Linear Muffin-tin Orbitals Research Articles

Experimental and theoretical investigations of the novel ternary compound Ca4InGe4

A new polar intermetallic compound with a novel structure type has been synthesized and characterized by both powder and single-crystal X-ray diffraction. Ca(4)InGe(4) crystallizes in the monoclinic crystal system (space group C2/c, Z = 4, Pearson symbol mS36) with five crystallographically unique atomic positions in the asymmetric unit. The corresponding lattice parameters at 200(2) K are a = 18.452(8) Å, b = 5.819(2) Å, c = 8.339(3) Å, and β = 99.330(6)°. The overall crystal structure can be described as a linear intergrowth of two imaginary fragments--Ca(2)InGe(2) with the Gd(2)AlGe(2) type- and CaGe with the FeB type-structures. Another way to rationalize the bonding is to focus on the polyanionic framework, which in this case is made up of unique nets of "seesaw"-shaped [InGe(4)] units. They are interconnected via Ge-Ge dimers into an open three-dimensional framework with Ca(2+) cations occupying the voids within it. Tight-binding linear muffin-tin orbital (LMTO) calculations provide a rationale for the unique local coordination geometry around In and the two distinct types of Ge-Ge bond distances.

Synthesis, Structure, and Bonding of Orthorhombic R5Au2Te2 (R = Lu, Ho, Dy, Y). Electronic Structure of the Binary Parent Valence Compound Eu5As4

Four examples of R(5)Au(2)Te(2) (vec = 29 e(-); R = Lu, Ho, Dy, Y) have been synthesized by high-temperature solid-state techniques, isotypic examples of Tm(5)Sb(2)Si(2) (vec = 33 e(-)) and binary Eu(5)As(4) (vec = 30 e(-)). The crystal structure was established for Lu(5)Au(2)Te(2), (orthorhombic Cmce (No. 64), a = 15.056(2), b = 7.749(1), c = 7.754(1) Å, and Z = 4), in which pairs of tellurium layers alternate with two-dimensional (2D) Lu(5)Au(2) slabs that are aggregated in such a way that each Au(2)-centered bi-trigonal prism (BTP) of Lu interconnects four other identical units, with the remaining cavities filled by nominal body-centered Lu cubes. The metal-metal aggregation in this structure provides a novel building unit in ternary rare-earth-metal-rich tellurides. Linear-muffin-tin-orbital (LMTO) electronic structure calculations and COHP analyses reveal that Lu(5)Au(2)Te(2) is a poor metal with Au(2) dimers and strong polar Lu-Au and Lu-Te interactions. The first theoretical analysis of the binary parent structure Eu(5)As(4) (vec = 30 e(-)) provides a simpler description of the equivalent orbital interactions and a closed shell gap in terms of the idealized (Eu(2+))(5)(As(2)(4-))(As(3-))(2) representation, particularly for the explicit filled As(2) levels σ(s), σ(s)*, σ(p), π, π*, plus empty σ(p)*. Crystal Orbital Hamilton Population (-COHP) data illuminate the prominent roles that polar bonding of Eu-As or Lu-Te and Lu-Au and relativistic effects with gold play in these, the former corresponding to 83% and 86% of the total Hamilton population for Eu(5)As(4) and Lu(5)Au(2)Te(2), respectively.

Structural, magnetic and magneto-optical properties of ultrathin films of nickel on iridium (001)

A detailed theoretical study of the structural, magnetic and magneto-optical behaviours of ultrathin films of nickel grown by pseudomorphic epitaxy on semi-infinite Ir(001) is given. The crystalline structure is found to be body centered tetragonal. The total energies are calculated by Spin-Polarized Relativistic Linear Muffin-Tin Orbitals with Atomic Sphere Approximation method. The calculation of the magnetic properties shows a ferromagnetic inter-layer coupling. The polar magneto-optical Kerr effect spectra are calculated over a photon energy range extended to 9 eV. The microscopic origin of the most interesting features is explained by the interband transitions between the localized spin projected states. These transitions are characterized by a spin-flip.

