Molecular Dynamics Study of the Lattice Vibration Contribution to the Frequency-Dependent Dielectric Constant of Lithium
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MOLECULAR DYNAMICS STUDY OF THE LATTICE VIBRATION CONTRIBUTION TO THE FREQUENCY-DEPENDENT DIELECTRIC CONSTANT OF LITHIUM IODIDE J. DEPPE*, M. BALKANSKI**, R. F. WALLIS* and A. McGURN*** *Physics Department, University of California, Irvine Irvine, California 92717 ** Laboratoire de Physique des Solides Universit6 Pierre et Marie Curie 4 Place Jussieu, 75252 Paris C6dex 05, France *** Department of Physics Western Michigan University Kalamazoo, Michigan 49008-5151
ABSTRACT A molecular dynamics simulation has been performed on the crystal lithium iodide, LiI. A rigid ion potential was used with parameters fit to thermal expansion, isothermal compressibility, lattice energy and the frequency of the transverse optical mode at the zone center. The current-current correlation function has been calculated at T = 200K and 400K, and from this the absorption and dispersion have been obtained. Anharmonic broadening is observed at the higher temperature.
1. INTRODUCTION In this paper we investigate the temperature and frequency dependent dielectric function of crystalline lithium iodide. We have modified a rigid ion model first proposed by Michielsen et al.[1] Although computer resources have now reached the stage such that more sophisticated potentials, i.e. including 3-body interactions[21 and explicit polarization terms[31, can be employed, the use of a central force interaction potential is justified in this case by the extremely small deviation from Cauchy behavior for lithium iodide[4]; (c12 - c 44)/(c 1 2 + c 44 ) = 0.005. In addition, although polarization certainly affects the dynamical properties of this crystal, the transverse optical mode at k = 0, which is the mode we will be probing, is not affected to a large extent. We are left with the large anharmonicity[51, which the molecular dynamics method is ideally suited to study. In the present treatment, the frequency and temperature dependent dielectric function (which is measured in the laboratory) is obtained from the current-current correlation function. We have calculated these quantities at two temperatures, 200K which we argue is on the order of the Debye temperature for this material, and 400K, at which we see large anharmonic broadening in the correlation functions. Lowndes[6] has measured the transverse optical frequency WTO using far infrared transmission through ultra thin films at T = 5K and T = T ,oom.However, we have not uncovered a measurement of the temperature dependent broadening with which we could compare our results. In the following section, the crystal structure and relevant properties of LiI will be reviewed. In section 3 we give the potential to be used in this simulation, a variation on one first used by Michielsen et al.[1] The equations of motion and simulation parameters,
Mat. Res. Soc. Symp. Proc. Vol. 210. Q1991 Materials Research Society
132
e.g. the timestep, numerical tolerances and temperatures investigated are also given. In addition, the determination of the correlation functions are discussed. In the next section, results
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