LMTO-ASA Method Research Articles

Positron lifetime spectroscopy applied to pure sulphur and selenium

High purity crystalline orthorhombic Sulphur and hexagonal Selenium have been investigated using Positron Lifetime Spectroscopy technique. Annealed for 3 h at 120 °C Selenium lifetime (308(1) ps) has been compared with previous Positron Lifetime Spectroscopy knowledge in order to validate the measurements and the analysis procedure, and also compared with theoretical calculations available in literature, including different methods and parametrizations, resulting on the best match of LMTO-ASA method with GGA parametrization. Annealed for 3 h at 80 °C Sulphur samples were studied by the same means providing a single lifetime component with value 300(1) ps that has been assigned to the positron annihilation in the bulk that is not present in the current literature.

Electron and phonon spectra dynamics and features of phase transitions in sodium at P=0-100 GPa

An electron and phonon spectra dynamics, features of structural phase transitions, and melting of sodium under the pressure ranging from 0 to 100 GPa are investigated. The electron and the phonon spectra of crystal sodium are calculated ab initio within the density functional theory by means of the software package LmtART-7 (see [1-3] and references herein), allowing the fully potential method of linear muffin-tin orbitals (FP-LMTO method). Earlier, this method was used in papers [4-6] for the calculation of a metals band structure within an atomic-spheres approximation (LMTO-ASA method). Using the Lindemann measure and the calculated phonon spectra, the theoretical values of melting points corresponding to the experimental data are obtained. Features of the electron and the phonon spectra dynamics in the melting curve maximum vicinity and within the structural transition range are discussed cI2 → cF4.

A new tetragonal structure type for Li2B2C.

The ternary dilithium diboron carbide, Li2B2C (tetragonal, space group P-4m2, tP10), crystallizes as a new structure type and consists of structural fragments which are typical for structures of elemental lithium and boron or binary borocarbide B13C2. The symmetries of the occupied sites are .m. and 2mm. for the B and C atoms, and -4m2 and 2mm. for the Li atoms. The coordination polyhedra around the Li atoms are cuboctahedra and 15-vertex distorted pseudo-Frank-Kasper polyhedra. The environment of the B atom is a ten-vertex polyhedron. The nearest neighbours of the C atom are two B atoms, and this group is surrounded by a deformed cuboctahedron with one centred lateral facet. Electronic structure calculations using the TB-LMTO-ASA method reveal strong B...C and B...B interactions.

Bonding in intermetallics may be deceptive – The case of the new type structure Au2InGa2

We have investigated the crystal and the electronic structure of a new ternary phase, Au2InGa2 using powder and single crystal X-Ray Diffraction (XRD) and Stuttgart Tight Binding LMTO-ASA methods, respectively. Complementary methods including Thermo-Gravimetric Differential Thermal Analysis (TG-DTA), Energy Dispersive X-ray Spectroscopy (EDS) and Scanning Electron Microscopy (SEM) were also employed. As a result, the complete structure was determined and the role of homoatomic and heteroatomic bonding is discussed.

First-principles theoretical studies of half-metallic ferromagnetism in CrTe

Using full-potential linear augmented plane-wave method (FP-LAPW) we have studied the stability and electronic properties of the chalcogenide CrTe in three competing structures: rocksalt (RS), zinc blende (ZB), and the NiAs-type (NA) hexagonal. Although the ground state is of NA structure, RS and ZB are interesting in that these fcc-based structures, which can perhaps be grown on various semiconductor substrates, exhibit half-metallic (HM) phases above some critical values of the lattice parameter. We find that the NA structure is not HM at its equilibrium volume while both ZB and RS structures are. The RS structure is more stable than the ZB with an energy that is lower by 0.25 eV/atom. While confirming previous results on the HM phase in ZB structure, we provide hitherto unreported results on the HM RS phase, with a gap in the minority channel and a magnetic moment of $4.0\text{ }{\ensuremath{\mu}}_{B}/\text{f}\text{.u}\text{.}$ A comparison of total energies for the ferromagnetic (FM), nonmagnetic, and antiferromagnetic (AFM) configurations shows the lowest energy configuration to be FM for CrTe in all the three structures. The exchange interactions in the RS and ZB structures are studied for a wide range of the lattice parameter using the linear-response method and a mapping of the total energy to the classical Heisenberg model. These linear-response calculations are performed in the linear muffin-tin orbitals (LMTOs) basis, using the atomic sphere approximation (ASA). We have verified that the results of the electronic structure obtained via the LMTO-ASA method under local-density approximation (LDA) and $\text{LDA}+U$ schemes are in close agreement with those obtained via the more accurate FP-LAPW method. The results show that the exchange interactions in the RS structure are much more short ranged than in the ZB structure. Hence, for the RS structure the exchange interactions are also studied by using a nearest- and next-nearest-neighbor $({J}_{1}\text{\ensuremath{-}}{J}_{2})$ model and the energy differences between FM and two AFM states. These ${J}_{1}\text{\ensuremath{-}}{J}_{2}$ model results are obtained by using both the FP-LAPW and LMTO-ASA methods and compared with the linear-response results. The calculated Curie temperatures for the RS phase are consistently higher than those for the ZB phase.

