Journal Article Nearly Constant Loss (NCL) in Lithium Metasilicate Glass at Low Temperatures – Anisotropic and Dynamical Caging from Molecular Dynamics Simulations

巾崎, 潤子  ,  HABASAKI, JUNKO  ,  Ngai, Kia

121pp.13729 - 13737 , 2017-06 , American Chemical Society
Ions in ionically conducting materials show complex but universal trends in the time development of the mean squared displacements (MSD) of ions, which correspond to the complex frequency dependence conductivity spectra or susceptibility. Molecular dynamics simulations enable us to characterize the details of such motions including the initial nearly logarithmic or weak power law time dependence of the MSD of caged ions, showing up in susceptibility as the nearly constant loss (NCL) and the important connections of the caged ion dynamics to ion hopping dynamics in ionically conducting materials. In previous works, the motion of ions in the NCL region for lithium metasilicate was characterized at 700 and 500 K. Since each time region of MSD becomes longer and longer with decreasing temperature, the NCL region will cover the entire observation time during the MD run at lower temperatures, and thus enabling a more detailed study of the properties of the NCL. This paper reports results from new simulations at temperatures lower than ever done before down to 100 K of long time scale. Temperature dependence of the time regime of the MSD corresponding to the NCL and the cage decay that terminates it are obtained from the self-part of the Van Hove function. The results at 100 K clearly show the behavior expected as well as new features. These include anisotropy of the shapes of cages, and some characteristics of the ionic motion within fluctuating cages, which are visualized by the simulations. The potential field in the NCL region is explained by the multipole expansions. The dynamics of ions in the caged regime are characterized by anharmonic motions within cages, and from which the NCL originates. Moreover, the dynamics shows strong intermittency. The relation to similar behaviors in glass forming liquids is discussed.

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