Linear Muffin-tin Orbitals Research Articles

Synthesis and structural characterization of A3In 2Ge 4 and A5In 3Ge 6 ( A=Ca, Sr, Eu, Yb)—New intermetallic compounds with complex structures, exhibiting Ge–Ge and In–In bonding

Reported are the synthesis and the structural characterization of four new polar intermetallic phases, which exist only with mixed alkaline-earth and rare-earth metal cations in narrow homogeneity ranges. (Sr 1– x Ca x ) 5In 3Ge 6 and (Eu 1– x Yb x ) 5In 3Ge 6 ( x≈0.7) crystallize in the orthorhombic space group Pnma with two formula units per unit cell (own structure type, Pearson symbol oP56). The lattice parameters are as follows: a=13.109(3)–13.266(3) Å, b=4.4089(9)–4.4703(12) Å, and c=23.316(5)–23.557(6) Å. (Sr 1– x Ca x ) 3In 2Ge 4 and (Sr 1– x Yb x ) 3In 2Ge 4 ( x≈0.4–0.5) adopt another novel monoclinic structure-type (space group C2 /m, Z=4, Pearson symbol mS36) with lattice parameters in the range a=19.978(2)–20.202(2) Å, b=4.5287(5)–4.5664(5) Å, c=10.3295(12)–10.3447(10) Å, and β=98.214(2)–98.470(2)°, depending on the metal cations and their ratio. The polyanionic sub-structures in both cases are based on chains of InGe 4 corner-shared tetrahedra. The A 5In 3Ge 6 structure ( A=Sr/Ca or Sr/Yb) also features Ge 4 tetramers, and isolated In atoms in nearly square-planar environment, while the A 3In 2Ge 4 structure ( A=Sr/Ca or Eu/Yb) contains zig-zag chains of In and Ge strings with intricate topology of cis- and trans-bonds. The experimental results have been complemented by tight-binding linear muffin-tin orbital (LMTO) band structure calculations.

Exchange interactions and Curie temperatures in Cr-based alloys in the zinc blende structure: Volume- and composition-dependence from first-principles calculations

We present calculations of the exchange interactions and Curie temperatures in Cr-based pnictides and chalcogenides of the form $\text{Cr}X$, with $X=\text{As}$, Sb, S, Se, and Te, and the mixed alloys ${\text{CrAs}}_{50}{X}_{50}$, with $X=\text{Sb}$, S, Se, and Te. The calculations are performed for zinc blende structure for 12 values of the lattice parameter between 5.44 and $6.62\text{ }\text{\AA{}}$, appropriate for some typical II-VI and III-V semiconducting substrates. Electronic structure is calculated via the linear muffin-tin-orbitals (LMTOs) method in the atomic sphere approximation (ASA) using empty spheres to optimize ASA-related errors. Whenever necessary, the results have been verified using the full-potential version of the method, FP-LMTO. The disorder effect in the As-sublattice for ${\text{CrAs}}_{50}{X}_{50}$ $(X=\text{Sb},\text{S},\text{Se},\text{Te})$ alloys is taken into account via the coherent-potential approximation. Exchange interactions are calculated using the linear-response method for the ferromagnetic (FM) reference states of the alloys as well as the disordered local moments (DLM) states. These results are then used to estimate the Curie temperature from the low- and high-temperature sides of the ferromagnetic-paramagnetic transition. Estimates of the Curie temperature are provided based on the mean field and the more accurate random-phase approximations. Dominant antiferromagnetic exchange interactions for some low values of the lattice parameter for the FM reference states in CrS, CrSe, and CrTe prompted us to look for antiferromagnetic (AFM) configurations for these systems with energies lower than the corresponding FM and DLM values. Results for a limited number of such AFM calculations are discussed, identifying the AFM[111] state as a likely candidate for the ground state for these cases.

Bandgaps and band bowing in semiconductor alloys

The bandgap and band bowing parameter of semiconductor alloys are calculated with a fast and realistic approach. The method is a dielectric scaling approximation that is based on a scissor approximation. It adds an energy shift to the bandgap provided by the local density approximation (LDA) of the density functional theory (DFT). The energy shift consists of a material-independent constant weighted by the inverse of the high-frequency dielectric constant. The salient feature of the approach is the fast calculation of the dielectric constant of alloys via the Green function (GF) of the TB-LMTOs (tight-binding linear muffin-tin orbitals) in the atomic sphere approximation (ASA). When it is applied to highly mismatched semiconductor alloys (HMAs) like Zn Te x Se 1− x , this method provides a band bowing parameter that is different from the band bowing parameter calculated with the LDA due to the bowing exhibited also by the high-frequency dielectric constant.

