Density Functional Theory of Interplane Cohesion in Graphite and Graphite Intercalation Compounds
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IELE
D.P. DiVINCENZO A;JD E.J.
Dept. of Physics, University of Pennsylvania, Philadelphia, PA
19104, USA.
ABSTRACT We employ density functional theory to study structural energies in pure graphite, Li-graphite, and K-graphite. Qualitative agreement with experiment is obtained for the carbon plane binding energy, lattice constants, compressibilities, and in-plane alkali-alkali potentials. Introduction As the papers of this Symposium show, the structural properties of graphite intercalation compounds have recently been the subject of intense study. Diffraction studies of phase transitions under temperature and pressure, determination of in-plane and out-of-plane phonon spectra, and heat capacity and other thermodynamic measurements, all are reported in this volume. This paper will present a theoretical study of the competing interactions which are responsible for some of these observed phenomena. We perform these calculations using the density functional theory (DFT)[1] , which permits us to examine the kinetic, electrostatic, exchange, and correlation contributions to structural energies. We first apply this theory to the interplanar binding properties of pure graphite. Graphite is both an intrinsically interesting material and a useful test of the DFT. For example, we show that the role of the van der Waals interaction has to date not been adequately explored in graphite. We obtain qualitatively correct predictions for the lattice constant, compressibility and binding energy for graphite, demonstrating the utility of the DFT for highly anisotropic systems. We have also applied a simple version of the theory to both the in-plane and out-of-plane binding properties of Li-graphite and Kgraphite. As for graphite, qualitatively correct predictions are obtained for the structural and elastic parameters of these materials as well. Calculation TheiT'Fin its various forms has been the most successful approach to the calculation of the structural and elastic properties of solids in recent years. In its most sophisticated form, DFT calculations employing the local density approximation have been able to yield very accurate predictions for the ground state properties of many solids 1] . However this approach is computationally difficult for highly anisotropic or low symmetry materials. Therefore, to study graphite and graphite intercalation compounds, we have used a simpler version of the DFT based on the Thomas Fermi approximation. In this approach, the total energy of the solid can be calculated for any arrangement of the atoms, assuming that the valence charge density p(r) is known, by the formula: 2
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124 Here Qo dfnotes integration over the unit cell, k are the crystal lattice vectors, ri the p•gsitions of atoms in the unit cell, Zi the charges of the ion cores, and VION(r) the ionic (ps
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