Density functional theory study of structural and thermodynamical stabilities of ferromagnetic MnX (X = P, As, Sb, Bi) compounds.

Density functional theory (DFT) calculations for deriving enthalpies of formation [Formula: see text] H for ferromagnetic MnX (X [Formula: see text] P, As, Sb, Bi) compounds were made for the two competing structures, hexagonal [Formula: see text] and orthorhombic [Formula: see text]. Standard calculations were performed by using pseudopotentials with the generalized-gradient-approximation (PBE) as exchange-correlation functional. Enhanced exchange-correlation interactions were included by making use of a so-called DFT[Formula: see text]U approach which requires [Formula: see text] as a parameter. Application of PBE potentials for all compounds and elementary phases (all-PBE) resulted in negative values of [Formula: see text] H for MnP and MnAs in both structures whereby the result for MnP [Formula: see text] agrees very well with experiment. For MnSb and MnBi the all-PBE calculation gives a positive nonbonding [Formula: see text] H disagreeing with experiment. To overcome this discrepancy for MnSb and MnBi a DFT[Formula: see text]U ansatz was employed for all compounds and elemental Mn. The values for [Formula: see text] ranging between 0.7 for MnBi and 1.4 eV for MnAs were determined by fitting the DFT results to measured data of [Formula: see text] H. As a reference for pure Mn the [Formula: see text]-Mn phase was taken with [Formula: see text] eV by which choice the experimental volume is fitted. Atomic volumes and ionicities were derived applying Bader's concept resulting in ionicities of Mn less than [Formula: see text].

Boron-phil and boron-phob structure units in novel borides Ni3Zn2B and Ni2ZnB: experiment and first principles calculations.

The crystal structures of two novel borides in the Ni-Zn-B system, τ5-Ni3Zn2B and τ6-Ni2ZnB, were determined by single crystal X-ray diffraction (XRSC) in combination with selected area electron diffraction in a transmission electron microscope (SAED-TEM) and electron probe microanalysis (EPMA). Both compounds crystallize in unique structure types (space group C2/m, a = 1.68942(8) nm, b = 0.26332(1) nm, c = 0.61904(3) nm, ß = 111.164(2)°, RF = 0.0219 for Ni3Zn2B, and space group C2/m, a = 0.95296(7) nm, b = 0.28371(2) nm, c = 0.59989(1) nm, ß = 93.009(4)°, RF = 0.0163 for Ni2ZnB). Both compounds have similar building blocks: two triangular prisms centered by boron atoms are arranged along the c-axis separated by Zn layers, which form empty octahedra connecting the boron centered polyhedra. Consistent with the (Ni+Zn)/B ratio, isolated boron atoms are found in τ5-Ni3Zn2B, while B-B pairs exist in τ6-Ni2ZnB. The crystal structure of Ni2ZnB is closely related to that of τ4-Ni3ZnB2, i.e. Ni2ZnB can be formed by removing the nearly planar nickel layer in Ni3ZnB2 and shifting the origin of the unit cell to the center of the B-B pair. The electrical resistivity and specific heat of τ5-Ni3Zn2B reveal the metallic behavior of this compound with an anomaly at low temperature, possibly arising from a Kondo-type interaction. Further analysis on the lattice contribution of the specific heat reveals similarity with τ4-Ni3ZnB2 with some indications of lattice softening in τ5-Ni3Zn2B, which could be related to the increasing metal content and the absence of B-B bonding in τ5-Ni3Zn2B. For the newly found phases, τ5-Ni3Zn2B and τ6-Ni2ZnB as well as for τ3-Ni21Zn2B20 and τ4-Ni3ZnB2 density functional theory (DFT) calculations were performed by means of the Vienna Ab initio Simulation Package (VASP). Total energies and forces were minimized in order to determine the fully relaxed structural parameters, which agree very well with experiment. Energies of formations in the range of -25.2 to -26.9 kJ mol-1 were calculated and bulk moduli in the range of 179.7 to 248.9 GPa were derived showing hardening by increasing the B concentration. Charge transfer is discussed in terms of Bader charges resulting in electronic transfer from Zn to the system and electronic charge gain by B. Ni charge contributions vary significantly with crystallographic position depending on B located in the neighbourhood. The electronic structure is presented in terms of densities of states, band structures and contour plots revealing Ni-B and Ni-Zn bonding features.

