Local Density Approximation Method Research Articles

Моделирование процесса формирования алмазоподобных фаз из структурных разновидностей тетрагонального графита

Theoretical investigations of the phase transformations of structural modifications of tetragonal L4-8 graphite in diamond-like phases was performed using the density functional theory method in local density approximation (LDA) and generalized gradient approximation (GGA). The varieties of L4-8 graphite were examined with the following packing of graphene layers: AA (P4/mmm), AB (I4/mmm), ABCD (I41/amd). The LA6 (Imma), LA7 (Cmcm), and LA10 (I41/amd) diamond-like phases can be obtained from these graphites under compression. The LA6 diamond-like phase can be obtained from L4-8 AA graphite at pressure of 44 GPa. Another phase, LA7, can be formed from tetragonal L4-8 AB graphite in the pressure range from 43 to 46 GPa. The LA10 structure can be obtained only from L4-8 graphite with the ABCD layer packing of at pressures from 32 to 40 GPa. It was established that «L4-8 AA graphite → LA6 phase», «L4-8 AB graphite → LA7 phase», and « L4-8 ABCD graphite → LA10 phase» structural transitions are exothermic phase transitions of the first kind, as a result of which an energy of 0.5, 0.5, and 0.3 eV/atom is released, respectively. The energy barriers that need to be overcome to observe reverse «LA6 → L4-8 AA graphite», «LA7 → L4-8 AB graphite» и «LA10 → L4-8 ABCD graphite» phase transitions are 0.38, 0.34, and 0.18 eV/atom for the LDA-calculations or 0.31, 0.28, and 0.13 eV/atom for the GGA-calculations, respectively. Possible ways of synthesizing LA6, LA7, and LA10 phases are a strong compression of graphite along the [001]-axis at low temperatures.

Density Function Theory Calculation of Crystal Properties and Electronic Structure of <i>δ</i>-Pu

In this paper, lattice constants, elastic properties and electronic structure of δ-Pu are investigated by means of plane wave pseudo-potential method (PWP) based on density function theory (DFT). A variety of density functional theory methods have been adopted to calculate the crystal structure and elastic property of δ-Pu, and it is found that the lattice constants, bulk modulus B, shear modulus G, Young's modulus E and Poisson's ratio ν calculated by spin polarization local density approximation (SP+LDA) method are in best agreement with experimental values. The electronic structure have been investigated within the framework of LDA+U, and the band structure, density of state (DOS) and partial density of state (PDOS) are calculated. Calculation results of elastic properties and electronic structure show that, δ-Pu shows obvious metallicity and well ductility, its electrons are strongly corrected and the DOS in the vicinity of the Fermi Level is mainly contributed by 5f electrons.

Structural, elastic and thermodynamic properties of A15-type compounds V3X (X = Ir, Pt and Au) from first-principles calculations

The structural, elastic and thermodynamic properties of the A15 structure V3Ir, V3Pt and V3Au were studied using first-principles calculations based on the density functional theory (DFT) within generalized gradient approximation (GGA) and local density approximation (LDA) methods. The results have shown that both GGA and LDA methods can process the structural optimization in good agreement with the available experimental parameters in the compounds. Furthermore, the elastic properties and Debye temperatures estimated by LDA method are typically larger than the GGA methods. However, the GGA methods can make better prediction with the experimental values of Debye temperature in V3Ir, V3Pt and V3Au, signifying the precision of the calculating work. Based on the E–V data derived from the GGA method, the variations of the Debye temperature, coefficient of thermal expansion and heat capacity under pressure ranging from 0 GPa to 50 GPa and at temperature ranging from 0 K to 1500 K were obtained and analyzed for all compounds using the quasi-harmonic Debye model.

