title: Atomistic characterisation of Li+ mobility and conductivity in Li(7-x)PS(6-x)Ix argyrodites from molecular dynamics simulations, solid-state NMR, and impedance spectroscopy.
abstract: The atomistic mechanisms of Li(+) ion mobility/conductivity in Li(7-x)PS(6-x)I(x) argyrodites are explored from both experimental and theoretical viewpoints. Ionic conductivity in the title compound is associated with a solid-solid phase transition, which was characterised by low-temperature differential scanning calorimetry, (7)Li and (127)I NMR investigations, impedance measurements and molecular dynamics simulations. The NMR signals of both isotopes are dominated by anisotropic interactions at low temperatures. A significant narrowing of the NMR signal indicates a motional averaging of the anisotropic interactions above 177+/-2 K. The activation energy to ionic conductivity was assessed from both impedance spectroscopy and molecular dynamics simulations. The latter revealed that a series of interstitial sites become accessible to the Li(+) ions, whilst the remaining ions stay at their respective sites in the argyrodite lattice. The interstitial positions each correspond to the centres of tetrahedra of S/I atoms, and differ only in terms of their common corners, edges, or faces with adjacent PS(4) tetrahedra. From connectivity analyses and free-energy rankings, a specific tetrahedron is identified as the key restriction to ionic conductivity, and is clearly differentiated from local mobility, which follows a different mechanism with much lower activation energy. Interpolation of the lattice parameters as derived from X-ray diffraction experiments indicates a homogeneity range for Li(7-x)PS(6-x)I(x) with 0.97 < or = x < or = 1.00. Within this range, molecular dynamics simulations predict Li(+) conductivity at ambient conditions to vary considerably.
authors.affiliation: Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Str. 40, 01187 Dresden, Germany.
authors.forename: Oliver
authors.initials: O
authors.lastname: Pecher
authors.forename: Shiao-Tong
authors.initials: ST
authors.lastname: Kong
authors.forename: Thorsten
authors.initials: T
authors.lastname: Goebel
authors.forename: Vera
authors.initials: V
authors.lastname: Nickel
authors.forename: Katja
authors.initials: K
authors.lastname: Weichert
authors.forename: Christof
authors.initials: C
authors.lastname: Reiner
authors.forename: Hans-Jörg
authors.initials: HJ
authors.lastname: Deiseroth
authors.forename: Joachim
authors.initials: J
authors.lastname: Maier
authors.forename: Frank
authors.initials: F
authors.lastname: Haarmann
authors.forename: Dirk
authors.initials: D
authors.lastname: Zahn