Tight-binding Linear Muffin-tin Orbital Method Research Articles

Nonlocal torque operators inab initiotheory of the Gilbert damping in random ferromagnetic alloys

We present an ab initio theory of the Gilbert damping in substitutionally disordered ferromagnetic alloys. The theory rests on introduced nonlocal torques which replace traditional local torque operators in the well-known torque-correlation formula and which can be formulated within the atomic-sphere approximation. The formalism is sketched in a simple tight-binding model and worked out in detail in the relativistic tight-binding linear muffin-tin orbital (TB-LMTO) method and the coherent potential approximation (CPA). The resulting nonlocal torques are represented by nonrandom, non-site-diagonal and spin-independent matrices, which simplifies the configuration averaging. The CPA-vertex corrections play a crucial role for the internal consistency of the theory and for its exact equivalence to other first-principles approaches based on the random local torques. This equivalence is also illustrated by the calculated Gilbert damping parameters for binary NiFe and FeCo random alloys, for pure iron with a model atomic-level disorder, and for stoichiometric FePt alloys with a varying degree of L10 atomic long-range order.

Elastic, Electronic, Optical and Thermal Properties of Na2Po: An Ab Initio Study

The structural, elastic, electronic, optical and thermodynamic properties of the sodium polonide Na2Po compound have been studied through the full potential linearized augmented plane wave plus local orbitals (FP-LAPW + lo) and tight-binding linear muffin-tin orbital (TB-LMTO) methods. The exchange–correlation potential was treated within the local density approximation for the TB-LMTO calculations and within the generalized gradient approximation for the FP-LAPW + lo calculations. In addition, Tran and Blaha-modified Becke–Johnson (TB-mBJ) potential and Engel–Vosko generalized gradient approximation were used for the electronic and optical properties. Ground state properties such as the equilibrium lattice constant, bulk modulus and its pressure derivative were calculated and compared with available data. The single-crystal and polycrystalline elastic constants of the considered compound were calculated via the total energy versus strain in the framework of the FP-LAPW + lo approach. The calculated electronic structure reveals that Na2Po is a direct band gap semiconductor. The frequency-dependent dielectric function, refractive index, extinction coefficient, reflectivity coefficient and electron energy loss function spectra are calculated for a wide energy range. The variations of the lattice constant, bulk modulus, heat capacity, volume expansion coefficient and Debye temperature with temperature and pressure were calculated successfully using the FP-LAPW + lo method in combination with the quasi-harmonic Debye model.

First-principles investigation of structural, mechanical, electronic, and bonding properties of NaZnSb

The structural, mechanical, electronic, and bonding properties and phase transition of NaZnSb are explored using the generalized gradient approximation based on ab initio plane-wave pseudopotential density functional theory.With the help of the quasi-harmonic Debye model, we probe the Gruneisen parameter, thermal expansivity, heat capacity, Debye temperature, and entropy of NaZnSb in the tetragonal phase. The results indicate that the lattice constants and the bulk modulus and its first pressure derivative agree well with the available theoretical and experimental data. NaZnSb in its ground state structure exhibits a distinct energy gap of about 0.41 eV, which increases with increasing pressure. Our conclusions are consistent with the theoretical predictions obtained by the ABINIT package, but are different from those obtained through the tight-binding linear muffin-tin orbital method. As a result, further experimental and theoretical researches need to be carried out. For the purpose of providing a comparative and complementary study for future research, we first investigate the thermodynamic properties of NaZnSb.

An ab initio approach on superconducting properties of Mo3X(X = Si,Ga,Ge) compounds

Self-consistent first principles calculations on type II weakly coupled superconducting [Formula: see text] compounds of A15 phase are performed to understand their fundamental characteristics of the electronic, thermal and superconducting properties. The bulk modulus [Formula: see text], Debye temperature [Formula: see text], density of states (DOS) [Formula: see text], electron–phonon coupling constant [Formula: see text], superconducting transition temperature [Formula: see text], and electronic specific heat coefficient [Formula: see text] have been computed in terms of the electronic structure results, obtained by using the tight-binding linear muffin-tin orbital method. It is observed that all the three materials have their electronic properties dominated by d-orbital at Fermi energy. Thermal and superconducting properties calculated here are found to corroborate well with the experimental results of literature.

