New insight on the unusually high ionic mobility in chevrel phases

Chevrel Phases (CPs), M _xMO ₆T ₈ (M = metal, T = S, Se) are unique materials, which allow for a fast and reversible insertion of various cations at RT. Earlier, CPs were divided into two major types: type I with large immobile cations, which block any ionic transport in the diffusion channels, and type II with small mobile cations. Our analysis of available experimental data shows that the transport behavior in CPs cannot be understood in the framework of the blocking concept; it is much more complex and includes: (i) apparent immobility of the large M cations like Pb ₂₊, Sn ₂₊, Ag ₊ in the ternary phases, MMo ₆T ₈; (ii) coupled M + M diffusion in the quaternary phases, M _xM _y,Mo ₆T ₈, where both large and small cations can assist; (iii) cation trapping in the Mg - Mo ₆S ₈, Cd - Mo ₆S ₈, and Na - Mo ₆T ₈ systems; (iv) a combination of low and high rate diffusion kinetics at the first and last intercalation stages, respectively, for the Cu - Mo ₆S ₈, Mn - Mo ₆S ₈, and Cd - Mo ₆Se ₈ systems; and (v) a fast ionic transport for small cations like Ni ²⁺, Zn ²⁺, and Li ⁺. A general structural approach (analysis of the polyhedral linkage in the diffusion channels of CPs and mapping of all the cation sites combined with their bond valence sum values and the distances from the adjacent Mo atoms) used for the first time for a variety of CPs shows two competing diffusion pathways of inserted ions for most of CPs: circular motion within the same cavity between the Mo ₆T ₈ blocks with activation energy E _c, and progressive diffusion from one cavity to the adjacent one with activation energy E _d. The character of the ionic transport depends mostly on the distribution of the repulsive forces for the inserted cations, as well as on the E _d/E _c ratio, affected in tum by the cation position, its size, cation - Mo interactions, and the anion nature.