Crystallography of Chevrel Phases, MMo6T8 (M = Cd, Na, Mn, and Zn, T = S, Se) and Their Cation Mobility

E. Levi (1) , Gregory Gershinsky (2) , Doron Aurbach (1) , Olivier Isnard (3)

1 Bar Ilan University Dept. Chemistry
2 Bar-Ilan University [Israël]
3 NEEL - MRS - Matériaux, Rayonnements, Structure

Chevrel phases (CPs), MxMo6T8 (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 Mo6T8 hosts: (i) apparent immobility of the large M cations such as Pb2+, Sn2+, Ag+ in the ternary phases, MMo6T8; (ii) coupled M+M' diffusion in the quaternary phases, MxM'yMo6T8, where both large and small cations can assist; (iii) cation trapping in the Mg-Mo6S8, 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-Mo6S8, Mn-Mo6S8, and Cd-Mo6Se8 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 crystallographic 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 Us 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 NaMo6T8, ZnMo6T8, CdMo6T8, 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.

Domaines: Science des matériaux [cond-mat.mtrl-sci]

E. Levi, Gregory Gershinsky, Doron Aurbach, Olivier Isnard. Crystallography of Chevrel Phases, MMo6T8 (M = Cd, Na, Mn, and Zn, T = S, Se) and Their Cation Mobility. Inorganic Chemistry, 2009, 48 (18), pp.8751-8758. ⟨10.1021/ic900805g⟩. ⟨hal-00988696⟩