A Theoretical Investigation on Structural and Electronic Properties of MgTl

Intermetallic compounds are innovative materials and are far superior to conventional metals and alloys. These intermetallic compounds have a great potential in industrial and technological fields because most of the intermetallic compounds are stronger as well asstiffer at elevated temperature and provide far better corrosion resistance than conventional metals and alloys.Over the past few years the scientific interest in the study of these intermetallic compounds emanates greatly because of their high-tech applications. Our motivation of the present studyMgTl mainly consernedon the physical data generation in context with its possible vast applications .We used a theoretical approach within the local density approximation method to study the structural and electronic properties of MgTl by calculating total energy. As far as our calculations are concerned, the band structure shows the overlapping of conduction and valence band thus itis clear that MgTl in its pure form is a good conductor of heat and electricity and falls in the category of metals. We have also calculated lattice parameters, bulkmodules, first order derivative electronic and lattice heat coefficient and Debye temperature.

The electronic and structural properties in Ca2TiMnO6 double perovskite: The first principle study

We studied the crystal structure of double perovskite Ca2TiMnO6 using ab initio density functional theory (DFT) calculations. We used the atomic positions given by the X-ray diffraction (XRD) data, in order to create a primitive lattice cell structure using visualization software Material Studio. Such sophisticated structure was found in tetragonal perovskite system, with space group with I4/m (#87) and lattice parameter of a = b = c = 5.32 Å. The exchange-correlation potential was treated with the Local Density Approximation (LDA) method. The calculations were performed with spin-polarization. The bandgaps for up-spin, net zero and down-spin were reported to be 1.051 eV, 0.988 eV and 1.806 eV respectively. The magnetic element Mn had an impact in tuning the bandstructure when the spin was induced. In summary, this double perovskite emits light when at the down spin state. The energy bandgap is opened, by 63 meV from net zero spin state to the up-spin state. Also, the PDOS illustration explains the reasons behind the changes in the bandstructure, between the up-spin, down-spin and net-zero spin, due to the shifts in the 3 d5 orbital states.

Effect of Si substitution on structural, electronic and optical properties of YNi4Si-type DyNi5−xSix (x=0, 1, 2) compounds

We employed first principle calculations for investigation of structural, electronic and optical properties of YNi4Si-type DyNi5−xSix (x=0, 1, 2) compounds. These properties are studied first time on YNi4Si-type DyNi5−xSix compounds. The exchange and correlation potential is treated by the Coulomb corrected local spin density approximation (LSDA+U) method for better accounting of the correlation between the 4f electrons. The optimized lattice constants and internal cell parameters are in agreement with the available data. Self consistence band structure calculations show that Ni-3d states remains in valance band and dominant below the EF, while Dy-5d and 4f states mainly contributes above Fermi Energy (EF) in DyNi5−xSix (x=0, 1, 2) compounds. We also find that when silicon for nickel substitution takes place (DyNi4Si), there is a gradual hybridization of Ni-3d and Si-3p states results, nickel moments decrease rapidly in agreement with the experiment. Optical spectra shows the main absorption peak around 4eV depends on the substituent concentration and could be due to transition from hybridized band (Ni-3d and Si-3p), below EF to free Dy-4d states. Frequency-dependent refractive index, n(ω), and the extinction coefficient, k(ω), of DyNi5−xSix (x=0, 1, 2) are also calculated for the radiation up to 14eV.

Connection between Hybrid Functionals and Importance of the Local Density Approximation.

The exchange-correlation (XC) local density approximation (LDA) is the original density functional used to investigate the electronic structure of molecules and solids within the formulation of Kohn and Sham. The LDA is fundamental for the development of density-functional approximations. In this work we consider the generalized Kohn-Sham (GKS) theory of hybrid functionals. The GKS formalism is an extension of the Kohn-Sham theory for electronic ground states and leads to a vast set of alternative density functionals, which can be estimated by the LDA and related methods. Herein we study auxiliary electronic systems with parametrized interactions and derive (i) a set of exact equations relating the GKS XC energies in the parameter space and (ii) a formal relation between the parameters and the standard XC derivative discontinuity. In view of the new results and previously reported findings, we discuss why the inclusion of Fock exchange, and its long-range-corrected form (in the ground-state calculations and in linear-response Kohn-Sham equations), dominate over the generalized gradient corrections to enhance the quality of the fundamental gap and to enhance excitation-energy estimations. As an example, we show that the adiabatic CAM-LDA0 (a functional with 1/4 global and 1/2 long-range Hartree-Fock interaction, respectively, a range separation factor of 1/3, and pure LDA exchange and correlation) works for electronic excitations as well as the adiabatic CAM-B3LYP functional.

