Full-potential Linear Muffin-tin Orbital Method Research Articles

Thermal properties of Al at high pressures and temperatures

Thermal properties of the face-centered-cubic (fcc) aluminum (Al) crystal including the linear thermal expansion coefficient, specific heat at constant volume, Hugoniot in the $P\text{\ensuremath{-}}V$ plane, thermodynamic Gr\"uneisen parameter and elastic constants at pressures up to $120\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and temperatures to $3300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ have been evaluated by using the full-potential linear muffin-tin orbital (FPLMTO) total-energy method combining with a mean-field model of the vibrational partition function. The mean-field is constructed from the sum of all the pair potentials between the reference atom and the others of the system. The calculated properties are in good agreement with available static and shock-wave experimental measurements.

Preparation, Electronic Structure and Optical Properties of the Electrochromic Thin Films

A new multilayer electrochromic device has been constructed according to the following pattern: glass1/ITO/WO3/gel electrolyte/BP/ITO/glass2, where ITO is a transparent conducting film made of indium and tin oxide and with the surface resistance equal 8–10 Ω/cm2 . The electrochromic devices obtained in the research are characterized by great (considerable) transmittance variation between coloration and bleaching state (25–40% at applied voltage of 1.5 to 3 V), and also high coloration efficiency (above 100 cm2 /C). Selfconsistent energy bands, dielectric permittivity and optical parameters are calculated using a full-potential linear muffin-tin orbital method. The numerical solution of the Debye-Smoluchowski equations is developed for simulating recombination probability of Li+ ions in amorphous electrolyte.

SrTiO3(001)(2×1)reconstructions: First-principles calculations of surface energy and atomic structure compared with scanning tunneling microscopy images

$(1\ifmmode\times\else\texttimes\fi{}1)$ and $(2\ifmmode\times\else\texttimes\fi{}1)$ reconstructions of the (001) $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ surface were studied using the first-principles full-potential linear muffin-tin orbital method. Surface energies were calculated as a function of $\mathrm{Ti}{\mathrm{O}}_{2}$ chemical potential, oxygen partial pressure ${p}_{{\mathrm{O}}_{}2}$and temperature. The $(1\ifmmode\times\else\texttimes\fi{}1)$ unreconstructed surfaces were found to be energetically stable for many of the conditions considered. Under conditions of very low oxygen partial pressure the $(2\ifmmode\times\else\texttimes\fi{}1)$ ${\mathrm{Ti}}_{2}{\mathrm{O}}_{3}$ reconstruction [Martin R. Castell, Surf. Sci. 505, 1 (2002)] is stable. The question as to why STM images of the $(1\ifmmode\times\else\texttimes\fi{}1)$ surfaces have not been obtained was addressed by calculating charge densities for each surface. These suggest that the $(2\ifmmode\times\else\texttimes\fi{}1)$ reconstructions would be easier to image than the $(1\ifmmode\times\else\texttimes\fi{}1)$ surfaces. The possibility that the presence of oxygen vacancies would destabilise the $(1\ifmmode\times\else\texttimes\fi{}1)$ surfaces was also investigated. If the $(1\ifmmode\times\else\texttimes\fi{}1)$ surfaces are unstable then there exists the further possibility that the $(2\ifmmode\times\else\texttimes\fi{}1)$ DL-$\mathrm{Ti}{\mathrm{O}}_{2}$ reconstruction [Natasha Erdman et al. Nature (London) 419, 55 (2002)] is stable in a $\mathrm{Ti}{\mathrm{O}}_{2}$-rich environment and for ${p}_{{\mathrm{O}}_{2}}>{10}^{\ensuremath{-}18}$ atm.

