K. Schwarz

Vienna University of Technology, Wien, Vienna, Austria

Are you K. Schwarz?

Claim your profile

Publications (261)580.01 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We investigate theoretically the site occupancy of Al3+ in the fast-ion-conducting cubic-garnet Li7–3xAl3+xLa3Zr2O12 (Ia-3d) using density functional theory. By comparing calculated and measured 27Al NMR chemical shifts an analysis shows that Al3+ prefers the tetrahedrally coordinated 24d site and a distorted 4-fold coordinated 96h site. The site energies for Al3+ ions, which are slightly displaced from the exact crystallographic sites (i.e., 24d and 96h), are similar leading to a distribution of slightly different local oxygen coordination environments. Thus, broad 27Al NMR resonances result reflecting the distribution of different isotropic chemical shifts and quadrupole coupling constants. From an energetic point of view, there is evidence that Al3+ could also occupy the 48g site with its almost regular octahedral coordination sphere. Although this has been reported by neutron powder diffraction, the NMR chemical shift calculated for such an Al3+ site has not been observed experimentally.
    Chemistry of Materials. 04/2014; 26(8):2617–2623.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We investigate theoretically the site occupancy of Al 3+ in the fast-ion-conducting cubic-garnet Li 7−3x Al 3+ x La 3 Zr 2 O 12 (Ia-3d) using density functional theory. By comparing calculated and measured 27 Al NMR chemical shifts an analysis shows that Al 3+ prefers the tetrahedrally coordinated 24d site and a distorted 4-fold coordinated 96h site. The site energies for Al 3+ ions, which are slightly displaced from the exact crystallographic sites (i.e., 24d and 96h), are similar leading to a distribution of slightly different local oxygen coordination environments. Thus, broad 27 Al NMR resonances result reflecting the distribution of different isotropic chemical shifts and quadrupole coupling constants. From an energetic point of view, there is evidence that Al 3+ could also occupy the 48g site with its almost regular octahedral coordination sphere. Although this has been reported by neutron powder diffraction, the NMR chemical shift calculated for such an Al 3+ site has not been observed experimentally.
  • Source
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Multiferroic materials, in which ferroelectric and magnetic ordering coexist, are of fundamental interest for the development of multi-state memory devices that allow for electrical writing and non-destructive magnetic readout operation. The great challenge is to create multiferroic materials that operate at room temperature and have a large ferroelectric polarization P. Cupric oxide, CuO, is promising because it exhibits a significant polarization, that is, P~0.1 μC cm(-2), for a spin-spiral multiferroic. Unfortunately, CuO is only ferroelectric in a temperature range of 20 K, from 210 to 230 K. Here, by using a combination of density functional theory and Monte Carlo calculations, we establish that pressure-driven phase competition induces a giant stabilization of the multiferroic phase of CuO, which at 20-40 GPa becomes stable in a domain larger than 300 K, from 0 to T>300 K. Thus, under high pressure, CuO is predicted to be a room-temperature multiferroic with large polarization.
    Nature Communications 09/2013; 4:2511. · 10.02 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Multiferroic materials, in which ferroelectric and magnetic ordering coexist, are of fundamental interest for the development of multi-state memory devices that allow for electrical writing and non-destructive magnetic read-out operation. The great challenge is to create multiferroic materials that operate at room-temperature and have a large ferroelectric polarization P. Cupric oxide, CuO, is promising because of its large P ~ 10^{2} {\mu}C.m^{-2}, but is unfortunately only multiferroic in a temperature range of 20 K, from 210 to 230 K. Here, using a combination of density functional theory and Monte Carlo calculations, we establish that pressure-driven phase competition induces a giant stabilization of the multiferroic phase of CuO, which at 20-40 GPa becomes stable in a domain larger than 300 K, from 0 to T > 300 K. Thus, under high-pressure, CuO is predicted to be a room-temperature multiferroic with large polarization.
    09/2013;
  • Source
  • [Show abstract] [Hide abstract]
    ABSTRACT: The adsorption of small Aun (n=2–4) clusters on the h-BN/Rh(111) nanomesh is studied by means of density functional theory calculations. We demonstrate that for these small gold clusters a linear geometry is most stable, where all Au atoms bind to underlying B atoms in the pores of the 13×12 h-BN/Rh(111) nanomesh. However, other geometries have similar adsorption energies of more than 2 eV/atom. All Au atoms in these clusters are still negatively charged and thus might be catalytically active. Their partial density of states are significantly narrower and at smaller binding energies as compared to a Au(111) surface.
