Experimental Lattice Constants Research Articles

Concentration-Dependent Half Metallicity in Mo6SxI10–x Nanowires

We have investigated the structural and electronic properties of Mo6SxI10–x nanowires for 0 < x < 2 using density functional theory calculations. The optimum atomic structures and lattice constants are determined by sequential S substitution of the fully I decorated Mo6I10 nanowire. We find a good agreement with the experimental lattice constants. The nanowires with increasing sulfur content change from metallic to half-metallic, and finally to semiconductors for x = 2. The Mo6SxI10–x nanowires may find use in spintronics applications with their tunable magnetic and electronic properties.

Influence of Calcium Ions on the Structural and Magnetic Properties of Cd–Mg Ferrites Nanoparticles

Cadmium magnesium ferrites doped with calcium having the chemical formula Cd0.5Mg0.5-x Ca(x)Fe2O4 (0.0 < or = x < or = 0.3) were prepared by the Co-precipitation method. X-ray diffraction analysis confirmed the formation of a single phase with spinel crystal structure for the samples. The lattice parameter is determined for each composition and has been found to increase from 8.505 angstroms to 8.626 angstroms with increasing calcium concentration. Cation distribution for the studied ferrite system is proposed in terms of the structural and magnetic properties by means of X-ray diffraction (XRD), infrared spectroscopy (IR), vibrating sample magnetometer (VSM) and is found to be reliable. The experimental and theoretical lattice constants show the same trend with increasing calcium concentration indicating the validity of the proposed cation distribution. The analysis of infrared spectra indicates the presence of splitting in the absorption band which may be attributed to the presence of small amounts of Fe2+ ions in the ferrite system. The appearance of a shoulder around 700 cm(-1) suggests the presence of calcium ions in the tetrahedral site. The addition of non magnetic calcium ions in the ferrites suppressed the A-interaction and developed a B-B interaction, which is reflected in reducing the saturation magnetization in the present samples. The coercive field (H(c)) is also found to increase by increasing of Ca2+ concentration and has been explained on the bases of direct relationship with anisotropy constant.

Electronic, crystal structure and morphological properties of the Sr2DyRuO6 double perovskite

Electronic, crystal structure and the surface morphology of the Sr2DyRuO6 double perovskite, has been investigated. Measurements of X-ray diffraction and Rietveld analysis show that this compound crystallizes in a perovskite monoclinic structure which belongs to the P21/n (#14) space group and 1:1 ordered arrangement of the Ru5+ and Dy3+-cations over the six-coordinate M′ sites, with lattice parameters a=5.7774(2)Å, b=5.7948(2)Å, c=8.1848(2)Å, angle β=90.181(3)° and V=276.88(1)Å3. A morphological characterization of material was performed by Scanning Electron Microscopy (SEM). Results show the granular feature of compound with particle size D=37.2nm. Semi-quantitative compositional studies were carried out from Energy Dispersive X-ray experiments (EDX), and calculations of electronic structure were performed by Density Functional Theory (DFT). Density of states (DOS) study was carried out considering the two spin polarizations, and spin-polarized DOS was obtained from the experimental lattice constants. Results show that the Ru5+ and Dy3+ are responsible of magnetic character of the Sr2DyRuO6.

Characteristic atom sequences of Nb–Mo alloys system in BCC structure and properties of disordered alloys

Comprehensively considering energy, volume and electronic structure of alloys, the ninth equation was determined as the interaction function of Nb-Mo alloys system in BCC structure on the basis of idea of systematic science of alloys, experimental lattice constants and heats of formation of disordered Nb(1-x)Mox alloys. The structural parameters and properties of Nb and Mo characteristic atoms sequences and corresponding characteristic crystals sequences were determined in Nb-Mo alloys system. The electronic structure and physical properties of disordered Nb(1-x)Mox alloys system were calculated according to concentration of characteristic atoms of disordered alloys. The change trend of physical properties is the same as that of electronic structure.

First principle electronic, structural, elastic, and optical properties of strontium titanate

We report self-consistent ab-initio electronic, structural, elastic, and optical properties of cubic SrTiO3 perovskite. Our non-relativistic calculations employed a generalized gradient approximation (GGA) potential and the linear combination of atomic orbitals (LCAO) formalism. The distinctive feature of our computations stem from solving self-consistently the system of equations describing the GGA, using the Bagayoko-Zhao-Williams (BZW) method. Our results are in agreement with experimental ones where the later are available. In particular, our theoretical, indirect band gap of 3.24 eV, at the experimental lattice constant of 3.91 Å, is in excellent agreement with experiment. Our predicted, equilibrium lattice constant is 3.92 Å, with a corresponding indirect band gap of 3.21 eV and bulk modulus of 183 GPa.