Electronic and structural properties of strontium chalcogenides SrS, SrSe and SrTe

AbstractWe present the results of a first‐principles study of the electronic and structural properties of strontium chalcogenides, SrS, SrSe and SrTe. The computational method is based on the full‐potential linear muffin‐tin orbitals method (FP‐LMTO) augmented by a plane‐wave basis (PLW). The exchange and correlation energy is described in the local density approximation (LDA) using the Perdew–Wang parameterization including a generalized gradient approximation (GGA). The calculated results of the structural properties are given for the NaCl (B1) and CsCl (B2) structures. We have also carried out band‐structure calculations for the three considered compound, but only for the NaCl (B1) structure. A reasonable agreement is found from the comparison of our results with other theoretical calculations and experimental data. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The bulk modulus of ThO 2—an experimental and theoretical study

In view of the large scatter in the literature, we have re-evaluated the high-pressure behaviour and the bulk modulus of thorium dioxide, experimentally using high-pressure X-ray diffraction (XRD) in conjunction with synchrotron radiation. To compare with the experimental measurements, theoretical calculations were performed using the full-potential linear muffin-tin orbital method (FP-LMTO) together with the local density approximation (LDA) as well as the generalised gradient approximation (GGA). The experimental value for the bulk modulus is found to be 195.3 ± 2.0 GPa, which compares well with the calculated value of 198 GPa obtained by using the generalised gradient approximation to the exchange correlation potential.

Calculation of uniaxial magnetic anisotropy energy of tetragonal and trigonal Fe, Co, and Ni

The magnetic anisotropy energy (MAE) of Fe, Co, and Ni is presented for tetragonal and trigonal structures along two paths of structural distortion connecting the bcc and the fcc structure. The MAE was calculated from first principles with the full-potential linear muffin-tin orbital method and the force theorem. As is expected from symmetry considerations, the MAE increases by orders of magnitude when the cubic symmetry is broken. For tetragonal structures of Co and Ni a regular behavior of the MAE is observed, i.e., only the symmetry dictated nodes at the cubic structures appear along this path of distortion. In the case of tetragonal Fe, additional reorientations of the easy axis occur that are attributed to a topological change of the Fermi surface upon distortion. For the trigonal structures of all three elements the strain dependence of the MAE is more complicated, with additional reorientations of the easy axis and an unexpectedly large MAE for certain distortions of Ni, and a strongly nonlinear behavior for trigonal structures of Co close to fcc. Furthermore, the linear magnetoelastic coupling coefficients are calculated from the MAE at small distortions from the cubic equilibrium structure of the three elements. Two different Brillouin-zone integration techniques were used to calculate the MAE. Since the Gaussian broadening method smears out details of the Fermi surface, it results in a different MAE as compared to the tetrahedron method in some cases.

Ab-initio study of interfacial strength and misfit dislocations in eutectic composites: NiAl/Mo

The electronic structure and fracture energy characteristics of the NiAl/Mo interface have been investigated by the full-potential linear muffin-tin orbital method. We optimized the crystal geometry of the coherent interface and calculated the ideal work of adhesion and interfacial shear energies. For the first time, based on ab-initio calculations, the structure and energy of misfit dislocations were determined within the Peierls–Nabarro model with a generalized restoring force law. The bonding at the coherent interface is found to be close to that of bulk NiAl, but the appearance of the misfit dislocations leads to a drastic weakening of the NiAl/Mo interface.

Electronic structure and magnetic interactions in MnN andMn3N2

Electronic structure calculations were performed for MnN and ${\mathrm{Mn}}_{3}{\mathrm{N}}_{2}$ using a full-potential linearized muffin-tin orbital method in the local spin density approximation. Structural relaxation by energy minimization and calculations for various different magnetic configurations were performed. Antiferromagnetic ordering along the [001] direction was found to have the lowest energy in both materials in agreement with experimental neutron diffraction data. The magnetic moments were found to be about $3{\ensuremath{\mu}}_{B}$ in good agreement with experiment. Analysis of the partial densities of states reveals that the magnetic moments arise primarily from the ${t}_{2g}$ orbitals. In ${\mathrm{Mn}}_{3}{\mathrm{N}}_{2},$ the magnetic moment was found to be slightly larger on the ${\mathrm{Mn}}_{1}$ atoms, i.e., the Mn atoms in planes without N than on ${\mathrm{Mn}}_{2}$ atoms. Band structures and densities of states are presented. The energy differences between different magnetic configurations were analyzed in terms of a Heisenberg Hamiltonian. We find strong second-nearest-neighbor ferromagnetic interactions for Mn connected collinearly with N, indicative of a double exchange mechanism and about four times weaker nearest-neighbor antiferromagnetic interactions in MnN. In a perfect rocksalt structure, this situation would lead to a multiply degenerate ground state. However, we find that the $c/a$ reduction of about 2% is due to the AFM coupling (it is absent in FM and nonmagnetic MnN) and leads to stronger inter-(001)-plane exchange interactions than intraplane exchange interaction. In ${\mathrm{Mn}}_{3}{\mathrm{N}}_{2},$ the Mn connected via a vacancy lack the strong ferromagnetic interaction. This would suggest a lower N\'eel temperature for MnN with increasing concentration of N vacancies. However, the nearest-neighbor direct exchange interaction in ${\mathrm{Mn}}_{3}{\mathrm{N}}_{2}$ is found to be about two times as large in ${\mathrm{Mn}}_{3}{\mathrm{N}}_{2}$ than in MnN which partially compensates for the lack of fewer indirect second-nearest exchange interactions. These changes in the exchange interactions are related to the structural relaxations. Nevertheless, the mean field approximation predicts a slightly lower N\'eel temperature in ${\mathrm{Mn}}_{3}{\mathrm{N}}_{2}$ than in MnN in contrast with experiment. Effects beyond mean field are discussed and are deemed not to be able to provide an explanation for this discrepancy. We suggest that the observed phase transition in ${\mathrm{Mn}}_{3}{\mathrm{N}}_{2}$ previously interpreted as the AFM-paramagnetic phase transition may in reality be an order-disorder transition of the N vacancies.

