Crystallography of chevrel phases, MMo₆T₈ (M = Cd, Na, Mn, and Zn, T = S, Se) and their cation mobility

Chevrel phases (CPs), M_xMo₆T₈ (M = metal, T = S, Se) are unique materials, which allow for a fast and reversible insertion of various cations at room temperature. In spite of extensive studies of these materials, the origin of their high ionic mobility remained unclear. In a previous paper we presented for the first time a proper classification of the very complex transport behavior of different cations in the Mo₆T ₈ hosts: (i) apparent immobility of the large M cations such as Pb²⁺, Sn²⁺, Ag⁺ in the ternary phases, MMo ₆T₈; (ii) coupled M+M' diffusion in the quaternary phases, M_xW'_yMo₆T₈, where both large and small cations can assist; (iii) cation trapping in the Mg-Mo₆S ₈, Cd-Mo₆S₈, and Na-Mo₆T₈ systems; (iv) a combination of low- and high-rate diffusion kinetics at the first and last intercalation stages, respectively, for the Cu-Mo ₆S₈, Mn-Mo₆S₈, and Cd-Mo ₆Se₈ systems, and (v) a fast ionic transport for small cations such as Ni²⁺, Zn²⁺, and Li⁺. It was shown that this behavior could be understood by a relatively simple crystallography analysis (mapping of all the cation sites and estimations of their potential energy according to the distances of these sites from adjacent anions and cations) of the diffusion routes, which differ for different cations. For this analysis, it was necessary to complete our knowledge about the cation location in the crystal structure for several CPs with known ionic mobility. This article presents the results of a combined Rietveld analysis of powder X-ray and high-resolution neutron diffraction profiles for NaMo ₆T₈, ZnMo₆T₈, CdMo₆T ₈, and MnMo₆S₈. All seven compounds can be defined as classic CPs, where cation delocalization from the center of the largest cavity between the Mo₆T₈ blocks decreases with the length of the ideal chemical bond, M-T. In addition, this work details the effect of the structural parameters on the ionic conductivity in CPs, namely, it shows how subtle changes in cation delocalization may be crucial for diffusion kinetics.