Quaternary pnictides with complex, noncentrosymmetric structures. Synthesis and structural characterization of the new Zintl phases Na11Ca2Al3Sb8, Na4CaGaSb3, and Na15Ca3In5Sb12.

Three new Zintl phases, Na11Ca2Al3Sb8, Na4CaGaSb3, and Na15Ca3In5Sb12, have been synthesized by solid-state reactions, and their structures have been determined by single-crystal X-ray diffraction. Na11Ca2Al3Sb8 crystallizes with its own structure type (Pearson index oP48) with the primitive orthorhombic space group Pmn2(1) (No. 31). The structure is best viewed as [Al3Sb8](15-) units of fused AlSb4 tetrahedra, a novel type of Zintl ion, with Na(+) and Ca(2+) cations that solvate them. Na4CaGaSb3 also crystallizes in its own type with the primitive monoclinic space group Pc (No. 7; Pearson index mP36), and its structure boasts one-dimensional [GaSb3](6-) helical chains of corner-shared GaSb4 tetrahedra. The third new compound, Na15Ca3In5Sb12, crystallizes with the recently reported K2BaCdSb2 structure type (space group Pmc2(1); Pearson index oP12). The Na15Ca3In5Sb12 structure is based on polyanionic layers made of corner-shared InSb4 tetrahedra. Approximately one-sixth of the In sites are vacant in a statistical manner. All three structures exhibit similarities to the TiNiSi structure type, and the corresponding relationships are discussed. Electronic band structure calculations performed using the tight-binding linear muffin-tin orbital atomic sphere approximation method show small band gaps for all three compounds, which suggests intrinsic semiconducting behavior for these materials.

Superconducting properties of Mo3Os, Mo3Pt, Mo3Ir from first principle calculations

Self-consistent first principle calculations were carried out to investigate the structural, electronic, thermal and superconducting properties of Mo 3 X ( X = Os , Ir , Pt ) compounds of A15 phase that are studied by using the tight-binding linear muffin-tin orbital method. The E and k convergence have been checked to analyze the ground state properties. The band structure and DOS histograms are plotted from the calculated equilibrium lattice parameter. The bulk modulus (B B ), Debye temperature (θ D ), density of states (N(E F )), electron–phonon coupling constant (λ), superconducting transition temperature (Tc) and electronic specific heat coefficient (γ) have been calculated from the electronic band structure results. The calculated values have been compared with the available experimental results of literature.

Experimental and theoretical investigations for site preference and anisotropic size change of RE11Ge4In6−xMx (RE = La, Ce; M = Li, Ge; x = 1, 1.96)

Two polar intermetallic compounds in the RE11Ge4In6−xMx (RE=La, Ce; M=Li, Ge; x=1, 1.96) series have been synthesized by conventional high-temperature reactions and characterized by both single-crystal and powder X-ray diffractions. Both compounds crystallized in the tetragonal crystal system (space group I4/mmm, Z=4, Pearson symbol tI84) with nine crystallographically independent atomic positions in the asymmetric unit and adopted the Sm11Ge4In6-type structure, which can be considered as an ordered version of the Ho11Ge10-type. The lattice parameters are a=11.8370(4)Å and c=17.2308(7)Å for La11Ge4In5.00(1)Li1.00; a=11.8892(4)Å, c=16.5736(7)Å for Ce11Ge5.96(3)In4.04. The overall crystal structures of two isotypic compounds can be described as a combination of the cage-shaped 3-dimensional (3-D) anionic framework and three different types of cationic polyhedra filling the inside of the 3-D frameworks. Anionic elements consisting of the frameworks indicate the particular site preference, which can be understood by QVAL values. Theoretical investigations using tight-binding linear muffin-tin orbital (LMTO) method provide rationales for the anisotropic size change of the unit cell of La11Ge4In5.00(1)Li1.00 using the various crystal orbital Hamilton population (COHP) curves and the possible short-range anionic ordering based on total electronic energy comparisons. Density of states (DOS) curves are also analyzed to explain the orbital interactions among components in the given crystal structure.