Peculiarities of antiferromagnetic ordering in orthorhombic LiMnO2

Data on the antiferromagnetic ordering in orthorhombic lithium manganite LiMnO2 are obtained from magnetic-susceptibility, calorimetry, and nuclear magnetic resonance studies. The minimal hysteresis and the absence of jumps in the temperature dependences of the sublattice magnetization M(T) and the magnetic susceptibility near TN indicate that the ordering occurs through a continuous second-order phase transition. Within the critical temperature range, the M(T−TN) variation is satisfactorily described by a power-law dependence with a critical exponent β = 0.25(4), which is substantially smaller than that predicted for 3D magnetic systems with isotropic Heisenberg exchange. The band structure of orthorhombic LiMnO2 is calculated using the LMTO-ASA method. Taking into account the spin states of manganese ions, an adequate pattern is obtained for the density-of-states distribution with an energy gap near the Fermi level (∼0.7 eV), which is in agreement with the measured electrical parameters of lithium manganite. The calculations demonstrate that the exchange interactions between Mn3+ ions leading to antiferromagnetic ordering are significantly anisotropic. It is found that small paramagnetic regions persist in the manganite below the Neel temperature, and it is concluded that the reason for this is partial structural disordering of LiMnO2. As a result, a certain fraction of the manganese positions is occupied by lithium ions (LiMn) and vise versa (MnLi). These defects are not involved in the formation of the ordered magnetic structure and compose a paramagnetic fraction.

Theoretical and experimental investigation of the influence of external effects on the electronic structure of Ce-based heavy-fermion systems

The electronic structure of Ce systems with different degrees of hybridization of f electrons with outer-shell electrons and the effect of pressure on the change in the electronic structure has been investigated. A correlation between the parameters of X-ray photoelectron spectra and the magnitude of the heavy-fermion state is established. The total and partial densities of states of atoms were calculated using the TB LMTO ASA method. X-ray photoelectron study of the effect of an X-ray induced vacancy in the inner levels on the shape of the Ce3d spectra for Ce-based systems with different degrees of covalence has been performed. It is shown that there is a relationship between the magnitude of the heavy-fermion state and the intensity of the shakedown satellites in X-ray photoelectron spectra and that the magnitude of the heavy-fermion state decreases under pressure.

Optical and electronic properties of (GaAs)n / (InAs)n(001) superlattices: LMTO-ASA approach

Abstract The optical and electronic properties of (GaAs)n/(InAs)n superlattices are calculated by means of LMTO-ASA method. The too small band gap problem of bulk material and superlattices is corrected by adding to the effective potentials an additional external potential that is sharply peaked at the atomic sites. The results show that the optical properties of GaAs/InAs(001) superlattices are about average of that of two bulks of GaAs and InAs.

Effect of unoxidized residual Al at the boundary of Co/Al-oxide/Co junction on TMR estimated by LMTO band calculation

The magneto-resistance effect of Co/Al-oxide/Co tunneling junction is evaluated by a first-principles band calculation. To clarify the effect of unoxidized Al in the Al-oxide layer on the Co layer, we calculated the electronic structures of the junction with a local density approximation using the LMTO-ASA method. Periodic superlattices which consist of layers separated by vacant layers were used for the conventional calculation method. The magneto-resistance ratio, Δ R/ R, was estimated by the equation Δ R/ R = 2 P 1 P 2/( P 1 + P 2), where P 1 is the polarization of a boundary Co atom on one side of a layer and P 2 is the polarization of a Co or Al atom with a vacant layer on the other side. The polarization of Co atoms near the boundary between the Co and Al layers decreases greatly due to the paramagnetic Al, although the Al was polarized by the ferromagnetic Co. These changes of polarization affect the magneto-resistance ratio with calculations indicating that the ratio is reduced by half for two atomic layers of residual Al.