The effect of oxygen non-stoichiometry and doping with vanadium on the nature of magnetism in titanium dioxide with the anatase structure

Abstract The effect of oxygen non-stoichiometry and doping with vanadium on the electronic structure and magnetic properties of the titanium dioxide with the anatase structure have been studied by employing the tight-binding linear muffin-tin orbitals (TB-LMTO) method in the local spin-density approximation with the account of on-site Coulomb correlations (LSDA+U). It has been shown that the appearance of ferromagnetism in Ti1−xVxO2−y, Ti1−2xV2xO2−y compounds with 0 ≤ x ≤ 1 / 16 , 0 ≤ y ≤ 1 / 16 is favored by the oxygen non-stoichiometry and location of the exchange-coupled vanadium atoms in the ac and bc planes of the crystal structure. At the vanadium concentration of 0.125 the super-exchange mechanism of the ferromagnetism is essential, with the Curie temperature (340 K) being in good relation to experimental data. The calculations demonstrate that the probability of formation of magnetic polarons is high at the vanadium concentration of 0.0625, but low at the concentration of 0.125. The reduction of magnetic moment with the rise of vanadium concentration corresponds to the general trend revealed for dilute magnetic semiconductors. It was established that the mechanisms of the high-temperature ferromagnetism in Ti1−xVxO2−y, Ti1−2xV2xO2−y compounds are different and depend on oxygen non-stoichiometry and vanadium concentrations.

K23Au12Sn9—An Intermetallic Compound Containing a Large Gold−Tin Cluster: Synthesis, Structure, and Bonding

A polyanionic unit {Au(12)Sn(9)} with a novel "corrugated sheet" shape occurs in K(23)Au(12)Sn(9). The compound was obtained by fusion of the pure elements in tantalum ampules at high temperatures followed by programmed cooling, and the structure was determined by X-ray diffraction: I42m (No. 121), a = 20.834(3), c = 6.818(1) A, Z = 2. The large heteroatomic cluster has D(2d) point symmetry and features a central four bonded (4b-) Sn, eight 3b- or 2b-Sn on the perimeter, and 24 linking nearly linear Sn-Au bonds at 12 Au atoms. Formula splitting according to the Zintl concept suggests that the compound is one electron deficient, and linear muffin-tin-orbital (LMTO) electronic structure calculations show that the Fermi level (E(F)) lies near a band gap at around 0.5 eV, that is, an incompletely filled valence band in concert with favorable atom packing. Large relative -ICOHP values for Au-Sn are consistent with the observed maximization of the number of heteroatomic bonds, whereas the numerous K-Sn and K-Au contacts contribute approximately 40 % of the total -ICOHP. Extended-Huckel population and molecular orbital analyses indicate that the open band feature originates from 5p states that are associated with the 2b-corner Sn atoms. In accord with the electronic structure calculations, magnetic susceptibility measurements show a nearly temperature-independent paramagnetic property.

Residual resistivity and its anisotropy in random CoNi and CuNi ferromagnetic alloys

Residual resistivities of random CoNi and CuNi ferromagnetic alloys are calculated by a recently developed ab initio technique based on the linear muffin-tin orbital (LMTO) method with perturbative inclusion of spin-orbit interaction. The obtained results compare well with those of existing fully relativistic techniques. Particular attention has been paid to the isotropic resistivity and the spontaneous magnetoresistance anisotropy; a high value of the resistance anisotropy is predicted for Ni-rich CoNi alloys.

Electronic properties of cubic ScGaAs and ScGaN ternaries and superlattices

The electronic properties of both Sc x Ga 1− x As and Sc x Ga 1− x N ternary alloy and superlattice systems are investigated within the first-principles full-potential linear muffin-tin orbitals method (FPLMTO) in its atomic sphere approximation (ASA) using the technique of empty spheres, which allows an accurate treatment of the interstitial regions. The phase transition from the rocksalt (B1) to the zinc blende (B3) structure is investigated and the possibility of zinc blende/zinc blende GaN/Sc x Ga 1− x N and GaAs/Sc x Ga 1− x As superlattices is expected. Wide and direct gaps are found to be possible in these systems, predicting them as good candidates for optoelectronic applications.