Electronic structure of cubic silicon carbide with substitutional 3d impurities at Si and C sites

The full-potential linear muffin-tin orbital method is used to calculate the electronic structure and cohesive energies for cubic silicon carbide doped with transition 3d metal impurities (Me=Ti, V, Cr, Mn, Fe, Co, Ni), substituting Si or C at the corresponding sublattice of the atomic matrix. It is established that all 3d impurities mainly occupy silicon sites. For the Ti → Si substitution, dopant impurity levels are located in the conduction band of SiC, whereas doping silicon carbide with other 3d impurities gives rise to additional donor or acceptor levels in the band gap. For 3C-SiC the effect of impurities on the lattice parameter (with the substitutions Me → Si) and on the impurity local magnetic moments (with the substitutions Me → Si, C) is studied.

Balanced crystal orbital overlap population—a tool for analysing chemical bonds insolids

A new tool for analysing theoretically the chemical bonding in solids is proposed. Abalanced crystal orbital overlap population (BCOOP) is an energy resolved quantity whichis positive for bonding states and negative for antibonding states, hence enabling adistinction between bonding and antibonding contributions to the chemical bond.Unlike the conventional crystal orbital overlap population (COOP), BCOOPhandles correctly the situation of crystal orbitals being nearly linear dependent,which is often the case in the solid state. Also, BCOOP is much less basis setdependent than COOP. A BCOOP implementation within the full-potential linearmuffin tin orbital method is presented and illustrated for Si, TiC and Ru. Thus,BCOOP is compared to the COOP and crystal orbital Hamilton population (COHP)for systems with chemical bonds ranging from metallic to covalent character.

Electronic structure of thin heterocrystalline superlattices in SiC and AlN

The spontaneous polarization, the valence band offset, and the quantum confinement effect for thin SiC and AlN cubic/hexagonal heterocrystalline superlattices are studied by use of a full potential linear muffin-tin orbital method (FP-LMTO). We find that the polarization is screened and suppressed while the length of the cubic region grows. The band offsets do not change with layer thickness. For thin superlattices, the quantum confinement effects dominate and result in a band gap which stays larger than the gap of the bulk cubic structure. Furthermore, the energy levels of the bound states in the quantum well resemble the pattern of the energy levels of conventional III-V based quantum wells and superlattices but at a much smaller length scale, which is due to the higher quantum well depth and the larger effective masses in SiC and AlN systems.

Calculated band structures and optical properties of lead chalcogenides PbX (X=S, Se, Te) under hydrostatic pressure

The hydrostatic pressure dependence of the principal energy gaps and of the optical properties of PbX (X=S, Se, and Te) has been calculated with the use of the self-consistent full-potential linear muffin-tin orbital method. The local density approximation and density functional theory are applied. Structural properties such as the equilibrium lattice constants, the bulk modulus, and its pressure derivatives were calculated for lead chalcogenides in the rock salt structure (NaCl). We have found that the results of the structural properties calculations are in agreement with those of ab initio and experimental data. In the rock salt structure, the pressure dependence of the energy gaps of these compounds is overestimated compared to the available experimental data. However, for the same structure, the comparison of our results with those of ab initio calculations shows good agreement. On the other hand, the effect of the applied pressure is clearly seen in the optical response spectrum, especially near the energy transition regions.