Galvanomagnetic properties of partially orderedL10FePt alloys

The effect of the long-range order (LRO) on the longitudinal $({\ensuremath{\rho}}_{xx},{\ensuremath{\rho}}_{zz})$ and anomalous Hall $({\ensuremath{\rho}}_{xy})$ resistivities as well as on the anisotropic magnetoresistance (AMR) in partially ordered $L{1}_{0}\phantom{\rule{0.28em}{0ex}}\mathrm{FePt}$ alloys is studied from first principles. The linear-response theory as formulated in the framework of the relativistic tight-binding linear muffin-tin orbital method which includes both the Fermi-surface and Fermi-sea terms is used. The effect of disorder is treated by means of the coherent-potential approximation. The main result is a weak dependence of the anomalous Hall conductivity ${\ensuremath{\sigma}}_{xy}$ on the LRO, which is, however, compatible with resistivities ${\ensuremath{\rho}}_{xx}$ and ${\ensuremath{\rho}}_{xy}$, which both depend strongly on the disorder present in the system. The resistivity and the AMR are predicted to increase with increasing degree of the LRO. We also investigate the effect of spin fluctuations on studied quantities using a simple model of the spin disorder. We have found good agreement between the theory and the recent experiment.

The Ce2Li0.39Ni1.61Si2structure as a new derivative of the AlB2family

A new quaternary dicerium lithium/nickel disilicide, Ce2Li0.39Ni1.61Si2, crystallizes as a new structure type of intermetallic compounds closely related to the AlB2 family. The crystal-chemical interrelationships between parent AlB2-type, BaLiSi, ZrBeSi and the title compound are discussed using the Bärnighausen formalism. Two Ce atoms occupy sites of 3m. symmetry. The remainder, i.e. Ni, mixed Ni/Li and Si atoms, occupy sites of -6m2 symmetry. The environment of the Ce atom is an 18-vertex polyhedron and the Ni, Ni/Li and Si atoms are enclosed in tricapped trigonal prisms. The title structure can be assigned to class No. 10 (trigonal prism and its derivatives) according to the Krypyakevich classification scheme [Krypyakevich (1977). In Structure Types of Intermetallic Compounds. Moscow: Nauka]. The electronic structure of the title compound was calculated using the tight-binding linear muffin-tin orbital method in the atomic spheres approximation (TB-LMTO-ASA). Metallic bonding is dominant in this compound. The strongest interactions are Ni-Si and Ce-Si.

Mechanism of Uniaxial Magnetocrystalline Anisotropy in Transition Metal Alloys

Magnetocrystalline anisotropy in transition metal alloys (FePt, CoPt, FePd, MnAl, MnGa, and FeCo) was studied using first-principles calculations to elucidate its specific mechanism. The tight-binding linear muffin-tin orbital method in the local spin-density approximation was employed to calculate the electronic structure of each compound, and the anisotropy energy was evaluated using the magnetic force theorem and the second-order perturbation theory in terms of spin-orbit interactions. We systematically describe the mechanism of uniaxial magnetocrystalline anisotropy in real materials and present the conditions under which the anisotropy energy can be increased. The large magnetocrystalline anisotropy energy in FePt and CoPt arises from the strong spin-orbit interaction of Pt. In contrast, even though the spin-orbit interaction in MnAl, MnGa, and FeCo is weak, the anisotropy energies of these compounds are comparable to that of FePd. We found that MnAl, MnGa, and FeCo have an electronic structure that is efficient in inducing the magnetocrystalline anisotropy in terms of the selection rule of spin-orbit interaction.

