Crystallography of Chevrel phases, MMo6T8 (M = Cd, Na, Mn, and Zn, T = S, Se) and their cation mobility.
Chevrel phases (CPs), M(x)Mo(6)T(8) (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 Mo(6)T(8) hosts: (i) apparent immobility of the large M cations such as Pb(2+), Sn(2+), Ag(+) in the ternary phases, MMo(6)T(8); (ii) coupled M+M' diffusion in the quaternary phases, M(x)M'(y)Mo(6)T(8), where both large and small cations can assist; (iii) cation trapping in the Mg-Mo(6)S(8), Cd-Mo(6)S(8), and Na-Mo(6)T(8) systems; (iv) a combination of low- and high-rate diffusion kinetics at the first and last intercalation stages, respectively, for the Cu-Mo(6)S(8), Mn-Mo(6)S(8), and Cd-Mo(6)Se(8) systems, and (v) a fast ionic transport for small cations such as Ni(2+), Zn(2+), 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 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(6)T(8), ZnMo(6)T(8), CdMo(6)T(8), and MnMo(6)S(8). All seven compounds can be defined as classic CPs, where cation delocalization from the center of the largest cavity between the Mo(6)T(8) 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.
Levi
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Department of Chemistry, Bar-Ilan University, Ramat-Gan, Israel 52900. elenal@mail.biu.ac.il
Gershinsky
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Aurbach
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Isnard
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