Room-temperature ferromagnetism in nanocrystalline (Na0.7K0.3)0.5Bi0.5TiO3

The nanocrystalline (Na0.7K0.3)0.5Bi0.5TiO3 plates present room temperature ferromagnetism. The reduction of ferromagnetism for the (Na0.7K0.3)0.5Bi0.5TiO3 plate with subsequent long time and high temperature air-annealing indicates that the observed ferromagnetism is connected with the cation vacancies at/near the surface of nanograins. The results of density functional theory calculation with the local density approximation plus on-site effect method on the magnetism of (Na2/3K1/3)0.5Bi0.5TiO3 supercell show that Na vacancies can introduce a nonzero magnetic moment. In addition, the electric field treatment leads to an enormous enhancement of saturation magnetization for (Na0.7K0.3)0.5Bi0.5TiO3 plates, showing a strong magnetoelectric coupling.

Pressure-induced phase transitions and structural properties of CoF2: An ab-initio molecular dynamics study

Abstract The crystal structure of CoF2 was studied theoretically using first-principles density functional theory (DFT) methods within the generalized gradient approximation (GGA) and local density approximation (LDA) under rapid hydrostatic pressure up to 144 GPa. CoF2 undergoes a structural phase transformation from the rutile-type tetragonal parent phase with space group P42/mnm to the CaCl2-type orthorhombic parent phase with space group Pnnm at 64 GPa with GGA and at 96 GPa with LDA methods. Another phase transformation occurs from the CaCl2-type structure to monoclinic parent phase with space group P21/c at 96 GPa with a GGA method. These phase transitions are also studied by enthalpy and total energy calculations. According to these calculations, we obtained the first phase transformation at about 6.5 GPa both GGA and LDA methods and the later phase transformation at about 45 GPa with the GGA method.

Isothermal compressibility determination across Bose-Einstein condensation

We apply the global thermodynamic variables approach to experimentally determine the isothermal compressibility parameter $\kappa_T$ of a trapped Bose gas across the phase transition. We demonstrate the behavior of $\kappa_T$ around the critical pressure, revealing the second order nature of the phase transition. Compressibility is the most important susceptibility to characterize the system. The use of global variables shows advantages with respect to the usual local density approximation method and can be applied to a broad range of situations.

First principles calculations of structural, elastic, electronic properties of Ir3Zr with L12 structure under high pressure

The effects of pressure on the structural, elastic and electronic properties of Ir3Zr are investigated by means of the first-principles calculations based on the density functional theory with generalized gradient approximation and local density approximation methods. The calculated lattice parameters and elastic modulus of Ir3Zr at zero pressure are in good agreement with available experimental and theoretical results. The values of elastic constants (C11, C12, C44), bulk modulus (B), shear modulus (G), Young modulus (E), Poisson's ratio (υ), anisotropy index (A) and Debye temperature (TD) present the linearly increasing dependences on the external pressure. Additionally, the B/G values exhibit an upward trend with increasing pressure, which means that higher pressure can improve its ductility. Ir3Zr exhibits a brittle characteristic at zero pressure. When the pressure reaches 10 GPa, the Cauchy pressure and B/G value show ductile feature. In addition, the pressure-dependence behavior of density of states, Mulliken charge and bond length are analyzed.