First Principles Study of Titanium Hydrides, TiHn: n = 1, 2, 3; Energetics and Phase Transition

The electronic structure and structural phase transition of TiH n (n = 1, 2 and 3) are investigated using the Tight-Binding Linear Muffin-Tin Orbital (TB-LMTO) method with Local density approximation (LDA) and Atomic sphere approximation (ASA). The equilibrium geometries, the electronic band structure, and the total and partial Density of States (DOS) are obtained under various pressures, and are analyzed in comparison with the available experimental and theoretical data. It is predicted that the most stable structure of Titanium hydride is a cubic structure at normal pressure. Both TiH and TiH 2 undergo a structural phase transition from a cubic to a hexagonal phase under high pressure. The stability of TiM 2 H and TiMH 2 is analyzed. doi: 10.14456/WJST.2014.7

Effect of Pressure on the Electronic and Magnetic Properties of a Few Mn-Based Half-Metallic Compounds

Spin-polarized calculations of three half-metallic, half-Heusler compounds, namely NiMnSb, PdMnSb and PtMnSb are presented. The calculations were performed using tight-binding linear muffin-tin orbital method within the local density approximation. In this work, the lattice constants, bulk modulus, band structures, density of states and energy gap in the minority spin direction and magnetic moment of these compounds were calculated at ambient conditions and under pressure. The effect of pressure on the magnetic properties and on the energy gap was studied. We have predicted that NiMnSb, PdMnSb and PtMnSb show the magnetic phase transition under pressure from the ferromagnetic to nonmagnetic phase at 195.6, 292.4 and 533 GPa, respectively. The magnetic phase transition is of second order in nature. The transition pressure increases from Ni(3d) to Pt(5d) in the compound. The calculated energy gap in the minority spin direction increases with pressure. The bulk modulus of these compounds have been predicted to be 140.2, 137.25 and 157.45 GPa, respectively.

Synthesis and Structural Characterization of RE7Zn21Tt2 (RE = La–Nd; Tt = Ge, Sn, and Pb): New Structure Type Among the Polar Intermetallic Phases

Reported are 11 new ternary phases with the general formula RE7Zn21Tt2 (RE = La-Nd; Tt = Ge, Sn, and Pb), synthesized from the respective elements by reactions at high temperature. Their structures, established on the basis of single-crystal and powder X-ray diffraction work, are shown to be a new structure type with the orthorhombic space group Pbam (No. 55, Pearson symbol oP60). This complex atomic arrangement features condensed polyhedra made up of Zn atoms, interspersed by Ge, Sn, or Pb atoms in trigonal-planar coordination. The structure bears resemblance with the La3Al11 and the LaRhSn2 structure types, which are compared and discussed. Temperature dependent dc magnetization measurements confirm RE(3+) ground states for all rare-earth elements, and the expected local-moment magnetism due to the partial filling of their 4f states for RE(3+) = Ce(3+), Pr(3+), and Nd(3+). Theoretical considerations of the electronic structure based on the tight-binding linear muffin-tin orbital (TB-LMTO-ASA) method are also presented: the calculations support the experimental observation of a small, but not negligible, homogeneity range in RE7Zn(21+x)Tt(2-x) (x < 0.5). The partial substitution of the tetrel atoms by the electron-poorer Zn appears to be an important attribute, leading to an optimal valence electron concentration and, thereby, to the overall electronic stability of the crystal structure of this family of polar intermetallics.

