9 research outputs found
Investigation of the Jahn-Teller Transition in TiF3 using Density Functional Theory
We use first principles density functional theory to calculate electronic and
magnetic properties of TiF3 using the full potential linearized augmented plane
wave method. The LDA approximation predicts a fully saturated ferromagnetic
metal and finds degenerate energy minima for high and low symmetry structures.
The experimentally observed Jahn-Teller phase transition at Tc=370K can not be
driven by the electron-phonon interaction alone, which is usually described
accurately by LDA.
Electron correlations beyond LDA are essential to lift the degeneracy of the
singly occupied Ti t2g orbital. Although the on-site Coulomb correlations are
important, the direction of the t2g-level splitting is determined by the
dipole-dipole interactions. The LDA+U functional predicts an aniferromagnetic
insulator with an orbitally ordered ground state. The input parameters U=8.1 eV
and J=0.9 eV for the Ti 3d orbital were found by varying the total charge on
the TiF ion using the molecular NRLMOL code. We estimate the
Heisenberg exchange constant for spin-1/2 on a cubic lattice to be
approximately 24 K. The symmetry lowering energy in LDA+U is about 900 K per
TiF3 formula unit.Comment: 7 pages, 9 figures, to appear in Phys. Rev.
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Bromoform (CHBr sub 3 ) -- A very high-pressure shock-wave analyzer
Bromoform, CHBr{sub 3}, appears to radiate like a black body. This means that the amount of radiation emitted from the shock front is extremely sensitive to temperature and hence even more sensitive to pressure. This feature has been exploited to locate overtake waves in impact experiments. Heretofore, Bromoform was used only for making timing measurements. However, if its P, V, E, and T EOS are known it could be used as high-pressure analyzer. Measurements to determine the Hugoniot, the Grueneisen parameter, {gamma}, and its optical radiation characterization are described, and preliminary data are presented. 8 refs., 7 figs., 1 tab
Elastic Properties of Polycrystalline Aluminum Oxynitride Spinel and Their Dependence on Pressure, Temperature, and Composition
Elastic constants of face-centered cubic and L12 Ni-Si alloys: Composition and temperature dependence
Electronic excitation energies in TiO 2 in the fluorite phase
The ab initio pseudopotential method within the generalized gradient approximation (GGA) and quasiparticle approximation has been used to investigate the electronic properties of titanium dioxide in the rutile, anatase, and fluorite structures, respectively. Here we present the GW approximation for the electronic self-energy, which allows to calculate excited-state properties, especially electronic band structures. For this calculation, good agreement with the experimental results for the minimum band gaps in rutile and anatase phase is obtained. In the fluorite phase we predict that titanium dioxide will be an indirect (Γ to X) wide band-gap semiconductor (2.367 or 2.369 eV) and the properties remain to be confirmed by experiment. Copyright EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg 2010