Eight-Coordinated Arsenic in the Zintl Phases RbCd4As3 and RbZn4As3: Synthesis and Structural Characterization

Reported are two new series of Zintl phases, ACd(4)Pn(3) and AZn(4)Pn(3) (A = Na, K, Rb, Cs; Pn = As, P), whose structures feature complex atomic arrangements based on four- and eight-coordinated arsenic and phosphorus. A total of 12 compounds have been synthesized from the corresponding elements via high temperature reactions, and their structures have been established by X-ray diffraction. RbCd(4)As(3), KCd(4)As(3), NaCd(4)As(3), NaZn(4)As(3), KCd(4)P(3), and KZn(4)P(3) crystallize with a new rhombohedral structure (space group R3m, Z = 3, Pearson symbol hR24), while the isoelectronic RbZn(4)As(3), CsCd(4)As(3), CsZn(4)As(3), KZn(4)As(3), CsZn(4)P(3), and RbZn(4)P(3) adopt the tetragonal KCu(4)S(3)-type structure (space group P4/mmm, Z = 1, Pearson symbol tP8). Both structures are very closely related to the ubiquitous CaAl(2)Si(2) and ThCr(2)Si(2) structure types, and the corresponding relationships are discussed. The experimental results have been complemented by linear muffin-tin orbital (LMTO) tight-binding band structure calculations. Preliminary transport properties measurements on polycrystalline samples suggest that the compounds of these families could be promising thermoelectric materials.

Structural properties and new phase transitions of ScN using FP-LMTO method

By full potential linear muffin-tin orbitals (FP-LMTO) method, we have studied the phase transitions of ScN under high pressures. The local density (LDA) approximation was used for the exchange and correlation energy density functional. The most important result is the prediction of the possibility of two phase transitions from the cubic rocksalt (NaCl) structure to the orthorhombic CaSi (Cmmc) structure above 252.5 GPa and to the tetragonal AuCu (P4/mmm) structure at 303.017 GPa, the first one (NaCl–CaSi) occurring at a lower pressure than the well known NaCl to CsCl transition (found here to be 324 GPa).

The effect of crystal lattice distortions on the electronic band structure and optical properties of the N,V- and N,Na-doped anatase

The crystal structure, energy band structure and optical absorption of the N,V-doped and N,Na-doped anatase are studied by means of the first-principle pseudo-potential plane wave and linear muffin-tin orbitals methods. We show that the nitrogen and vanadium atoms have a tendency to form covalently bonded pairs. The crystal lattice distortions associated with doping essentially affect the optical absorption. With doping the impurity bands emerge in the band gap of the host anatase, however, a noticeable increase of optical absorption takes place at the energy only above 3 eV. Possible impact of this effect on the photocatalytic activity of the doped anatase is outlined.

Relativistic Effects and Gold Site Distributions: Synthesis, Structure, and Bonding in a Polar Intermetallic Na6Cd16Au7

Na(6)Cd(16)Au(7) has been synthesized via typical high-temperature reactions, and its structure refined by single crystal X-ray diffraction as cubic, Fm ̅3m, a = 13.589(1) Å, Z = 4. The structure consists of Cd(8) tetrahedral star (TS) building blocks that are face capped by six shared gold (Au2) vertexes and further diagonally bridged via Au1 to generate an orthogonal, three-dimensional framework [Cd(8)(Au2)(6/2)(Au1)(4/8)], an ordered ternary derivative of Mn(6)Th(23). Linear muffin-tin-orbital (LMTO)-atomic sphere approximation (ASA) electronic structure calculations indicate that Na(6)Cd(16)Au(7) is metallic and that ∼76% of the total crystal orbital Hamilton populations (-ICOHP) originate from polar Cd-Au bonding with 18% more from fewer Cd-Cd contacts. Na(6)Cd(16)Au(7) (45 valence electron count (vec)) is isotypic with the older electron-richer Mg(6)Cu(16)Si(7) (56 vec) in which the atom types are switched and bonding characteristics among the network elements are altered considerably (Si for Au, Cu for Cd, Mg for Na). The earlier and more electronegative element Au now occupies the Si site, in accord with the larger relativistic bonding contributions from polar Cd-Au versus Cu-Si bonds with the neighboring Cd in the former Cu positions. Substantial electronic differences in partial densities-of-states (PDOS) and COHP data for all atoms emphasize these. Strong contributions of nearby Au 5d(10) to bonding states without altering the formal vec are the likely origin of these effects.

Band gap of InxGa1−xN: A first principles analysis

We report first principles analysis of the band gap Eg of ternary group-III nitride InxGa1−xN in both the wurtzite and zincblende form, within the linear muffin-tin orbital (LMTO) density functional theory method. We have implemented the semilocal modified Becke–Johnson (MBJ) exchange potential to accurately determine the band gap. The doping of In atoms into the GaN crystal is handled by the InxGa1−xN alloy model within the coherent potential approximation (CPA). The LMTO-CPA-MBJ approach allows us to predict Eg as a function of arbitrary In concentration x. Quantitative comparison to the experimental data is made.