Cage-forming compounds in the Ba-Rh-Ge system: from thermoelectrics to superconductivity.

Phase relations and solidification behavior in the Ge-rich part of the phase diagram have been determined in two isothermal sections at 700 and 750 °C and in a liquidus projection. A reaction scheme has been derived in the form of a Schulz-Scheil diagram. Phase equilibria are characterized by three ternary compounds: τ(1)-BaRhGe(3) (BaNiSn(3)-type) and two novel phases, τ(2)-Ba(3)Rh(4)Ge(16) and τ(3)-Ba(5)Rh(15)Ge(36-x), both forming in peritectic reactions. The crystal structures of τ(2) and τ(3) have been elucidated from single-crystal X-ray intensity data and were found to crystallize in unique structure types: Ba(3)Rh(4)Ge(16) is tetragonal (I4/mmm, a = 0.65643(2) nm, c = 2.20367(8) nm, and R(F) = 0.0273), whereas atoms in Ba(5)Rh(15)Ge(36-x) (x = 0.25) arrange in a large orthorhombic unit cell (Fddd, a = 0.84570(2) nm, b = 1.4725(2) nm, c = 6.644(3) nm, and R(F) = 0.034). The body-centered-cubic superstructure of binary Ba(8)Ge(43)□(3) was observed to extend at 800 °C to Ba(8)Rh(0.6)Ge(43)□(2.4), while the clathrate type I phase, κ(I)-Ba(8)Rh(x)Ge(46-x-y)□(y), reveals a maximum solubility of x = 1.2 Rh atoms in the structure at a vacancy level of y = 2.0. The cubic lattice parameter increases with increasing Rh content. Clathrate I decomposes eutectoidally at 740 °C: κ(I) ⇔ (Ge) + κ(IX) + τ(2). A very small solubility range is observed at 750 °C for the clathrate IX, κ(IX)-Ba(6)Rh(x)Ge(25-x) (x ∼ 0.16). Density functional theory calculations have been performed to derive the enthalpies of formation and densities of states for various compositions Ba(8)Rh(x)Ge(46-x) (x = 0-6). The physical properties have been investigated for the phases κ(I), τ(1), τ(2), and τ(3), documenting a change from thermoelectric (κ(I)) to superconducting behavior (τ(2)). The electrical resistivity of κ(I)-Ba(8)Rh(1.2)Ge(42.8)□(2.0) increases almost linearly with the temperature from room temperature to 730 K, and the Seebeck coefficient is negative throughout the same temperature range. τ(1)-BaRhGe(3) has a typical metallic electrical resistivity. A superconducting transition at T(C) = 6.5 K was observed for τ(2)-Ba(3)Rh(4)Ge(16), whereas τ(3)-Ba(5)Rh(15)Ge(35.75) showed metallic-like behavior down to 4 K.

Strain and structure driven complex magnetic ordering of a CoO overlayer on Ir(100).

We investigate the magnetic ordering in the ultrathin c(10×2) CoO(111) film supported on Ir(100) on the basis of ab initio calculations. We find a close relationship between the local structural properties of the oxide film and the induced magnetic order, leading to alternating ferromagnetically and antiferromagnetically ordered segments. While the local magnetic order is directly related to the geometric position of the Co atoms, the mismatch between the CoO film and the Ir substrate leads to a complex long-range order of the oxide.

Type-I clathrate Ba8Ni(x)Si(46-x): phase relations, crystal chemistry and thermoelectric properties.

The phase relations, crystal structure and thermoelectric properties of the type-I solid solution Ba(8)Ni(x)Si(46-x) were investigated. Based on X-ray diffraction, differential thermal analysis and electron probe microanalysis data, a partial phase diagram was constructed for the Si-rich part of ternary system Ba-Ni-Si at 800 °C. The solubility range of Ni in the clathrate-I phase at 800 °C was determined (2.9 ≤x≤ 3.8) and thermoelectric properties, namely electrical resistivity, Seebeck-coefficient and thermal conductivity, were measured in the temperature range from 300 to 850 K. A shift of the thermoelectric properties from a predominantly metallic to a more semiconducting behavior was observed for an increasing Ni-content. Density functional calculations revealed a significant decrease of the gap width in the density of states induced by the incorporation of Ni. Electrical resistivity and Seebeck coefficients for Ba(8)Ni(x)Si(46-x) with 3.3 ≤x≤ 3.8 have been modeled within the rigid band approximation.