New Polar Intermetallic Phases RE2Zn5Tt (RE = La–Nd; Tt = Sn and Pb): Synthesis, Structure, Chemical Bonding, and Magnetic Properties

Reported are the synthesis, crystal structure, electronic structure, and magnetic properties of a series of zinc-rich ternary phases with formulas RE2Zn5Tt (RE = La-Nd; Tt = Sn and Pb). The structures of these compounds have been established by single-crystal and powder X-ray diffraction. They crystallize in the orthorhombic space group Cmcm (No. 63, LaRhSn2 structure type, Pearson symbol oC32). The most prominent structural feature is the trigonal-planar coordination of the Sn(Pb) atoms; the latter interconnect layers of Zn atoms to comprise a complex [Zn5Tt] polyanionic framework. The structural relationships between the structure of the title compounds and the EuIn4, La3Al11, and YIrGe2 structure types are highlighted. Temperature-dependent DC magnetization measurements indicate Pauli-like paramagnetism for La2Zn5Sn, while Ce2Zn5Sn, Pr2Zn5Sn, and Nd2Zn5Sn display Curie-Weiss behavior in the high-temperature regime. At cryogenic temperatures, the magnetic responses of Ce2Zn5Sn, Pr2Zn5Sn, and Nd2Zn5Sn appear to deviate from the Curie-Weiss law; however, no magnetic orderings could be observed down to 5 K. Theoretical considerations of the electronic structure on the basis of the tight-binding linear muffin-tin orbital (TB-LMTO-ASA) method are also presented and discussed.

First Principles Study of Electronic Structure and Magnetic Properties of TMH (TM = Cr, Mn, Fe, Co)

First principles calculations are performed using a tight-binding linear muffin-tin orbital (TB-LMTO) method with local density approximation (LDA) and atomic sphere approximation (ASA) to understand the electronic properties of transition metal hydrides (TMH) (TM = Cr, Mn, Fe, Co). The structural property, electronic structure, and magnetic properties are investigated. A pressure induced structural phase transition from cubic to hexagonal phase is predicted at the pressures of 50 GPa for CrH and 23 GPa for CoH. Also, magnetic phase transition is observed in FeH and CoH at the pressures of 10 GPa and 180 GPa, respectively.

Structural Characterization of the Intermetallic Phase EuZnxIn4-x(x ≈ 1.1-1.2). Zn and In Site-Preferences in the BaAl4Structure-Type from Computational Analysis

, respectively. In this structure, the Eu atoms are situated at the center of 18-vertexFedorov polyhedra made of Zn and In atoms, where the 4 d site is preferentially occupied by In and the 4 e siteis occupied by randomly mixed Zn and In atoms. Theoretical investigations using tight-binding linear muffin-tin orbital (TB-LMTO) method provide rationale for the observed site preferences and suggest potentiallywider homogeneity range than the experimentally established for EuZn

Accurate determination of band gaps within density functional formalism

In this paper, we report an adaptation of the Harbola-Sahni (HS) exchange potential to the tight-binding linear muffin-tin orbital (TB-LMTO) method to determine band gaps (BGs) of solids accurately. We show that the electrostatic basis of derivation of the Harbola-Sahni potential allows this nonvariational approach to improve substantially over local-density approximation derived BGs, bringing them very close to experimental values. That the accuracy of the HS potential is directly responsible for the determination of correct BGs is demonstrated by performing similar calculations with an accurate model potential that too leads to BGs close to their experimental values. Moreover, ground-state properties like equilibrium lattice parameters and bulk moduli (BM) for various semiconductors like C, Si, AlN, AlP, BP, and 3C-SiC calculated with the HS approach are in close agreement with the experiments. The clear physical interpretation of HS potential leads us to suggest exploring its use for calculating various properties of solids.