Gallium Pnictides of the Alkaline Earth Metals, Synthesized by Means of the Flux Method: Crystal Structures and Properties of CaGa2Pn2, SrGa2As2, Ba2Ga5As5, and Ba4Ga5Pn8 (Pn = P or As)

AbstractThe focus of this paper is on the structural characterization of the new Zintl phases CaGa2P2, CaGa2As2, SrGa2As2, and Ba2Ga5As5, and the solid solution (Ba0.85(1)Sr0.15)2Ga5As5, all of which were synthesized from molten metal fluxes.CaGa2P2, CaGa2As2, and SrGa2As2 have layered structures with polyanionic layers made of ethane‐like Ga2P6 and Ga2As6 motifs fused through common edges; the polyanionic substructure in Ba2Ga5As5 consists of condensed Ga2As6 units and GaAs4 tetrahedra. Ba4Ga5P8 and Ba4Ga5As8, another pair of new compounds with channel‐like 3D structures, were also synthesized from metal fluxes, and their structures were established from single‐crystal X‐ray and synchrotron powder diffraction. They are based on GaP4 and GaAs4 tetrahedra, with parts of their structures being heavily disordered. The electronic structures computed with the linear muffin‐tin orbital (LMTO) method are discussed as well, alongside the thermopower and the electrical conductivity, measured on single crystals of Ba2Ga5As5 and the solid solution (Ba0.85(1)Sr0.15)2 Ga5As5. They demonstrate that such an approach would be an effective way to fine‐tune the transport properties.

Roles of the band filling and the d orbitals hybridization on the conduction and magnetism of some Sr-based double perovskites: An LSDA+ U study

We have systematically investigated the electronic structure and magnetic properties of the double perovskite oxides Sr 2CrZrO 6, Sr 2MnNbO 6, Sr 2FeMoO 6 and Sr 2NiRuO 6 in terms of band filling of different t 2g and e g orbitals. Calculations of density of the states (DOS) were performed within the frame of the local spin density approximation and Hubbard potential (LSDA+ U) employed in the Linear Muffin Tin Orbitals with Atomic Sphere Approximation (LMTO-ASA) method. The DOS of Sr 2FeMoO 6 was obtained and found to agree very well with published reports. Half-metallic character through the spin-down channel and the ferrimagnetic ordering was observed with notable hybridization among the 3d, p and 4d orbitals within Fermi level. A difference between the theoretically expected values and the calculated ones for the magnetic moment (Δ m) of the 3d orbitals is determined and had been found to increase on going from Sr 2CrZrO 6 to Sr 2NiRuO 6. This increment in Δ m indicates increment in the hybridization strength, which leads to long range ordering in Sr 2FeMoO 6 and Sr 2NiRuO 6 compounds. In comparison with reported experiments, the frustrated magnetism and weak ferrimagnetism states are the most possible ones in Sr 2CrZrO 6 in Sr 2MnNbO 6, respectively. The half-metallic character was also seen in Sr 2MnNbO 6 and Sr 2CrZrO 6 compounds while Sr 2NiRuO 6 shows normal metallic conduction and ferromagnetism in accordance with the superexchange interaction e g 2–o–e g 0 of Goodenough–Kanamori–Anderson (GKA).

Thermoelectric Properties of New Thallium Tellurides

ABSTRACTTernary thallium chalcogenides of the formula Tl4MQ4, whereM= Zr and Hf andQ= S, Se, and Te were synthesized and characterized. Our X-ray diffraction studies on suitable single crystals reveal that the sulphides and selenides are isostructural, with monoclinic space groupP, whereas the corresponding tellurides crystallize in the rhombohedral crystal system (R). The structures of the sulphides and selenides are comprised of zigzag chains of edge-sharingMQ6octahedra, whereas theMTe6octahedra are interconnected via common faces to form linear trimeric units. In all cases, the atoms adopt common oxidation states, namely Tl+,M4+, andQ2–. The electronic structure calculations using the linear muffin tin orbital (LMTO) method predicted band gaps of 1.7 eV, 1.3 eV and 0.3 eV for the sulphides, selenides and tellurides, respectively, implying sulphides and selenides are large band gap materials, and the tellurides narrow gap semiconductors. Their electronic transport properties are also evaluated with respect to the thermoelectric energy conversion.