HSE hybrid functional within the FLAPW method and its application to GdN

We present an implementation of the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional within the full-potential linearized augmented-plane-wave (FLAPW) method. Pivotal to the HSE functional is the screened electron-electron interaction, which we separate into the bare Coulomb interaction and the remainder. Both terms give rise to exchange potentials, which sum up to the screened nonlocal exchange potential of HSE. We evaluate the former with the help of an auxiliary basis, defined in such a way that the bare Coulomb matrix becomes sparse. The latter, which is a slowly varying function in real space, is computed efficiently in reciprocal space. This approach is general and can be applied to a whole class of screened hybrid functionals. We obtain excellent agreement of band gaps and lattice constants for prototypical semiconductors and insulators with electronic-structure calculations using plane-wave or Gaussian basis sets. We apply the HSE hybrid functional to examine the ground-state properties of rocksalt GdN, which have been controversially discussed in literature. Our results indicate that there is a half-metal to insulator transition occurring between the theoretically optimized lattice constant at $0\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and the experimental lattice constant at room temperature. Overall, we attain good agreement with experimental data for band transitions, magnetic moments, and the Curie temperature.

Assessing the accuracy of hybrid functionals in the determination of defect levels: Application to the As antisite in GaAs

The accuracy of nonscreened and screened hybrid functionals for the calculation of defect levels within the band gap is assessed for the As antisite in GaAs, the nature of which is well characterized experimentally. A set of functionals differing by the fraction of nonlocal Fock exchange or by the screening length are examined. The $+2/\phantom{\rule{-0.16em}{0ex}}\phantom{\rule{-0.16em}{0ex}}+\phantom{\rule{-0.16em}{0ex}}1$ and the $+1/0$ charge transition levels as determined with any of the considered functionals line up when referred to the average electrostatic potential, with a defect-level separation in fair agreement with the experimental value. When the band gap is well reproduced, the calculated defect levels are found within 0.2 eV of the experimental levels. The use of the theoretical rather than the experimental lattice constant for the bulk host is found to affect the defect levels by less than 0.1 eV.

Volume dependence of the exchange interaction and Curie temperature in Co2MGa (M = Ti and Fe): A first-principles study

Magnetic moment, exchange interaction and Curie temperature (TC) have been calculated for Co2TiGa and Co2FeGa by a first-principles density functional calculation combined with a linear response method. The exchange interaction is dominated by Co-Co pairs in Co2TiGa while that of Co2FeGa is mainly contributed by Fe-Co pairs. Based on the mean field multiple-sublattices model, the estimated TC is about 114 K for M = Ti and 1270 K for M = Fe, calculated with the experimental lattice constant, in good agreement with the experimental values (128 K and 1093 K for M = Ti and Fe, respectively). With increasing lattice constant, a, from 95% to 105% of the experimental value (aexp.), the moment per formula unit mf.u. changes from 0.43 μB to 1.0 μB and TC increases from 27 K to 142 K in Co2TiGa. However, mf.u. increases slightly from 4.98 μB to 5.40 μB while TC decreases from 1330 K to 1190 K with increasing a from 95% to 105% of aexp. in Co2FeGa. These different volume dependences of TC are ascribed to the weak ferromagnetism in Co2TiGa and the strong ferromagnetism in Co2FeGa.

Structural and Elastic Properties of Li-Ni Ferrite

Li-Nickel ferrites with the chemical formula () have been prepared by the ceramic method. The spinel structure in homogenous state was realized by X-ray diffraction analysis. The lattice parameter has been determined for each composition and found to be nearly constant over the whole range of Ni concentration (= 0.83 nm ± 0.01). The cation distribution for each composition has been suggested. The experimental and theoretical lattice constants were found to be in good agreement with each other confirming the agreeability of the suggested cation distribution. The analysis of IR spectra indicates the presence of splitting in the absorption band due to the presence of small amounts of Fe2+ ions in the ferrite system. The force constants for tetrahedral and octahedral sites have been determined. Young’s modulus (E), rigidity modulus (G), bulk modulus (B), Debye temperature (), and transverse () and longitudinal () wave velocities have been determined. The variation of elastic moduli with composition has been interpreted in terms of binding forces between the atoms of spinal lattice.

Exchange interaction in GdGa from first-principles

The electronic structure and magnetic properties for GdGa have been studied from a first-principles density functional calculation. The energy band structure has been calculated in a local spin density approximation (LSDA), plus Hubbard U approach (LSDA+U). For Gd atoms, seven spin up 4f bands are fully occupied and situated at the bottom of Ga 4s states, while the spin down 4f hole levels are completely unoccupied and well above the Fermi level ( E f ). The p- and d-like states dominate at E f . The calculated magnetic moment is 7.37 μ B per formula unit (f.u.) and is not sensitive to the change of the unit cell volume. The effective exchange parameters, J 0, decrease from 2.6 to 0.9 mRy with increasing lattice volume from 35.7 to 55.3 Å 3/f.u., resulting in a pressure induced enhancement of the Curie temperature ( T C ). With the experimental lattice constants, the calculated mean field T C is 187 K, in good agreement with the experimental value ( T C exp. =183 K).