Computational study of hydroxyapatite structures, properties and defects

Hydroxyapatite (HAp) was studied from a first principle approach using the local density approximation (LDA) method in AIMPRO code, in combination with various quantum mechanical (QM) and molecular mechanical (MM) methods from HypemChem 7.5/8.0. The data obtained were used for studies of HAp structures, the physical properties of HAp (density of electronic states—DOS, bulk modulus etc) and defects in HAp. Computed data confirmed that HAp can co-exist in different phases—hexagonal and monoclinic. Ordered monoclinic structures, which could reveal piezoelectric properties, are of special interest. The data obtained allow us to characterize the properties of the following defects in HAp: O, H and OH vacancies; H and OH interstitials; substitutions of Ca by Mg, Sr, Mn or Se, and P by Si. These properties reveal the appearance of additional energy levels inside the forbidden zone, shifts of the top of the valence band or the bottom of the conduction band, and subsequent changes in the width of the forbidden zone. The data computed are compared with other known data, both calculated and experimental, such as alteration of the electron work functions under different influences of various defects and treatments, obtained by photoelectron emission. The obtained data are very useful, and there is an urgent need for such analysis of modified HAp interactions with living cells and tissues, improvement of implant techniques and development of new nanomedical applications.

Modified Hydroxyapatite Structure and Properties: Modeling and Synchrotron Data Analysis of Modified Hydroxyapatite Structure

First principle modeling and calculations of hydroxyapatite both native and surface modified and having various defects (OH vacancies, H inter-nodes) were performed. Local Density Approximation method used with calculations of Density of States allows us to analyze the experimental obtained work function data. Molecular modeling was confirmed by photo-electron measurements up to 6.5 eV and photoluminescence experimental data from synchrotron DESY up to 30 eV. Brief analysis of the influence of heating, microwave radiation, hydrogenation, and synchrotron radiation on hydroxyapatite surface is presented in this work. New data on the structure of modified hydroxyapatite are obtained.

Piezoelectricity in the Ordered Monoclinic Hydroxyapatite

The results are based on the first principle modeling and calculations of hydroxyapatite (HAP) nanostructures, especially, in ordered monoclinic phase. HAP structures were studied using local density approximation (LDA) method with calculations of density of states (DOS) in combination with molecular modeling by HyperChem. Computed data show that monoclinic and hexagonal phases can co-exist, especially in their ordered (along OH groups) states. Calculated piezoelectric coefficient dyy ∼ d33 ∼ 15.7 pm/V is consistent with recently obtained experimental data (effective longitudinal piezoelectric coefficient d33eff about 8 pm/V), which vary significantly from grain to grain and may correspond to coexisting phases.

Room-temperature magnetoelectric coupling in nanocrystalline (Na1−xKx)0.5Bi0.5TiO3 (x = 0.1, 0.16, 0.20, 0.25)

Nanocrystalline (Na1−xKx)0.5Bi0.5TiO3 (x = 0.1, 0.16, 0.20, 0.25) plates exhibit ferromagnetism at room temperature.

Neutral collective modes in spin-polarized fractional quantum Hall states at filling factors13,25,37, and49

We determine the lowest and higher order collective modes in both spin-conserving and spin-reversed sectors by calculating energy differences of the appropriate linear combinations of different levels of composite-fermion-excitons and the fully spin-polarized ground states at filling factors $\nu=1/3$, 2/5, 3/7, and 4/9. Apart from providing the detailed study of previously reported modes that have also been observed in the experiments, we predict additional higher energy modes at different filling factors. The lowest and the next higher spin-conserving modes have equal number of "magneto-rotons" and the number is the same as the number of filled effective Landau-like levels of composite fermions. The higher energy modes at $\nu =1/3$ merge with the lowest mode at long-wavelength. The spin-conserving modes do not merge at other filling factors. Apart from showing zero-energy spin-wave mode at zero momentum, thanks to Larmor's theorem, the lowest spin-reversed modes at the ferromagnetic ground states of $\nu=2/5$, 3/7, and 4/9 display one or more "spin-rotons" at negative energies signalling the unstable fully polarized ground states at sufficiently small Zeeman energies. The high energy spin-reversed modes also have spin-rotons but at positive energies. The energies of these excitations depend on the finite width of the quantum well as the Coulomb interaction gets screened. We determine finite thickness correction to the Coulomb interaction by the standard method of local density approximation and use them to calculate the critical energies such as rotons, long-wavelength, and short-wavelength modes which are detectable in inelastic light scattering experiments.