Tailor-made ultrathin manganese oxide nanostripes: 'magic widths' on Pd(1 1 N) terraces.

The growth of ultrathin two-dimensional manganese oxide nanostripes on vicinal Pd(1 1 N) surfaces leads to particular stable configurations for certain combinations of oxide stripe and substrate terrace widths. Scanning tunneling microscopy and high-resolution low-energy electron diffraction measurements reveal highly ordered nanostructured surfaces with excellent local and long-range order. Density functional theory calculations provide the physical origin of the stabilization mechanism of 'magic width' stripes in terms of a finite-size effect, caused by the significant relaxations observed at the stripe boundaries.

Thickness dependent structural and electronic properties of CuO grown on SrTiO3(100): a hybrid density functional theory study.

We discuss the structural and electronic properties of tetragonal CuO grown on SrTiO3(100) by means of hybrid density functional theory. Our analysis explains the anomalously large Cu-O vertical distance observed in the experiments (≈2.7 Å) in terms of a peculiar frustration between two competing local Cu-O environments characterized by different in-plane and out-of-plane bond lengths and Cu electronic populations. The proper inclusion of substrate effects is crucial to understanding the tetragonal expansion and to reproduce correctly the measured valence band spectrum for a CuO thickness of 3-3.5 unit cells, in agreement with the experimentally estimated thickness.

Polaronic hole trapping in doped BaBiO3.

The present ab initio study shows that in BaBiO3, Bi3+ sites can trap two holes from the valence band to form Bi5+ cations. The trapping is accompanied by large local lattice distortions; therefore the composite particle consisting of the electronic hole and the local lattice phonon field forms a polaron. Our study clearly shows that even sp elements can trap carriers at lattice sites, if local lattice relaxations are sufficiently large to screen the localized hole. The derived model describes all relevant experimental results, and settles the issue of why hole-doped BaBiO3 remains semiconducting upon moderate hole doping.

Interplay between magnetic, electronic, and vibrational effects in monolayer Mn3O4 grown on Pd(100).

The surface stabilized MnO(100)-like monolayer, characterized by a regular c(4 x 2) distribution of Mn vacancies, is studied by hybrid functionals and discussed in the light of available scanning tunneling microscopy and high-resolution electron energy loss spectroscopy data. We show that the use of hybrid functionals is crucial to account for the intermingled nature of magnetic interactions, electron localization, structural distortions, and surface phonons. The proposed Pd(100) supported Mn(3)O(4) structure is excellently compatible with the experiments previously reported in literature.

FLAPW: applications and implementations.

Modern material design involves a close collaboration between experimental and computational materials scientists. To be useful, the theory must be able to accurately predict the stability and properties of new materials, describe the physics of the experiments, and be applicable to new and complex structures-the all-electron full-potential linearized augmented plane wave (FLAPW) is one such method that provides the requisite level of numerical accuracy, albeit at the cost of complexity. Technical aspects and modifications related to the choice of basis functions (energy parameters, core-valence orthogonality, extended local orbitals) that affect the applicability and accuracy of the method are described, as well as an approach for obtaining k-independent matrix elements. The inclusion of external electric fields is illustrated by results for the induced densities at the surfaces of both magnetic and non-magnetic metals, and the relationship to image planes and to nonlinear effects such as second harmonic generation. The magnetic coupling of core hole excitations in Fe, the calculation of intrinsic defect formation energies, the concentration-dependent chemical potentials, entropic contributions, and the relative phase stability of Zr-rich Zr-Al alloys are also discussed.

Two-dimensional manganese oxide nanolayers on Pd(100): the surface phase diagram.

Two-dimensional manganese oxide layers have been grown on Pd(100) and have been characterized by means of scanning tunnelling microscopy, low energy electron diffraction and x-ray photoelectron spectroscopy (XPS). The complex surface phase diagram of MnO(x) on Pd(100) is reported, where nine different novel Mn oxide phases have been detected as a function of the chemical potential of oxygen µ(O). Three regions of the chemical potential of oxygen can be identified, in which structurally related oxide phases are formed, often in coexistence at the surface. The different regions of µ(O) are reflected in the oxidation states of the respective Mn oxide nanolayers as revealed by the Mn 2p and O 1s XPS binding energies. The MnO(x) nanolayers form two-dimensional wetting layers and it is speculated that they mediate the epitaxial growth of MnO on Pd(100) by providing structurally graded interfaces.

