First principles investigation of the superionic electrolyte Li$_7$P$_3$S$_{11}$

Li$_7$P$_3$S$_{11}$ has been shown to be a promising superionic conductor for solid state rechargeable batteries with a room temperature conductivity as high as $10^{-3}$ S/cm and a thermal activation energy as small as $E_A$=0.12 eV.\footnote {F. Mizuno et al., {\em{Solid State Ionics}} {\bf{177}}, 2721 (2006).} We have performed first principles modeling studies\footnote{N. A. W. Holzwarth, N. D. Lepley, Y. A. Du, {\em{J. Power Sources}} {\bf{196}}, 6870 (2011).} on this material in order to explain its stability and Li ion migration properties. Our investigation considers optimized crystal structures, migration involving both vacancy and interstitial mechanisms, as well as related materials. We find optimized crystal structures in reasonable agreement with experiment,\footnote{H. Yamane et al., {\em{Solid State Ionics}} {\bf{178}}, 1162 (2007); Y. Onodera et al., {\em{J. Phys. Soc. Jpn.}} {\bf{79}}, 87 (2010), suppl. A. } and the lowest calculated activation energy barrier was found to be $E_A$=0.15 eV in good agreement with the experimental value.