AB INITIO APPROACH TO STRUCTURAL, ELECTRONIC AND OPTICAL PROPERTIES OF B-, C- AND N-DOPED ANATASE

The effect of doping with boron, carbon and nitrogen on crystal lattice parameters, electronic band structure and optical absorption of anatase has been studied by means of an ab initio density functional theory approach. The investigations included optimization of crystal structure based on the pseudo-potential plane-wave approach. The spin-polarized calculations of the band structure with the account of on-site Coulomb correlations (LSDA+U) employing the tight-binding linear muffin-tin orbitals method (TB-LMTO) have also been performed. The evaluations of optical absorption were based on the calculations of dielectric function with local field effects taken into account. We find that the crystal lattice relaxation around the doping atoms produces noticeable changes in the band structure, magnetic state and optical properties of the doped compounds. The most considerable effects are the collapse of magnetic moment on carbon atom and an essential reduction of the optical absorption in the region of the impurity band — impurity band transitions. Comparing optical absorption for different kinds of doping and taking into account the intensity distribution of the solar light we come to the conclusion than the doping with boron is the most promising kind of doping for photocatalytic applications of the doped anatase.

Structural and electronic properties of bulk GaP and AlP and their [formula omitted] superlattices

The structural and the electronic properties of binary GaP and AlP compounds and their ( GaP ) n / ( AlP ) n superlattices are investigated using the recent version of the first-principles full potential linear muffin-tin orbitals method (FP-LMTO) (Lmtart 7.0). The structural parameters and the pressures at which these compounds undergo structural phase transition from zinc-blende (B3) to the rocksalt (B1) are determined. From the results of the electronic properties we find that the parent materials (GaP, AlP) have indirect bandgaps. The resemblances between GaP and AlP and their small lattice mismatch led us to perform investigations on zinc-blende/zinc-blende ( GaP ) n / ( AlP ) n for n = 1 , 2 and 3 monolayer. Our calculations performed for band structure and density of state show an indirect band gap superlattices for n = 1 and 2 and a direct band gap for n = 3 . Details of the electronic structure of superlattices are discussed. An excellent agreement was found between our results and those of other theoretical predictions and experimental measurements.

Contrasts in Structural and Bonding Representations among Polar Intermetallic Compounds. Strongly Differentiated Hamilton Populations for Three Related Condensed Cluster Halides of the Rare-Earth Elements

The crystal and electronic structures of three related R(3)TnX(3) phases (R = rare-earth metal, Tn = transition metal, X = Cl, I) containing extended mixed-metal chains are compared and contrasted: (1) Pr(3)RuI(3) (P2(1)/m), (2) Gd(3)MnI(3) (P2(1)/m), and (3) Pr(3)RuCl(3) (Pnma). The structures all feature double chains built of pairs of condensed R(6)(Tn) octahedral chains encased by halogen atoms. Pr(3)RuI(3) (1) lacks significant Ru-Ru bonding, evidently because of packing restrictions imposed by the large closed-shell size of iodine. However, the vertex Pr2 atoms on the chain exhibit a marked electronic differentiation. These are strongly bound to the central Ru (and to four I), but very little to four neighboring Pr in the cluster according to bond populations, in contrast to Pr2-Pr "bond" distances that are very comparable to those elsewhere. In Gd(3)MnI(3) (2), the smaller metal atoms allow substantial distortions and Mn-Mn bonding. Pr(3)RuCl(3) (3), in contrast to the iodide (1), can be described in terms of a more tightly bound superstructure of (2) in which both substantial Ru-Ru bonding and an increased number of Pr-Cl contacts in very similar mixed-metal chains are favored by the smaller closed-shell contacts of chlorine. Local Spin Density Approximation (LSDA) Linearized Muffin-Tin Orbital (LMTO)-ASA calculations and Crystal Orbital Hamilton Population (COHP) analyses show that the customary structural descriptions in terms of condensed, Tn-stuffed, R-R bonded polyhedral frameworks are poor representations of the bonding in all. Hamilton bond populations (-ICOHP) for the polar mixed-metal R-Tn and the somewhat smaller R-X interactions account for 75-90% of the total populations in each of these phases, together with smaller contributions and variations for R-R and Tn-Tn interactions. The strength of such R-Tn contributions in polar intermetallics was first established or anticipated by Brewer. The rare-earth-metal atoms are significantly oxidized in these compounds.