Room-temperature magnetoelectric coupling in nanocrystalline Na0.5Bi0.5TiO3

Nanocrystalline Na0.5Bi0.5TiO3 was prepared by sol-gel method. The nanocrystalline Na0.5Bi0.5TiO3 plates present room temperature ferromagnetism (FM). The reduction of FM for the plate with subsequent long time and high temperature air-annealing indicating that the observed ferromagnetism is connected with the vacancies at/near the surface of nanograins. For Na0.5Bi0.5TiO3 powders after annealing at 900 °C in air, a subsequent annealing in vacuum at 900 °C for 20 min weakens the room-temperature FM, but a subsequent annealing in oxygen atmosphere at 900 °C for 20 min enhances the room-temperature FM, which means that the observed FM in nanocrystalline Na0.5Bi0.5TiO3 may originate from cation vacancy at/near the surface of nanograins. The results of density functional theory calculation with the local density approximation plus on-site effect (LDA + U) method on the magnetism of Na0.5Bi0.5TiO3 (100) surface show that Na vacancies can introduce a nonzero magnetic moment. The Na0.5Bi0.5TiO3 plates annealed at 900 °C for 1 h present d0 multiferroicity with the coexistence of ferromagnetism and ferroelectricity at room temperature. The room-temperature magnetodielectric effect was observed in Na0.5Bi0.5TiO3 plates. In addition, the electric field treatment leads to an enormous enhancement of saturation magnetization for Na0.5Bi0.5TiO3 multiferroic plate, showing a strong magnetoelectric coupling.

Photoionization of bonding and antibonding-type atom-fullerene hybrid states in Cd@C60 vs Zn@C60

Powerful hybridization of the Cd 4d state with the d-angular momentum state of π symmetry is found in the local density approximation (LDA) structure of Cd@ ground state. The photoionization of the resulting symmetric and antisymmetric levels are computed using the time dependent LDA method to include electron correlations. Cross sections exhibit effects of the plasmonic motion coherently coupled to the diffraction-type cavity oscillations induced by local emissions from . The Cd@ results exhibit a substantial difference from our previous results for Zn@.

First-Principles Investigations on Structural, Phonon, and Thermodynamic Properties of Cubic $$\text {CeO}_{2}$$ CeO 2

The structural, phonon, and thermodynamic properties of the cubic \(\hbox {CeO}_{2}\) are investigated from first-principles calculations. The calculated lattice parameters, bulk modulus, and phonon dispersion curves are in agreement with available experimental data and other calculations. It is shown that the local density approximation (LDA)+\(U\) method is more suitable for describing the properties of \(\hbox {CeO}_{2}\) compared with the LDA method. The pressure and temperature dependences of the specific heat, Debye temperature, and the thermal expansion coefficient are successfully obtained from the Debye–Gruneisen model by combining with the phonon density of states.

Crystal electric field and the ground state properties of heavy fermion Ce3Ru4Sn13

We report on the electronic structure, electric transport and basic thermodynamic properties of the skutterudite-related Ce3Ru4Sn13 and La3Ru4Sn13. The calculated density of electronic states within the local spin density approximation (LSDA) method well fit the X-ray photoelectron spectroscopy (XPS) valence band spectra of Ce3Ru4Sn13. Magnetic susceptibility and specific heat measurements reveal that the sixfold degenerated multiplet of Ce3+ ions splits into three doublets, due to the tetragonal Ce point local symmetry in the cubic Ce3Ru4Sn13 system. Ce3Ru4Sn13 exhibits a large increase in the specific heat, C/T, data due to Kondo effect and strong electron and short-range magnetic correlations, we also suggest significant contribution of the crystal field effect. La3Ru4Sn13 is typical BCS superconductor, however, specific heat and electrical resistivity data show that La3Ru4Sn13 also exhibits a second superconducting phase at higher temperatures, which is characteristic of inhomogeneous superconductors.