Magnetic properties of Cr–CrH and YFe 2–YFe 2H 4

Modifications of the electronic and magnetic properties of Cr and YFe 2 due to hydrogen absorption were studied using band structure calculations with the LMTO–ASA method. For chromium, we show that the presence of hydrogen leads to a disappearance of antiferromagnetism. The paramagnetic state is the stable state of the hexagonal hydride CrH, in agreement with experimental results. We analyzed the role of the structural modifications and volume expansion of the Cr lattice and also the role of hydrogen on the electronic and magnetic properties. The increase of the density of states at the Fermi level from Cr to CrH is in agreement with measurements of heat capacity and magnetic susceptibility. The stronger magnetization in ferromagnetic YFe 2H 4 compared to YFe 2 was explained by two antagonistic effects: (i) the lattice expansion due to hydrogen insertion which increases the magnetic moments M S at the Fe sites, (ii) the metal-hydrogen interaction which slightly reduces M S . In the hydride, the majority spin states of Fe are fully filled and the Fermi energy falls in a peak of the minority spin density of states. We found a small ferrimagnetic coupling of the 4d states of yttrium. Our results are in satisfactory agreement with experimental magnetic data.

Evolution of the d ∥ band across the metal–insulator transition in VO 2

We studied the evolution of the electronic structure of VO 2 across the metal–insulator transition. The electronic structure was calculated using the standard TB–LMTO–ASA method. The calculated DOS was compared to previous photoemission and X-ray absorption spectra. The electronic structure is discussed in terms of the usual molecular–orbital scheme. In the metallic phase, the d ∥ band appears at the bottom of the V 3d bands and crosses the Fermi level. In the insulating phase, the d ∥ band is split around 2 eV opening a pseudo band gap at the Fermi level. The largest effect of the splitting appears in the unoccupied part of the d ∥ band. The calculated value of the splitting accounts for 77% of the experimental value, 2.6 eV. The results suggest that electron–lattice interaction seems to be the dominant factor in the splitting of the d ∥ band.

Magnetic states of MgCo 2 and CaCo 2 with the cubic and hexagonal Laves phase structures

Electronic structures of MgCo 2 and CaCo 2 in the cubic C15-type and hexagonal C14-type Laves phases are calculated in a self-consistent LMTO-ASA method, including the correction of the GGA, for the paramagnetic, ferromagnetic and antiferromagnetic states. By the comparison of calculated total energies, the ground states of MgCo 2 and CaCo 2 are shown to be ferromagnetic in the C14-type structure and in the C15-type one, respectively, being consistent with the observed results. A good agreement between the calculated and observed values of lattice constant and magnetic moment is obtained for MgCo 2, but the agreement between them is not so good for CaCo 2.

First-principles calculations of energetics of sigma phase formation and thermodynamic modelling in Fe–Ni–Cr system

A combination of full-potential linear augmented plane-wave (FLAPW) electronic structure method (for elemental constituents) with simpler linear muffin-tin orbital method in the atomic-sphere approximation (LMTO-ASA method, for mixtures) was employed for nickel systems (Ni–Fe and Ni–Cr) to calculate the energy of formation of sigma phase with respect to standard element reference (SER) structures of pure constituents. In all cases, the optimisation of the equilibrium volume of sigma phase and of the SER structures was performed. Calculated energies of formation of sigma phase with respect to SER state of pure constituents in Ni–Cr and Ni–Fe systems were compared with the results of thermodynamic modelling. Phase diagram of ternary Fe–Ni–Cr system determined with the help of ab initio calculated sigma phase parameters in pure elements are in a good agreement with experimental data. Enthalpies of formation of sigma phase in Ni–Cr and in Fe–Ni systems obtained by thermodynamic modelling agree reasonably well with the energy of formation in these systems, calculated ab initio.

Atomic and electronic structure determination for a series of Al–Re–Si MI-type 1/1-cubic approximants

The crystalline structure of the Mackay-Icosahedral type Al–Re–Si 1/1-cubic approximants was determined by Rietveld analysis on the powder diffraction spectra taken with synchrotron radiation. The electronic structure of this approximant was calculated by the LMTO-ASA method with the experimentally determined atomic structure. One of the Al–Re–Si 1/1-cubic approximants, Al 73. 6Re 17. 4Si 9, was found to have a disorder-free structure in sharp contrast with other 1/1-approximants. This disorder-free structure leads to a positive temperature coefficient of the electrical resistivity and a relatively low residual resistivity. We evaluated the mean free path by substituting the electronic density of states at the Fermi level, the averaged Fermi velocity and the measured resistivity into the Boltzmann transport equation. The resulting mean free paths of 58 Å at 5 K and 21 Å at room temperature show a good consistency with the Boltzmann transport mechanism for this disorder-free complex compound.

Electronic calculations on rutile VO 2 by the LMTO-ASA method

In order to get a good overall description of the electronic structure of VO 2, it has been calculated self-consistently from first principles using the semirelativistic linear muffin tin method (LMTO) in the atomic sphere approximation (ASA), including the 2s states of oxygen as strongly bound band states participating in the charge transfer. The basis set is completed with s and p functions in ‘empty spheres’ inserted in the open parts of the structure. The band results, analysed in terms of the site and angular momentum decomposed density of states and joint photoemission spectra, are quite consistent with the experimental observations.