Cis-trans Germanium chains in the intermetallic compounds ALi 1– xIn xGe 2 and A2(Li 1– xIn x) 2Ge 3 ( A=Sr, Ba, Eu)—experimental and theoreticalstudies

Two types of strontium-, barium- and europium-containing germanides have been synthesized using high temperature reactions and characterized by single-crystal X-ray diffraction. All reported compounds also contain mixed-occupied Li and In atoms, resulting in quaternary phases with narrow homogeneity ranges. The first type comprises EuLi 0.91(1)In 0.09Ge 2, SrLi 0.95(1)In 0.05Ge 2 and BaLi 0.99(1)In 0.01Ge 2, which crystallize in the orthorhombic space group Pnma (BaLi 0.9Mg 0.1Si 2 structure type, Pearson code oP16). The lattice parameters are a=7.129(4)–7.405(4) Å; b=4.426(3)–4.638(2) Å; and c=11.462(7)–11.872(6) Å. The second type includes Eu 2Li 1.36(1)In 0.64Ge 3 and Sr 2Li 1.45(1)In 0.55Ge 3, which adopt the orthorhombic space group Cmcm (Ce 2Li 2Ge 3 structure type, Pearson code oC28) with lattice parameters a=4.534(2)–4.618(2) Å; b=19.347(8)–19.685(9) Å; and c=7.164(3)–7.260(3) Å. The polyanionic sub-structures in both cases feature one-dimensional Ge chains with alternating Ge–Ge bonds in cis- and trans-conformation. Theoretical studies using the tight-binding linear muffin-tin orbital (LMTO) method provide the rationale for optimizing the overall bonding by diminishing the π– p delocalization along the Ge chains, accounting for the experimentally confirmed substitution of Li forIn.

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.

Effect of mono vacancy on electron and positron properties of 3C SiC

AbstractFirst principle calculation of the electron and positron band structure in the perfect and defected 3C silicon carbide (SiC) is performed within the linear muffin‐tin orbital (LMTO) method in the atomic spheres approximation (ASA). For Si and C vacancy the insulator‐metal transition is observed. Effect of the electron–positron interaction on the resulting positron distribution is studied. Theoretical electron and electron–positron momentum densities for Si and C mono vacancies, obtained within various approximations to the electron–positron correlations, are discussed in comparison with the relevant characteristics in the perfect 3C SiC.

Studies regarding the homogeneity range of the zirconium phosphide telluride Zr 2+δPTe 2

The phosphide tellurides Zr 2+ δ PTe 2 (0 ≤ δ ≤ 1) can be synthesized from the elements in a solid state reaction or by thermal decomposition of Z. Zr 2PTe 2 decomposes under release of Te 2(g) + P 4(g) forming the homogeneity range Zr 2+ δ PTe 2. The growth of single crystals of Zr 2+ δPTe 2 succeeded by chemical vapour transport using iodine as transport agent from 830 °C in direction of higher temperatures up to 900 °C. Zr 2+ δ PTe 2 crystallizes in the rhombohedral space group R 3 ¯ m (no. 166) with lattice parameters a = 383(1)…386(1) pm and c = 2935(4)…2970(4) pm for δ = 0…1, respectively. Single crystal data have been determined for Zr 2.40(2)PTe 2 with lattice parameters a = 385.24(4) pm and c = 2967.8(4) pm. The electronic structure and chemical bonding in Zr 2+ δ PTe 2 was investigated by the linear muffin–tin orbital (LMTO) method. Both Zr 2PTe 2 and Zr 3PTe 2 show non-vanishing DOS values at the Fermi level ( E F) indicating metallic character. According to COHP bonding analyses, mainly the heteroatomic Zr–P and Zr–Te bonds are responsible for the structural stability of Zr 3PTe 2. The new Zr2–Te bond, which is not present in Zr 2PTe 2, is stronger than Zr1–Te and is thought to be responsible for the stability of phases having Zr in excess.

Magnetism in FeNiW disordered alloys: Experiment and theory

We report here an experimental study of magnetization of FeNiW alloys at different compositions. We have studied variation of magnetization with temperature (at low external fields) and magnetic field (at low temperatures). The alloy shows para to ferromagnetic transitions across the composition range. We do not find any indication of the spin-glass phase. We have supplemented the experimental work with theoretical analysis using the first-principles tight-binding linear muffin-tin orbitals based augmented space recursion method. Our theoretical estimates of magnetic moment and Curie temperatures agree well with experiment. Our mean-field phase analysis also does not indicate the possibility of a spin-glass phase.

Calculated valence electronic structure of 3d metals for use in the X-ray intensity ratio studies

3d occupation numbers of the transition elements corresponding to various types of atomic configurations are calculated by means of the linear muffin-tin orbital (LMTO) method. This data is used with the multiconfiguration Dirac–Fock (MCDF) X-ray intensity ratios to estimate the electron populations of the 3d metals in alloys.