Structural modification and optical reflectivity of new gold–indide intermetallic compounds

Abstract The gold–indide SrAu 3.36(2) In 4.64 contains a novel 3D Au/In network, which can be described in terms of complex ∞ 3 [Au,In] 8 2− polyanions with strontium atoms filling hexagonal cages that are part of highly condensed infinite channels. This new intermetallic compound represents a small decrease in the indium content, i.e., as compared to SrAu 3.76(4) In 4.24 , and results in a significant orthorhombic to monoclinic structural transformation. Electronic structure calculations by Linear Muffin-Tin-Orbital (LMTO) methods show that Au–In interactions are predominant in its structural stabilization. UV–vis measurements show a generally lower reflectivity and higher absorption, as compared to elemental Au.

Electronic structure and magnetic properties of RuFe 3N nitride

Self-consistent band structure calculations were performed on nitride RuFe 3N in order to investigate its magnetic and ground state properties. The Linear Muffin-Tin Orbital (LMTO) method was employed and calculations were performed at several lattice parameters so as to obtain the RuFe 3N equilibrium volume. Nonmagnetic and ferromagnetic LMTO calculations have shown that the RuFe 3N stable stage is ferromagnetic with constant lattice equilibrium of 7.2502 atomic units (a.u.). At equilibrium volume the LMTO calculations have given magnetic moments of 1.25 and 1.63 μ B at Ru and Fe sites, respectively, and no magnetic moment at N sites. The analysis of states density at equilibrium volume as well as the results for charge transfer illustrates why this ruthenium nitride is ferromagnetic. The LMTO calculations anticipate that the magnetic moment, the hyperfine field (the Fermi contact) and the isomer shift show a strong dependence on the lattice spacing.

Chemistry and Physical Properties of the Phosphide Telluride Zr2PTe2

AbstractThe synthesis of the phosphide telluride Zr2PTe2 by solid‐state reaction from the elements at 850 °C or by thermite‐type reaction of Zr and Te8O10(PO4)4 was accomplished. Crystals were grown by chemical vapour transport by using iodine from 800 °C in the direction of higher temperatures up to 900 °C. The chemical bonding in Zr2PTe2 was investigated by the linear muffin‐tin orbital (LMTO) method. The non‐vanishing DOS of zirconium and tellurium states at the Fermi level (EF) complies with the metallic conductivity (ρ ≈ 40 μΩ cm at 300 K) and temperature‐independent Pauli paramagnetism.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

New quaternary Zintl phases – Synthesis, crystal and electronic structures of K A2Cd 2Sb 3 ( A = Ca, Sr, Ba, Eu, Yb)

The isostructural and isoelectronic Zintl phases K A 2Cd 2Sb 3 ( A = Ca, Sr, Ba, Eu, Yb) are the first in the corresponding quaternary systems. They have been synthesized by high-temperature reactions of the respective elements and their structures have been established by single-crystal X-ray diffraction. The orthorhombic structure (space group Pnma, no. 62, Z = 4) is complex and is best described following the Zintl formalism as a channel-like three-dimensional ∞ 3 [Cd 2Sb 3] 5− framework, with K + and A 2+ cations filling the space within it. The polyanionic framework is based on corner-shared antimony-tetrahedra, centered by the Cd. Electronic structures calculations using the linear muffin-tin orbital (LMTO) method suggest that the title compounds are electron-precise Zintl phases, evidenced by the small energy gaps at the Fermi level. Magnetic susceptibility data for KEu 2Cd 2Sb 3, confirming divalent Eu are also presented.

