-p states. The analysis of the site and momentum-projected densities shows that the bonding possesses covalent nature. The dielectric optical properties were also calculated and discussed in detail.     

Electron momentum density and phase transition in SrO

In this article, we present electron momentum density distribution and phase transition in SrO. The experimental values of momentum density have been measured using 5Ci ²⁴¹Am Compton spectrometer and analyzed using theoretical data obtained from the ab-initio linear combination of atomic orbitals method. The first-principles calculations of the total energy of SrO as a function of cell volume have also been carried out for the cubic rocksalt (B1) and cesium chloride (B2) phases. Several structural parameters, i.e. equilibrium lattice constant, transition pressure, bulk modulus, etc. of B1 and B2 phases have been calculated and compared with the previous investigations. We conclude that the stable phase of SrO is B1 and the phase transition from B1 to B2 occurs at 35.8?GPa.     

To the theory of magnetic-moment anisotropy of shallow acceptor centers in diamond-like semiconductors

The spin quartet splitting of the ground state sublevels of shallow acceptor centers in a magnetic field has been calculated for diamond-like semiconductors with a strong spin-orbit coupling, such as Ge and GaAs. The anisotropy of this splitting has been shown to depend strongly on the binding energy and to be very sensitive to small changes in the Luttinger band parameters. These strong dependences permit one to use calculated ground-state g factors to determine the Luttinger magnetic band-structure parameters κ and q. A new method is proposed for determination of these parameters, and their values for Ge and GaAs are calculated.     

无序对金属超晶格电子结构的影响——CPA方法

本文用一种简单的哈密顿模型,在紧束缚近似下,只计及对角矩阵元的无规分布,用CPA方法自洽地计算了无序所引起的金属超晶格电子结构的变化,给出了某些参数下局部态密度和自能曲线。 关键词：     

Electrostatics of the ferroelectric phases of BaTiO3

Extensive theoretical calculations are reported here on the electrostatics of the three ferroelectric phases of BaTiO₃ using the static polarizable point-ion model. For each structure the following quantities have been calculated: monopole and dipole fields at ionic sites, field gradients, spontaneous polarization, high frequency dielectric constant and the Coulomb energy terms. The parameters of the model have been fitted on the basis of the experimental information on the tetragonal structure and then treated as constants for all the studied structures. The use of the experimental positional parameters of the orthorhombic structure gives such exceptional behaviour for all the calculated quantities that a redetermination of the structural parameters for this phase is suggested. Numerical study of the changes in the Coulomb energy as a function of the macroscopic depolarization potential demonstrated that the most stable state is achieved with the value zero for the potential.     

X-ray, scanning electron microscopy and electrical properties of synthetic fresnoite (Ba2TiSi2O8) ceramics

Polycrystalline ceramic samples of fresnoite (Ba₂TiSi₂O₈ or BTS) have been prepared by a standard solid-state reaction method using high-purity oxides and carbonates. For one set of compounds, in stoichiometric ratio BaCO₃, TiO₂ and SiO₂ were melted at 1300°C and then sintered into pellet form, whereas the other set of compounds have been prepared without melting and sintered into pellet form at 1250°C. The formation of the single-phase compound and its structural parameters were investigated by X-ray diffraction followed by Rietveld refinement and scanning electron microscopic (SEM) techniques. A better agreement between the observed and calculated X-ray diffraction patterns was obtained by performing the Rietveld refinement with a structural model using the non-centrosymmetric space group P4bm. A better agreement between observed and calculated d-values shows that the lattice parameters calculated using the Rietveld refinement analysis are better than that of the earlier report so far. The activation energies of both compounds were calculated by measuring its dc electrical conductivities. The results are discussed in detail.     

Electronic structure and structural phase stability of CuAlX2 (X=S, Se, Te) under pressure

The electronic and structural properties of chalcopyrite compounds CuAlX₂ (X=S, Se, Te) have been studied using the first principle self-consistent Tight Binding Linear Muffin-Tin Orbital (TBLMTO) method within the local density approximation. The present study deals with the ground state properties, structural phase transition, equations of state and pressure dependence of band gap of CuAlX₂ (S, Se, Te) compounds.Electronic structure and hence total energies of these compounds have been computed as a function of reduced volume. The calculated lattice parameters are in good agreement with the available experimental results. At high pressures, structural phase transition from bct structure (chalcopyrite) to cubic structure (rock salt) is observed. The pressure induced structural phase transitions for CuAlS₂, CuAlSe₂, and CuAlTe₂ are observed at 18.01, 14.4 and 8.29 GPa, respectively. Band structures at normal as well as for high-pressure phases have been calculated. The energy band gaps for the above compounds have been calculated as a function of pressure, which indicates the metallic character of these compounds at high-pressure fcc phase. There is a large downshift in band gaps due to hybridatization of the noble-metal d levels with p levels of the other atoms.     

α-C:H薄膜及其在硅太阳电池上作增透膜的研究

采用高频等离子体汽相淀积法在不同材料衬底上淀积出α-C:H薄膜。Raman谱分析表明,该薄膜既具有类金刚石相,又具有金刚石相。测量了该薄膜可见光范围的透射率和折射率。本文提出把该薄膜涂覆在硅太阳电池上作光学增透膜,使其在0.55—1.0μm波长范围内光谱响应明显提高,并使该电池短路电流增加率达38％。 关键词：     

Physical properties of molybdenum monoboride: Ab-initio study

The Ab initio investigations on structural, electronic, optical and thermal properties of MoB have been reported using full potential linearised-augmented plane wave method within the framework of density functional theory. The exchange and correlation potentials were calculated using the Perdew-Burke-Ernzerhof-Sol generalised gradient approximation. The calculated equilibrium lattice constants and cell volume are in excellent agreement with the experimental results as compared to the available theoretical data. Electronic band structure shows that MoB is metallic in nature. From the partial densities of states of MoB it has been found that major contribution on the Fermi level is due to Mo-4d states. Among the reported optical parameters the large value of reflectivity at low energy shows that MoB can be used as a coating material in IR region. Maximum absorption in extreme UV region shows that it can be used in production of electricity through solar power in space vehicles. Various thermal properties have been calculated in a wide temperature range at high pressures. Change in thermal expansion coefficient with respect to temperature shows that anharmonic effect in MoB is very weak at high temperature. The optical and thermal properties of MoB are presented for the first time in this work.