    Physical review. B, Condensed matter 10/2012; 86(15). · 3.77 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The site occupation of binary Fe-Cr, Co-Cr, Re-W and Fe-V sigma phases is studied in the present work with a first-principles-based single-site mean field theory. We show that the alloy components in these systems exhibit similar site preferences except for the Re-W system, where the occupation of two sites is reversed in agreement with previously published works. In case of the FeV sigma phase, for which the size mismatch between the alloy components is large, we also include into our consideration the effect of local lattice relaxations. The obtained results are found in good agreement with the experimental data and previous theoretical studies.
    Applied Sciences. 09/2012; 2:654-668.
  • P.mohn, K.schwarz
    [Show abstract] [Hide abstract]
    ABSTRACT: Based on the spin-density functional theory we discuss the essential mechanism of spin-split itinerant electrons which cause the formation of spin-magnetic moments in a solid. The success and the difficulties of the Stoner model of itinerant magnetism is shown for hcp Co. The FSM (fixed spin moment) method allows us to compute the total energy as a function of volume and magnetic moment, E(M, V). These energy surfaces contain the crucial information about magneto-volume instabilities and related phenomena. At finite temperatures collective phenomena such as spin fluctuations are important which can be treated with a Landau—Ginzburg formalism. Results are given for the finite temperature properties of the strongly enhanced Pauli paramagnet fcc Pd and the metamagnetic system YCo2.
    International Journal of Modern Physics B 01/2012; 07(01n03). · 0.36 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Fixed-spin-moment band-structure calculations of ordered fcc Fe3Ni reveal Fermisurface nesting, which can give rise to a Kohn-like anomaly. This nesting behaviour is very similar to what has recently been seen in nonmagnetic NixAl1–x.1 Using the method of Varma and Weber2 we discuss the renormalized phonon spectrum of Fe3Ni and investigate how the phonon softening and the martensitic transformation depend on the Fermi-surface nesting.
    International Journal of Modern Physics B 01/2012; 07(01n03). · 0.36 Impact Factor
  • V.l.sliwko, P.blaha, P.mohn, K.schwarz
    [Show abstract] [Hide abstract]
    ABSTRACT: We have performed Augmented Spherical Wave (ASW) calculations on bcc Mn and confirmed previous results by Moruzzi and Marcus who reported a ferrimagnetic phase. Independent computations employing the full-potential linearized-augmented-plane-wave (LAPW) method led to similar results. Our ASW results yield the total energy and the magnetic moments as a function of volume assuming different (ferro-, ferri-, antiferro-and non-magnetic) bcc related structures with type I, II, and III spin alignments. We relate the relative stability of various phases to band gaps that open at the Fermi energy for certain volumes. The LAPW symmetry-decomposed partial charges allow to analyze the binding mechanism.
    International Journal of Modern Physics B 01/2012; 07(01n03). · 0.36 Impact Factor
  • Source
    Xavier Rocquefelte, Karlheinz Schwarz, Peter Blaha
    [Show abstract] [Hide abstract]
    ABSTRACT: It remains a challenge to understand the unconventional mechanisms that cause high-T(C) superconductivity in cuprate superconductors, high-T(C) multiferroicity in CuO, or low-dimensional magnetism in the spin-Peierls transition compounds such as CuGeO(3). A common feature of all these copper oxide compounds (containing Cu(2+) ions) is the presence of large magnetic superexchange interactions J. It is a general strategy to apply chemical and/or physical pressure in order to tune these exotic properties. Here we show theoretically, for the first time, the impact of physical pressure on J on CuO, for which we predict a strong enhancement of the low-dimensionality of the magnetic interactions and the spin-frustration at high-pressures. Such modifications are expected to strongly influence the multiferroic properties of CuO. We finally demonstrate that PBE0 hybrid DFT calculations provide reliable J values for a wide range of copper(II) oxides compounds, i.e. CuGeO(3), BaCu(2)Si(2)O(7), BaCu(2)Ge(2)O(7), and La(2)CuO(4.).
    Scientific Reports 01/2012; 2:759. · 5.08 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We have studied the angular-dependent de Haas-van Alphen oscillations of LaRu2P2 using magnetic torque in pulsed magnetic fields up to 60 T. The observed oscillation frequencies are in excellent agreement with the geometry of the calculated Fermi surface. The temperature dependence of the oscillation amplitudes reveals effective masses m*(α)=0.71 and m*(β)=0.99 me, which are enhanced over the calculated band mass by λcyc of 0.8. We find a similar enhancement of λγ≈1 in comparing the measured electronic specific heat (γ=11.5 mJ/mol K2) with the total density of states from band-structure calculations. Remarkably, very similar mass enhancements have been reported in other pnictides, LaFe2P2, LaFePO (Tc≈4 K), and LaRuPO, independent of whether they are superconducting or not. This is contrary to the common perceptions that the normal-state quasiparticle renormalizations reflect the strength of the superconducting pairing mechanism and leads to new questions about pairing in isostructural and isoelectronic Ru- and Fe-pnictide superconductors.