Convergence of the Møller-Plesset perturbation series for the fcc lattices of neon and argon

Complete basis set limit calculations are carried out for the fcc lattices of solid neon and argon, using second- to fourth-order Moller-Plesset theory, MP2–MP4, and coupled-cluster calculations, CCSDT ,t o describe electron correlation within a many-body expansion of the interaction potential up to third order. A correct description of the three-body Axilrod-Teller-Muto term for the solid state is only obtained from third order on in the many-body expansion of the correlation energy, correcting the severe underestimation of long-range three-body effects at the MP2 level of theory. MP4 shows good agreement with the CCSDT results, and the latter are in good agreement with experimental lattice constants, cohesive energies, and bulk moduli. However, with increasing pressures the convergence of the Moller-Plesset series deteriorates as the electronic band gap decreases, resulting in rather large deviations for the equation of state pressure-volume dependence. For neon, however, the errors in the MP2 two- and three-body terms almost cancel, i.e., at a volume of V =3 cm 3 / mol the MP2 pressure is underestimated by only 1 GPa compared to the pressure of P = 251 GPa calculated at the CCSDT level of theory. In contrast, for argon this is not the case, and at V = 5.5 cm 3 / mol the calculated MP2 pressure of 228 GPa deviates substantially from the CCSDT result of 252 GPa.

Lattice dynamics in the itinerant helical magnet MnSi

The phonon dispersion relations in MnSi were measured using inelastic neutron scattering. At the same time, calculations of the lattice dynamics of MnSi were carried out in the framework of the density-functional theory using both the local-density and the generalized gradient approximations. The calculations match most of the phonon modes in the frequency range up to 40 meV, if they are performed using the experimental lattice constant. Spin-polarized calculations for ferromagnetic ground states result in subtle modifications of the dispersion curves, which further improve the description, in particular, for the acoustic branches.

Ab initio calculations for properties of Ti 2AlN and Cr 2AlC

The electronic structure, dielectric function, Debye temperature and elastic properties of Ti 2AlN and Cr 2AlC were studied by means of pseudo-potential plane-waves method using the density function theory. The exchange-correlation approximation energy was determined by generalized gradient approximation method. The experimental lattice constants and the internal parameters were used in the calculation. The dielectric function was calculated by energy band theory, and the elastic constants were obtained using the static finite strain technique. The bulk and shear module, Young’s module and Poisson’s ratio for ideal polycrystalline Ti 2AlN and Cr 2AlC aggregates were derived in this paper. The Debye temperature of Ti 2AlN and Cr 2AlC from the average sound velocity was also estimated. The obtained results are in agreement with the available theoretical data.

Bond-Energy and Surface-Energy Calculations in Metals

A simple technique appropriate for introductory materials science courses is outlined for the calculation of bond energies in metals from lattice energies. The approach is applied to body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal-closest-packed (hcp) metals. The strength of these bonds is tabulated for a variety metals and is seen to be somewhat smaller than those typically found for the covalent bonds in molecular substances or polymers. The calculated bond energies are then combined with experimental lattice constants to illustrate the estimation of the surface energy of metals with these three crystal structures. The agreement with experiment is reasonably good, given the difficulties inherent in measuring the surface tension of metals and the considerable extrapolation required for surface-energy calculation.

Design of a Versatile Force Field for the Large-Scale Molecular Simulation of Solid and Liquid OMCTS

We developed a new, versatile force field for the molecular simulation of octamethylcyclotetrasiloxane (OMCTS) both in the solid and liquid phases. From a series of molecular dynamics simulations, we obtain good agreement with the experimental lattice constants, sublimation enthalphy, and molecular packing of the crystal. The experimental density, diffusion coefficient, and shear viscosity of this van der Waals liquid in the range 300−440 K are well reproduced as well. The new force field can be thus employed in the large-scale molecular simulation of liquid OMCTS where structural details are important in determining the collective properties of the system.