X-ray absorption near-edge structure of selenium inthe Cu-In-Se system

The x-ray absorption near edge structure (XANES) of selenium is investigatedin the crystals with compositions from the pseudobinary cut lineCu2Se-In2Se3. This includes CuInSe2, indium-rich ternary compounds (Cu2In4Se7,CuIn3Se5, CuIn5Se8, CuIn7Se11) and α-In2Se3. The absorption at the K andL3/L2edges of selenium has been measured using synchrotron radiation. Twotheoretical approaches are used to the interpretation of the experimentaldata: the band structure calculation and the real-space multiple-scattering(RSMS) method. In the first one, the angular momentum projected densities ofstates at Se sites are calculated for CuInSe2 and α-In2Se3 for the energies upto 17 eV above the conduction band minimum by the LMTO-ASA method. The RSMSapproach represented by the FEFF8 code is used to calculate the XANES spectrafor the phases with tetragonal symmetry. Clusters up to 160 atoms are used inthe calculations. The influence of different structural factors on theselenium XANES is studied.

Magnetism and Electronic Structure for (Sc1-xTix)Fe2with the Hexagonal Laves Phase (C14) Structure

The electronic structure of (Sc 1- x Ti x )Fe 2 with the hexagonal Laves phase (C14) structure is calculated self-consistently by LMTO-ASA method and the fixed spin-moment procedure. Ferromagnetic solutions with high- and low-moments are obtained together with the antiferromagnetic one over a wide range of composition and volume. It is shown that the magnetic moment of Fe on the 2a site in TiFe 2 vanishes in the low-moment ferromagnetic state, just as in the antiferromagnetic state, and grows up nearly to the size in the high-moment ferromagnetic state with increasing Sc concentration. The energy difference between the two ferromagnetic states is small and depends sensitively on the change in volume and leads to a considerable magnetovolume effect. A metamagnetic transition from the antiferromagnetic state into the high-moment ferromagnetic state is expected at the critical magnetic field of 145 T for TiFe 2 . The composition dependence of the ferromagnetic moment for Ti 1- y Fe 2+ y near the stoichiometr...

Optical interband spectra and band structure of Ru2Si3 and Ru2Ge3

Among the semiconducting silicides Ru2Si3 is considered to be an attractive material for optoelectronic and thermoelectric applications. However, only few papers on the electronic properties exist. Here we report results of optical investigations of this semiconducting material and its analogon Ru2Ge3 by spectroscopic ellipsometry in the spectral range of 0.7–10 eV and by UV–vis–near infrared (NIR) transmission and reflectivity measurements. The optical spectra for Ru2Si3 and Ru2Ge3 exhibit semiconducting character with dominant interband features around 2 and 5 eV and a relative minimum in the joint density of interband states between 3 and 4 eV. More pronounced structures were found for single crystalline samples than for solid phase crystallized thin films. The dielectric functions are compared with theoretical calculations of the optical properties which have been carried out using the LMTO-ASA method. Good agreement has been found between theory and experiment for the spectral distribution of optical constants.

Electronic structure and stability of Li1 + xMn2 − xO4 spinels

Trends in the electronic structure of the spinel-type manganese oxide Li1 + xMn2 − xO4, 0 ≤ x ≤ 1/4, are studied using the LMTO–ASA method. The stability of the system is discussed in terms of composition (x) and the position of lithium in the structure; the influence of the latter on the total valence energy and the density-of-states (DOS) is also probed systematically. The 'extra' lithium atoms are shown to prefer the octahedral 16d position for x < 2/16, and the octahedral 16c position for higher degrees of substitution. The open-circuit voltage reaches a maximum for x = 1/16.

Determination of Spatially Hybridized Charge Distribution and its Effect on Electron Transport in the Al-Cu-Ru-Si 1/1-Approximant ---Theoretical Basis for the Hume-Rothery rule---

AbstractThe origin of the pseudogap across the Fermi level was investigated by analyzing the electronic structure calculated in the framework of the LMTO-ASA method for the RT-type Al-Mg-Zn and MI-type Al-Cu-Ru-Si 1/1-approximants. The pseudogap in the former is proved to originate from the interaction of electronic states with the Brillouin zone planes associated with reciprocal lattice vectors matching with the Fermi sphere in the extended zone scheme. In the latter, the Fermi surface-Brillouin zone interaction coupled with the hybridization effect between the Al-3p and transition metal d-states produces a deep pseudogap at the Fermi level. The real-space charge distribution for electrons at the Fermi level is calculated for the Al-Cu-Ru-Si 1/1-approximant. The charge distribution thus obtained could explain not only the possession of a large resistivity of this approximant but also evidenced that the icosahedral clusters play a key role in reducing the electronic energy in favor of quasicrystalline and approximant phases.