    Physical review. B, Condensed matter 12/2011; 84(22). · 3.77 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present density functional calculations of the adsorption of a single Au atom on the h-BN/Rh(111) nanomesh and compare it to Au adsorption on a pure h-BN surface. While Au binds only weakly to pure h-BN (similar to graphene) or to BN at the “wires” of the nanomesh, the subsurface Rh atoms in the “pores” of the nanomesh modify the electronic structure of h-BN considerably and Au adsorbs strongly on top of the B atoms. A large outward relaxation of the B atom accompanies the strong covalent interaction and, in addition, the Au atoms are significantly charged. The results provide a first explanation of the ability of the h-BN nanomesh to trap small metal clusters in its pores.
    Physical review. B, Condensed matter 12/2011; 84(24). · 3.77 Impact Factor
  • Xavier Rocquefelte, Karlheinz Schwarz, Peter Blaha
    [Show abstract] [Hide abstract]
    ABSTRACT: A Comment on the Letter by Gianluca Giovannetti et al., Phys. Rev. Lett. 106, 026401 (2011). The authors of the Letter offer a Reply.
    Physical Review Letters 12/2011; 107(23):239701; discussion 239702. · 7.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Configurational thermodynamics of the Fe-Cr sigma phase is investigated on the basis of an Ising-type configurational Hamiltonian and a single-site mean-field model for the free energy. The parameters of the statistical models are obtained from efficient first-principles calculations using different computational techniques. We demonstrate that the effective pair and multisite interactions in the σ phase are relatively small, which allows using a simplified model for distributing Fe and Cr atoms among sublattices. We also show that this system exhibits a nontrivial magnetic behavior at high temperatures, which affects the site occupation by Fe and Cr atoms. The structural variation (volume and c/a) that might be present due to neutron irradiation and thermal expansion can lead to an additional atomic redistribution.
    Physical Review B 11/2011; 84(18):184206. · 3.66 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: In this contribution we present ab initio density-functional-theory (DFT) calculations for CeO2 monolayers on the Pt(111) surface. The ceria surface and ceria-metal interface are of great interest because of the oxygen-storage and release capabilities of ceria, which are widely utilized in catalysis. Both the experimentally reported 3:4 [(4×4)] and 5:7 [(1.4×1.4)] matching geometries of the CeO2/Pt(111) system are studied using the GGA + U exchange-correlation scheme. Geometry optimizations of the structures are performed, resulting in a significant corrugation of both the Pt surface as well as the ceria adlayer surface. The total energies and adsorption energies of three different adsorption geometries for the 3:4 type are compared to the 5:7 structure in terms of stability. The electronic properties and the bonding character are studied by analysis of the density of states (DOS) and of the electron density. The charge transfer occurring during adsorption is calculated using the atoms-in-molecules (AIM) method. Strong interactions are detected, which are mainly based on electrostatic interactions between the topmost Pt layer and the oxygen atoms at the interface, but also include small contributions from hybridization.
    Physical review. B, Condensed matter 07/2011; 84(4). · 3.77 Impact Factor
  • Philipp Haas, Fabien Tran, Peter Blaha, Karlheinz Schwarz
    [Show abstract] [Hide abstract]
    ABSTRACT: The specialized exchange-correlation functionals of Wu and Cohen [ Phys. Rev. B 73 235116 (2006)] (WC) and Hammer et al. [ Phys. Rev. B 59 7413 (1999)] [revised Perdew, Burke, and Ernzerhof (RPBE)] yield good results for either the lattice constants of solids or the atomization energies of molecules, respectively, but are rather poor for the opposite quantity. By combining the WC and RPBE functionals, we construct a functional that performs equally well for both molecules and solids. Our proposed functional, which is still in the form of the simple generalized gradient approximation, can thus be applied efficiently to systems that involve both finite and infinite systems, a case that is crucial, for example, in heterogeneous catalysis studies. Therefore, the chemisorption of CO on transition-metal surfaces was considered, and it is shown that our functional gives improved results.
    Physical review. B, Condensed matter 05/2011; 83(20). · 3.77 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The atomic site distribution of the complex σ-phase structure (P42/mnm) has been studied using density functional theory (within the EMTO and WIEN2k codes) applying the cluster expansion method in a mean field approximation at finite temperatures. We found that at low temperatures Fe atoms predominantly occupy the icosahedrally coordinated (A,D) sites, Cr atoms prefer the (B,E) sites with the high coordination numbers, while the C site remains mixed. However, at higher temperature close to 1000 K all occupations become more and more mixed and reproduce well the available experimental data.
    Solid State Phenomena 04/2011; 170:13-16.
  • [Show abstract] [Hide abstract]
    ABSTRACT: An efficient first-principles-based mean-field theory is applied to study the site occupations in the Fe-Cr σ phase. It is based on the expansion of the total energy of a random alloy close to the equiatomic composition in the paramagnetic state in terms of the effective on-site interactions and takes into account longitudinal spin fluctuations at high temperatures. The calculated site occupation is in very good agreement with the existing experimental data.
    Physical Review B 03/2011; 83(9):092201. · 3.66 Impact Factor