Ab initio phase diagram of oxygen adsorption on W(110).

The phase diagram of oxygen adsorption on the W(110) surface is derived without any empirical parameters by a combination of density functional theory (DFT) calculations, the cluster expansion (CE) technique and Monte Carlo (MC) applications. Coverages up to 1 monolayer are considered corresponding to the range of oxygen concentrations, 0≤x(O)≤1. DFT results for single-site adsorption and in particular for full coverage reveal that adsorption at threefold hollow (H3) sites is by far the most stable one. Therefore, the CE is done for an atomic layer with the two H3 sublattices of the W(110) surface. Based on 60 DFT calculations with fully relaxed atomic geometries of lateral unit cells containing 12 atoms, and a ground state search for 80 394 structures, four ground state structures are found with the lateral unit cells (2 × 5) for x(O) = 0.20, (2 × 2) (a) for x(O) = 0.25, (2 × 1) for x(O) = 0.50 and (2 × 2) (b) for x(O) = 0.75. In agreement with experiments the most stable structures are (2 × 1) and (2 × 2) (b), which correspond to higher coverages. The thermodynamical stability of the two ground states at lower coverages is very weak. Detailed analysis of the relaxation of the (2 × 1) structure reveals sizeable lateral stresses acting on the surface tungsten atoms. On the basis of the effective cluster interactions MC simulations are performed in order to derive the critical temperatures by which the phase diagram is finally constructed.

Spin transition in a four-coordinate iron oxide.

Spin transition has attracted the interest of researchers in various fields since the early 1930s, with thousands of examples now recognized, including those in minerals and biomolecules. However, so far the metal centres in which it has been found to occur are almost always octahedral six-coordinate 3d(4) to 3d(7) metals, such as Fe(II). A five-coordinate centre is only rarely seen. Here we report that under pressure SrFe(II)O(2), which features a four-fold square-planar coordination, exhibits a transition from high spin (S = 2) to intermediate spin (S = 1). This is accompanied by a transition from an antiferromagnetic insulating state to a ferromagnetic so-called half-metallic state: only half of the spin-down (d(xz),d(yz)) states are filled. These results highlight the square-planar coordinated iron oxides as a new class of magnetic and electric materials.

Superconductivity in novel Ge-based skutterudites: {Sr,Ba}pt4Ge12.

Combining experiments and ab initio models we report on SrPt4Ge12 and BaPt4Ge12 as members of a novel class of superconducting skutterudites, where Sr or Ba atoms stabilize a framework entirely formed by Ge atoms. Below T(c)=5.35 and 5.10 K for BaPt4Ge12 and SrPt4Ge12, respectively, electron-phonon coupled superconductivity emerges, ascribed to intrinsic features of the Pt-Ge framework, where Ge-p states dominate the electronic structure at the Fermi energy.

Influence of impurities on localized transition metal surface states: scanning tunneling spectroscopy on V(001).

The first scanning tunneling spectroscopy measurements on V(001) are reported. A strong surface state is detected which is very sensitive to the presence of segregated carbon impurities. The surface state energy shifts from 0.03 eV below the Fermi level at clean areas towards higher energies (up to approximately 0.2 eV) at contaminated areas. Because of the negative dispersion of this state, the upward shift cannot be described in a simple confinement picture. Rather, the surface state energy is governed by vanadium surface s- d interactions which are altered by carbon coverage.

Bulk terminated NaCl(111) on aluminum: a polar surface of an ionic crystal?

Atomically resolved scanning tunneling microscopy reveals the existence of triangular (111) bulk terminated NaCl islands. The islands can be grown by subsequent adsorption of Na and Cl2 on Al(111) and Al(100) or by conversion of stoichiometric NaCl(100) islands to NaCl(111) via additional Na adsorption. The NaCl(111) islands have Na-Cl-Na sandwich structure. Ab initio calculations of the electronic structure of these islands show that each of the Na atoms carries half a positive elementary charge, leaving the islands neutral and explaining the existence of an otherwise unstable surface.