Centric and Non-centric Ca3Au∼7.5Ge∼3.5: Electron-Poor Derivatives of La3Al11. Syntheses, Structures, and Bonding Analyses

Two La(3)Al(11) type derivatives have been discovered in the Ca-Au-Ge system and structurally characterized by single-crystal X-ray diffraction. Compositions Ca(3)Au(7.16(6))Ge(3.84(6)) (1) and Ca(3)Au(7.43(9))Ge(3.57(9)) (2) lie within a non-centric Imm2 phase region with a approximately 4.40 A, b approximately 13.06 A, c approximately 9.60 A. The Au-richer and electron-poorer Ca(3)Au(7.50(1))Ge(3.50(1)) (3) and Ca(3)Au(8.01(1))Ge(2.99(1)) (4) occur within a centric Pnnm phase region with a approximately 9.50 A, b approximately 13.20 A, c approximately 4.43 A. Both phases contain complex [Au,Ge](11)(6-) polyanionic networks made up of hexagonal and pentagonal prisms that are filled with the electropositive Ca atoms. Both 3:11 phases represent opposed 1 x 3 x 1 superstructure distortions of CaAu(2)Ge(2) (ThCr(2)Si(2) type, I4/mmm), the structure of which has also been re-determined in this work. Linear muffin-tin-orbital (LMTO) calculations reveal that the symmetry variations induced by changes of the Au contents in the present 3:11 phases are consequences of bonding and structural optimizations. The hypothetical "CaAu(2.33-2.67)Ge(1.33-1.00)cube(0.33)" compositions, which are close to those of 1-4, follow through creation and elimination of vacancies within the electronegative networks of CaAu(2)Ge(2).

An augmented space approach to the study of random ternary alloys: I. Electronicstructure with uncorrelated disorder and short ranged order

We present here a generalized augmented space recursive technique which includes theeffects of diagonal and environmental disorder explicitly: an analytic, lattice translationalinvariant, multiple scattering theory for the study of short range ordering in randomternary alloys. Our generalized augmented space formalism includes atomic correlationsover a finite cluster including short range order (SRO). We propose the augmentedspace recursion (ASR), a computationally fast and accurate technique whichincorporates configuration fluctuations over a large local environment. We apply theformalism to a tight-binding linear muffin-tin orbital (LMTO) study of stainless steelFe80−xNixCr20 (x = 14 and 17). We have demonstrated the effects of short range ordering by calculating theconfiguration averaged density of states with and without SRO and with different kinds ofcluster environment embedded in an averaged medium.

Out-of-plane spin-transfer torques: First-principles study

Spin-transfer torques in layered structures are studied with particular attention to their direction. Their direction is in spin valves determined mainly by the properties of the free layer and its interfaces. Situations which lead to a significant component of the torque in the direction perpendicular to both magnetizations present in the spin valve (out-of-plane torque) are described and analyzed. This component may arise from the transmitted spin current, as is demonstrated for a Cu | Ni | Cu ( 0 0 1 ) based system, or from the reflected spin current, which is found to be particularly high for systems with Cr | Co 2 MnSi interface. Ab initio calculations were performed for these systems employing the local spin density approximation (LSDA) and the linear muffin-tin orbital (LMTO) method. Information about the direction of the torque was obtained from the spin-mixing conductance.

Electron–phonon coupling and spin fluctuations in 3d and 4d transition metals:implications for superconductivity and its pressure dependence

We have calculated the electron–phonon coupling for the complete 4d series and thenonmagnetic 3d transition metals using the linear response method and the linearmuffin-tin orbitals’ basis. A comparison of the linear response results and those obtainedvia the rigid muffin-tin approximation is provided. Based on the calculated values of theelectron–phonon coupling constants, band density of states and the measured values of theelectronic specific heat constants, we estimate the spin-fluctuation effects, i.e. theelectron–spin-fluctuation (electron–paramagnon) coupling constants in these systems. Forthe sake of comparison, several other metals, Cu, Zn, Ag, Cd, Al and Pb, are also studied.Alternative estimates of the electron–paramagnon coupling constants are obtainedfrom the values of the Stoner parameters and the band densities of states atthe Fermi level. Implications of these results on the superconductivity and itspressure dependence as well as the alloying effects of superconductivity in thesesystems are discussed. It is pointed out that spin fluctuations play an importantrole in the validity of the Matthias rule that in metallic systems the optimumconditions for (electron–phonon) superconductivity occur for 5 and 7 valenceelectrons/atom.