Tm2.22Co6Sn20and TmLi2Co6Sn20stannides as disordered derivatives of the Cr23C6structure type

A new ternary dithulium hexacobalt icosastannide, Tm2.22Co6Sn20, and a new quaternary thulium dilithium hexacobalt icosastannide, TmLi2Co6Sn20, crystallize as disordered variants of the binary cubic Cr23C6 structure type (cF116). 48 Sn atoms occupy sites of m.m2 symmetry, 32 Sn atoms sites of .3m symmetry, 24 Co atoms sites of 4m.m symmetry, eight Li (or Tm in the case of the ternary phase) atoms sites of -3 symmetry and four Tm atoms sites of m3m symmetry. The environment of one Tm atom is an 18-vertex polyhedron and that of the second Tm (or Li) atom is a 16-vertex polyhedron. Tetragonal antiprismatic coordination is observed for the Co atoms. Two Sn atoms are enclosed in a heavily deformed bicapped hexagonal prism and a monocapped hexagonal prism, respectively, and the environment of the third Sn atom is a 12-vertex polyhedron. The electronic structures of both title compounds were calculated using the tight-binding linear muffin-tin orbital method in the atomic spheres approximation (TB-LMTO-ASA). Metallic bonding is dominant in these compounds, but the presence of Sn-Sn covalent dumbbells is also observed.

A Ferrimagnetic Zintl Phase Pr4MnSb9: Synthesis, Structure, and Physical Properties

A new valence precise Zintl phase, Pr4MnSb9, has been successfully synthesized by solid-state reaction at high temperature. The single-crystal X-ray diffraction data reveal its monoclinic symmetry in the space group C2/m (No. 12) with a = 24.12(2) Å, b = 4.203(3) Å, c = 15.67(2) Å, β = 98.05(1)°, and Z = 4. The structure is characterized by the covalent three-dimensional network constructed by two types of five-atom-wide Sb5(7-) ribbons that are joined by 6-fold coordinated Mn(3+) cations, through which the narrower three-atom-wide Sb3(5-) ribbons are attached as a tag, and interstitial Pr(3+) cations and single Sb(3-) anions locate within the tunnels. Its magnetic susceptibility and isothermal hysteresis suggest ferrimagnetic behavior. The electrical conductivity and Seebeck coefficient of the cold-pressed pellet suggest a semimetal feature that agrees with the spin-polarized calculation results using the tight-binding linear muffin-tin orbital (TB-LMTO) method.

From One to Three Dimensions: Corrugated ∞1[NiGe] Ribbons as a Building Block in Alkaline Earth Metal Ae/Ni/Ge Phases with Crystal Structure and Chemical Bonding in AeNiGe (Ae = Mg, Sr, Ba)

The new equiatomic nickel germanides MgNiGe, SrNiGe, and BaNiGe have been synthesized from the elements in sealed tantalum tubes using a high-frequency furnace. The compounds were investigated by X-ray diffraction both on powders and single crystals. MgNiGe crystallizes with TiNiSi-type structure, space group Pnma, Z = 4, a = 6.4742(2) Å, b = 4.0716(1) Å, c = 6.9426(2) Å, wR2 = 0.033, 305 F(2) values, 20 variable parameters. SrNiGe and BaNiGe are isotypic and crystallize with anti-SnFCl-type structure (Z = 4, Pnma) with a = 5.727(1) Å, b = 4.174(1) Å, c = 11.400(3) Å, wR2 = 0.078, 354 F(2) values, 20 variable parameters for SrNiGe, and a = 5.969(4) Å, b = 4.195(1) Å, c = 11.993(5) Å, wR2 = 0.048, 393 F(2) values, 20 variable parameters for BaNiGe. The increase of the cation size leads to a reduction of the dimensionality of the [NiGe] polyanions. In the MgNiGe structure the nickel and germanium atoms build a ∞(3)[NiGe] network with magnesium atoms in the channels. In SrNiGe and BaNiGe the ∞(1)[NiGe] ribbons are separated by strontium/barium atoms, whereas in the known CaNiGe structure the ribbons are fused to two-dimmensional atom slabs. The crystal chemistry and chemical bonding in AeNiGe (Ae = Mg, Ca, Sr, Ba) are discussed. The experimental results are reconciled with electronic structure calculations performed using the tight-binding linear muffin-tin orbital (TB-LMTO-ASA) method.