Periodic density functional theory study of spin crossover in the cesium iron hexacyanochromate prussian blue analog

This paper presents a detailed theoretical analysis of the electronic structure of the CsFe[Cr(CN)(6)] prussian blue analog with emphasis on the structural origin of the experimentally observed spin crossover transition in this material. Periodic density functional calculations using generalized gradient approximation (GGA)+U and nonlocal hybrid exchange-correlation potentials show that, for the experimental low temperature crystal structure, the t(2g)(6) e(g)(0) low spin configuration of Fe(II) is the most stable and Cr(III) (S = 3/2, t(2g)(3) e(g)(0)) remains the same in all cases. This is also found to be the case for the low spin GGA+U fully relaxed structure with the optimized unit cell. A completely different situation emerges when calculations are carried out using the experimental high temperature structure. Here, GGA+U and hybrid density functional theory calculations consistently predict that the t(2g)(4) e(g)(2) Fe(II) high spin configuration is the ground state. However, the two spin configurations appear to be nearly degenerate when calculations are carried out for the geometries arising from a GGA+U full relaxation of the atomic structure carried out at experimental high temperature lattice constant. A detailed analysis of the energy difference between the two spin configurations as a function of the lattice constant strongly suggests that the observed spin crossover transition has a structural origin with non-negligible entropic contributions of the high spin state.

Long-range Finnis-Sinclair potential for molecular dynamics simulation of α-Al2O3

The molecular dynamics （MD） simulation of metal-metal oxide interface requires a uniform potential to simultaneously describe metal and metal oxide. Accordingly, we obtained a set of long-range Finnis-Sinclair （F-S） potential parameters of α-Al2O3. All of the parameters were fitted to the targets, i.e. the experimental lattice energy, lattice constants, and elastic constants of α-Al2O3. Meanwhile, we compared our results with those reported by EAM, Glue and modified Matsui （m-Matsui） potentials and found that our rcsults are equivalent or better. After that, MD simulation of α-Al2O3 at 300 K with our long-range F-S potential was performed and the pair correlation functions, coordination numbers were calculated. The good agreement between calculation results and experiments validated the feasibility of this set of F-S potential parameters to the description of α-Al2O3 system.

Ab initioinvestigation of intermolecular interactions in solid benzene

A computational strategy for the evaluation of the crystal lattice constants and cohesive energy of the weakly bound molecular solids is proposed. The strategy is based on the high level ab initio coupled-cluster determination of the pairwise additive contribution to the interaction energy. The zero-point-energy correction and non-additive contributions to the interaction energy are treated using density functional methods. The experimental crystal lattice constants of the solid benzene are reproduced, and the value of 480 meV/molecule is calculated for its cohesive energy.

Calculated electronic properties and structural phase transitions of GdN pnictide under hydrostatic pressure

The full-potential linear augmented plane wave method has been used to study the electronic structure, the structural phase transitions, the ground-state x-ray absorption spectroscopy XAS , and the x-ray magnetic circular dichroism XMCD of GdN pnictide under hydrostatic pressure. To produce a satisfactory excited state used to calculate the XAS, it was necessary to describe the Gd 4f electron-electron interaction within the so-called LDA+U method in the mean field approximation. We have shown that the band structure of GdN pnictide is that of a half-metal for the experimental lattice constant. The 5d orbitals of the Gd sites which dominate the GdN low-lying conduction bands are investigated by means of XAS and XMCD calculations, and the results of the L2,3 XAS and XMCD spectra are found to be in good agreement with the experimental results. Total energy calculations performed for the rocksalt, the wurtzite, the zinc blende, and the CsCl structures reveal the richness of the electronic structure of GdN and predict two structural phase transitions: from rocksalt to zinc blende structure for the extended lattice parameter, and from half-metallic rocksalt to a semiconducting wurtzite structure under hydrostatic pressure.

Equation of state and thermodynamic functions for the fcc soft-core multiple-Yukawa solid and application to fullerenes with compressible molecular radius

The generalized free volume theory is applied to the soft-core multiple-Yukawa solid, and the hard-core multiple-Yukawa is included in as a special case. The expressions for equation of state and internal energy are derived. The formalism developed is applied to the ${\mathrm{C}}_{60}$, ${\mathrm{C}}_{76}$, and ${\mathrm{C}}_{84}$ solids. The effective diameter of ${\mathrm{C}}_{60}$ molecule is taken as the experimental value; the parameters of the double Yukawa (DY) potential for carbon-carbon atoms are determined through fitting the experimental data of cohesive energy, the lattice constant, and the compression curve of ${\mathrm{C}}_{60}$ solid at ambient temperature. The effective diameter of ${\mathrm{C}}_{76}$ and ${\mathrm{C}}_{84}$ molecules are determined through fitting the experimental lattice constants at ambient temperature. The numerical results of ${\mathrm{C}}_{60}$ solid from the soft-core DY potential are in good agreement with the experiments, including the lattice constant and compression curve. The lattice constant versus temperature relationship for ${\mathrm{C}}_{76}$ and ${\mathrm{C}}_{84}$ solids calculated from the DY potential is qualitatively in accordance with experimental data as same as the Girifalco potential. The compression curve of the ${\mathrm{C}}_{84}$ solid calculated from the DY potential deviates from and is softer than the experimental data available. The reason for deviation is discussed, and it is concluded that the influence of compressibility of fullerene molecules to thermophysical quantities is important at high-pressure conditions.