Nitridogermanate Nitrides Sr7[GeN4]N2 and Ca7[GeN4]N2: Synthesis Employing Sodium Melts, Crystal Structure, and Density-Functional Theory Calculations

The alkaline earth nitridogermanate nitrides AE(7)[GeN(4)]N(2) (AE = Ca, Sr) have been synthesized using a Na flux technique in sealed Ta tubes. According to single-crystal X-ray diffraction the isotypic compounds crystallize in space group Pbcn (No. 60) with Z = 4, (Sr(7)[GeN(4)]N(2): a = 1152.6(2), b = 658.66(13), c = 1383.6(3) pm, V = 1050.5(4) x 10(6) pm(3), R1 = 0.049; Ca(7)[GeN(4)]N(2): a = 1082.6(2), b = 619.40(12), c = 1312.1(3) pm, V = 879.8(3) x 10(6) pm(3), R1 = 0.016). Owing to the high N/Ge ratio, the compounds contain discrete N(3-) ions coordinated by six AE(2+) besides discrete [GeN(4)](8-) tetrahedrons. One of the AE(2+) ion is coordinated by only four N(3-) ions, which is rather an unusual low coordination number for Sr(2+). Together with the isolated [GeN(4)](8-) tetrahedrons, these Sr(2+) ions form chains of alternating cation centered edge sharing tetrahedrons. The electronic structure and chemical bonding in Sr(7)[GeN(4)]N(2) has been analyzed employing linear muffin-tin orbital (LMTO) band structure calculations.

Evidence of the correlation between a strong 4d-As/2p-N orbitals coupling and the bowing effect in GaAsN

By means of a simple physical argumentation, we found that the giant bowing observed in GaAsN is correlated to the interaction between 4d-As and 2p-N orbitals. The calculations were carried out within the first principles full potential linear muffin-tin orbitals (FPLMTO) method in its plane wave approximation (PLW) which enables an accurate treatment of the interstitial regions. The choice of this method ensures our work to be free from adjustable parameters and enabled us to perform a microscopic study.

Dynamical Coherent-Potential Approximation and Tight-Binding Linear Muffintin Orbital Approach to Correlated Electron System

Dynamical coherent-potential approximation (CPA) to correlated electrons has been extended to a system with realistic Hamiltonian which consists of the first-principles tight-binding linear muffintin orbital (LMTO) bands and intraatomic Coulomb interactions. Thermodynamic potential and self-consistent equations for Green function are obtained on the basis of the functional integral method and the harmonic approximation which neglects the mode–mode couplings between the dynamical potentials with different frequency. Numerical calculations have been performed for Fe and Ni within the second-order dynamical corrections to the static approximation. The band narrowing of the quasiparticle states and the 6 eV satellite are obtained for Ni at finite temperatures. The theory leads to the Curie–Weiss law for both Fe and Ni. Calculated effective Bohr magneton numbers are 3.0 µ B for Fe and 1.2 µ B for Ni, explaining the experimental data. But calculated Curie temperatures are 2020 K for Fe and 1260 K for Ni, being ...

Combined Experimental and Density Functional Theory Studies on the Crystal Structures and Magnetic Properties of Mg(Mg1–xMnx)2Sb2 (x ≈ 0.25) and BaMn2Sb2

AbstractReported are the synthesis and structural characterization of two ternary transition metal intermetallic compounds, Mg(Mg1–xMnx)2Sb2 (where x is approximately 0.25) and BaMn2Sb2·Mg(Mg2.50(3)Mn0.5)2Sb2 crystallizes in the trigonal space group P$\bar {3}$m1 (no. 164) and belongs to the CaAl2Si2‐type, while BaMn2Sb2 is isostructural with the body‐centred tetragonal ThCr2Si2‐type (I4/mmm, no. 139). Both structures have been refined from single‐crystal X‐ray diffraction data and their magnetic properties determined with the aid of a SQUID‐magnetometry. The experimental results have been corroborated by electronic‐structure calculations using the linear muffin‐tin orbital (LMTO) method. Spin‐polarized density‐functional theory calculations confirm the antiferromagntically ordered magnetic structures. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Study of the Fermi Surface ofZrB12Using the de Haas-van Alphen Effect

The de Haas-van Alphen (dHvA) effect in the cluster superconductor ZrB12 was studied by magnetic torque measurements in magnetic fields up to 28 T at temperatures down to 0.07 K. The dHvA oscillations due to orbits from the neck sections and "cubic box" of the Fermi surface were detected. The dHvA frequencies as well as the cyclotron effective masses were calculated using the full-potential linear muffin-tin orbitals method within the generalized gradient approximation. A comparison of the experimental and calculated cyclotron mass shows unusually large electron-phonon interaction on the neck (lambdaep=0.95) and box (lambdaep=1.07) sections of the Fermi surface on the Brillouin zone boundaries.