Quantum size effects in free-standing MgB 2 films

Using the full-potential Linearized Muffin-tin Orbital method, we calculate the electronic structure of free-standing MgB 2 (0001) films with 2 to 10 Mg/B bilayers. Owing to the quantum size effects, the electron energy of the occupied quantum well states and the density of states at the Fermi level exhibit oscillatory behavior with the number of Mg/B bilayers. The incremental energy with the bilayer added and the quantum-well state energy are used to study the MgB 2 film stability, suggesting the existence of the “magic numbers” in film growth.

Pairing and hyperfine interactions of Cd-P complexes in silicon

Theoretical calculations of formation energies, binding energies of Cd-P pairs in Si, and hyperfine parameters at the Cd site are performed. The ab initio all electron full-potential linear muffin-tin orbital method based on smooth Hankel functions, local-density-functional theory, and the supercell technique are utilized. Cd-P pairs with different numbers of P atoms, in several charge states, are studied. In each complex, the phosphorus atom is placed at the substitutional site while Cd is localized at the substitutional or interstitial sites. We find that substitutionals ${\mathrm{CdP}}^{0,\ensuremath{-}1}$ and ${\mathrm{CdP}}_{2}^{0}$ are stable complexes with binding energies around 0.7--0.8 eV. The calculated electric-field gradients of these complexes at the Cd site give quadrupolar coupling constants in agreement with previous theoretical and experimental assessments. However, Cd at interstitial positions weakly binds to P for n-type samples, and gives hyperfine parameters much lower than previous tentative experimental assignations. We find the cluster ${\mathrm{CdP}}_{3},$ being as stable as CdP and ${\mathrm{CdP}}_{2},$ can explain the origin of the third quadrupolar coupling constant found at a high P level doping. These facts suggest that a segregation process of P atoms around Cd could take place. Calculated charge states, transition state levels, and equilibrium concentrations are in accord with experimental measurements.

Structural properties, electronic structure, Fermi surface, and mechanical behavior of bcc Cr-Re alloys

The electronic structure and ground-state properties (lattice parameter, cohesive energy, bulk and tetragonal shear moduli) were calculated for bcc Cr and Cr-Re alloys over a wide range of Re concentration by the full-potential linear muffin-tin orbital method (FLMTO) with generalized gradient approximation (GGA). We show that the GGA predicts the commensurate antiferromagnetic Cr state as a ground state for the experimental volume with a small lower energy compared with the nonmagnetic state, but gives a large moment of $0.92{\ensuremath{\mu}}_{B}.$ In the Cr-Re system the magnetism becomes weak with Re concentration and for 25% Re the mean Cr moment is about $0.05{\ensuremath{\mu}}_{B}.$ Examining the Fermi surface of Cr-Re alloys within the modified rigid-band approximation, the electronic topological transition (ETT) is found to occur for 6% Re. A weakening of the Cr-Cr bonds near this ETT is considered as a possible microscopic reason for the ``small'' rhenium effect---the ductility enhancement in Cr(Mo,W)-Re alloys with small Re additions. As the mechanism for the ``large'' rhenium effect (near 25--35% Re) the formation of Cr-Re close-packed particles should play an important role together with the significantly weakened Cr-Cr bonding.

Structural and Electronic Properties of a Thin Metal-Silicide Film on Silicon Surface: A First-Principles Study

The structural stability of metal (M) silicide (M = Co, Ni) phases and magnetic properties for the deposited one and two monolayers (ML) of M on the Si(001) surface was studied by the use of the ab initio full-potential linear muffin-tin orbital (FP-LMTO) method. The diffusion of M atoms on Si into the tetrahedral (fourfold M) sites was found to be energetically favorable. The deposited M atoms easily migrate into the bulk because the diffusion or interchange is energetically more favorable than the formation of cluster in a surface layer. The energetics for the structural stability of the "fourfold Si surface" model was compared with that of the sixfold structural model. Our results for the electronic properties of the MSi2/Si(001) surface and interface were compared with experimental and other theoretical results.

