Diffusion paths formation for Cu ions in superionic Cu6PS5I single crystals studied in terms of structural phase transition

In order to understand the structural transformations leading to high ionic conductivity of Cu⁺ ions in Cu₆PS₅I argyrodite compound, the detailed structure analysis based on single-crystal X-ray diffraction has been performed. Below the phase transition at Cu₆PS₅I belongs to monoclinic, ferroelastic phase (space group Cc) with ordered copper sublattice. Above T_c delocalization of copper ions begins and crystal changes the symmetry to cubic superstructure with space group F-43c (, z=32). Finally, above increasing disordering of the Cu⁺ ions heightens the symmetry to F-43m (, z=4). In this work, the final structural model of two cubic phases is presented including the detailed temperature evolution of positions and site occupation factors of copper ions (R₁=0.0397 for F-43c phase, and 0.0245 for F-43m phase). Possible diffusion paths for the copper ions are represented by means of the atomic displacement factors and split model. The structural results coincide well with the previously reported non-Arrhenius behavior of conductivity and indicate significant change in conduction mechanism.     

Li6PO5Br and Li6PO5Cl: The first Lithium‐Oxide‐Argyrodites

The title compounds Li₆PO₅Br (Fand Li₆PO₅Cl (F represent the first oxidic argyrodites in general and the first lithiumoxoargyrodites in particular. The overall crystal structure corresponds to the cubic high temperature (HT) modification of all known cubic argyrodites, however, with a seemingly small but important difference concerning the lithium positions. In all other HT argyrodites with similar lithium content the 24 lithium atoms per unit cell are disordered over a 48 fold position in close vicinity to a 24 fold one causing a high mobility of the Li⁺. In the title compounds, however, they occupy the 24 fold one in a strictly ordered manner thus establishing a planar triangular first sphere coordination environment. This detail is of great importance for the amount of the specific lithium ionic conductivity and for the possible phase transition to an LT (low temperature) modification accompanied by an ordering of the disordered lithium atoms. Apparently the latter transition is suppressed in the title compounds because the Li⁺ are already frozen out in the cubic (HT = LT) form. The initially open question how this structural peculiarity influences the ionic conductivity (strengthening or weakening in comparison to oxygen free argyrodites?) is answered by a series of impedance measurements. The specific lithium ionic conductivity of the title compounds in the range 313 K < T < 518 K is significantly lower than in oxygen free argyrodites.     

Li7PS6 and Li6PS5X (X: Cl,Br, I): Possible Three‐dimensional Diffusion Pathways for Lithium Ions and Temperature Dependence of the Ionic Conductivity by Impedance Measurements

Possible three‐dimensional diffusion pathways of lithium ions in crystalline lithium argyrodites are discussed based on earlier studies of local dynamics and site preferences. The specific Li‐ionic conductivities of the lithium argyrodites Li₇PS₆ and Li₆PS₅X (X: Cl, Br, I) and their temperature dependences are measured by impedance spectroscopy using different electron‐blocking and ion‐blocking electrode systems. Measurements were carried out between 160 K and 550 K depending on the respective sample. Bulk and grain boundary contributions and the influence of sample preparation are discussed. Typical values for the ionic conductivities at room temperature are in the range 10^–7 to 10^–5 S ·  cm^–1 and at 500 K between 10^–6 and 10^–3 S ·  cm^–1. Thermal activation energies are in the range 0.16 to 0.56 eV. The electronic conductivity at room temperature was measured by polarization measurements for the samples Li₆PS₅X (X: Cl, Br) and was shown to be in the order of magnitude of 10^–8 S ·  cm^–1. Chemical diffusion coefficients of lithium were calculated based on the polarization measurements. For Li₆PS₅Br a high value of 3.5 × 10^–6 cm² · s^–1 was found.