Probability density and effective one-particle potential of octahedral Ag in the fast ion conductor Ag0.39 TiS2

The structure of Ag0.39TiS2 has been determined in the temperature range 300-600 K using single crystal X-ray data. The parameters of the hexagonal unit cell (space group P3mlBAR) at 300 K are: a = 3.4359(2) angstrom, c = 6.4151(6) angstrom. The structure factor is extended to include fourth order tensor elements for Ag to account for the softening of the vibration potential at higher temperatures. wR(F2) factors range from 0.012 to 0.031. It is shown that the surface of constant displacement probability of Ag is an ellipsoid, displaying the anisotropic nature of the softening of the vibration potential. From the temperature dependence of the harmonic tensor elements and the effective one-particle potential it is deduced that only occupational disorder is present in the Ag distribution, i.e. without accompanying static displacement disorder. Thus, Ag vibrates in a strongly anharmonic vibration potential around only one equilibrium site in the center of a sulfur octahedron.