First-principles investigations of orbital magnetic moments and electronic structures of the double perovskitesSr2FeMoO6,Sr2FeReO6,andSr2CrWO6

The electronic structures and magnetic properties of double perovskites ${\mathrm{Sr}}_{2}{\mathrm{FeMoO}}_{6},$ ${\mathrm{Sr}}_{2}{\mathrm{FeReO}}_{6},$ and ${\mathrm{Sr}}_{2}{\mathrm{CrWO}}_{6}$ have been studied by using the full-potential linear muffin-tin orbital method within the local-spin-density approximation (LSDA) and the generalized gradient approximation (GGA). The on-site Coulomb energy U has also been taken into account in both schemes $(\mathrm{LSDA}+U$ and $\mathrm{GGA}+U).$ The results predict a half-metallic ferrimagnetic band structure with total spin magnetic moment of $4{\ensuremath{\mu}}_{B},$ $3{\ensuremath{\mu}}_{B},$ and $2{\ensuremath{\mu}}_{B}$ per formula unit for ${\mathrm{Sr}}_{2}{\mathrm{FeMoO}}_{6},$ ${\mathrm{Sr}}_{2}{\mathrm{FeReO}}_{6},$ and ${\mathrm{Sr}}_{2}{\mathrm{CrWO}}_{6},$ respectively. By including the spin-orbit coupling in the self-consistent calculations, we find that the $5d$ transition-metal atoms W and Re exhibit large unquenched orbital magnetic moments because of significant spin-orbit interaction in $5d$ orbitals. On the other hand, in Fe and Cr $3d$ and Mo $4d$ orbitals, the orbital moments are all quenched even though the on-site Coulomb energy U is taken into account. This is in strong contrast to the enhanced large $3d$ orbital moments in CoO and NiO but similar to the quenched orbital moment in ${\mathrm{CrO}}_{2}.$

Electronic Structure of Native Point Defects in Zngep2

ABSTRACTFirst-principles calculations are presented for various native point defects in ZnGeP2 us-ing a full-potential linearized muffin-tin orbital method in the local density approximation to density functional theory. Under Zn-poor conditions, the lowest Gibbs energy defects are found to be the Gezn antisite and Vzn. The Vae is found to have high energy of formation under any chemical potential conditions and is unstable towards formation of a Vzn and ZnGe pair. It is shown that the V−Zn cannot account for the ALI EPR spectrum commonly associated with this vacancy and an alternative model consisting of a Vzn – GeZn – Vzn is tentatively proposed.

The electronic properties of native interstitials in ZnO

To investigate the role of interstitial geometry, we reported ab initio calculations of octahedrally and tetrahedrally native interstitials in ZnO, using the full-potential linear muffin-tin orbital method. The results show that both zinc interstitials could contribute to the native n-type conduction in ZnO, meanwhile, the zinc vacancy and octahedral oxygen interstitial may contribute to the p-type conduction in ZnO. We also suggest that the 31 and 61 meV donors found by temperature-dependent Hall experiment originate from tetrahedral- and octahedral zinc interstitials, respectively, based on their charge distribution. It is noticeable that tetrahedral interstitials cause stronger interaction between interstitials and its neighboring atoms than that of octahedral interstitials.

Systematic first-principles study of impurity hybridization in NiAl

We have performed a systematic first-principles computational study of the effects of impurity atoms (boron, carbon, nitrogen, oxygen, silicon, phosporus, and sulfur) on the orbital hybridization and bonding properties in the intermetallic alloy NiAl using a full-potential linear muffin-tin orbital method. The matrix elements in momentum space were used to calculate real-space properties: onsite parameters, partial densities of states, and local charges. In impurity atoms that are empirically known to be embrittler (N and O) we found that the 2s orbital is bound to the impurity and therefore does not participate in the covalent bonding. In contrast, the corresponding 2s orbital is found to be delocalized in the cohesion enhancers (B and C). Each of these impurity atoms is found to acquire a net negative local charge in NiAl irrespective of whether they sit in the Ni or Al site. The embrittler therefore reduces the total number of electrons available for covalent bonding by removing some of the electrons from the neighboring Ni or Al atoms and localizing them at the impurity site. We show that these correlations also hold for silicon, phosporus, and sulfur.