Publication Stats

8k Citations
580.01 Total Impact Points

Institutions

  • 2–2014
    • Vienna University of Technology
      • • Institute of Materials Chemistry
      • • Institute of Chemical Technology, Environmental Technology and Life Sciences
      • • Institute of Theoretical Physics
      • • Institute of Applied Physics
      Wien, Vienna, Austria
    • University of Florida
      Gainesville, Florida, United States
  • 2012
    • French National Centre for Scientific Research
      • Institut des Matériaux Jean Rouxel
      Paris, Ile-de-France, France
  • 2011
    • University of Nantes
      • Institut des Matériaux Jean Rouxel (IMN)
      Nantes, Pays de la Loire, France
  • 2010
    • Institut des Materiaux Jean Rouxel
      Naoned, Pays de la Loire, France
    • University of Duisburg-Essen
      Essen, North Rhine-Westphalia, Germany
    • Christian-Albrechts-Universität zu Kiel
      Kiel, Schleswig-Holstein, Germany
  • 1971–2010
    • University of Vienna
      • • Department of Organic Chemistry
      • • Institut für Physikalische Chemie
      Wien, Vienna, Austria
  • 2004–2006
    • Technische Universität Clausthal
      • Department of Theoretical Physics
      Bergstadt-Clausthal-Zellerfeld, Lower Saxony, Germany
    • The University of Warwick
      • Department of Physics
      Coventry, England, United Kingdom
  • 2005
    • Korea Advanced Institute of Science and Technology
      • Department of Materials Science and Engineering
      Seoul, Seoul, South Korea
  • 2001
    • Pädagogische Hochschule Wien
      Wien, Vienna, Austria
    • Medical School Hamburg
      Hamburg, Hamburg, Germany
  • 1999
    • Technical University Darmstadt
      • Research Area of Materials Science
      Darmstadt, Hesse, Germany
  • 1998
    • University of Amsterdam
      • Van der Waals-Zeeman Institute
      Amsterdamo, North Holland, Netherlands
    • Karl-Franzens-Universität Graz
      • Institute of Physics
      Graz, Styria, Austria
  • 1997
    • Arizona State University
      Phoenix, Arizona, United States
  • 1976–1993
    • Technische Universität München
      • Faculty of Physics
      München, Bavaria, Germany