Phase Diagram of Mg Insertion into Chevrel Phases, MgxMo6T8(T S, Se). 2. The Crystal Structure of Triclinic MgMo6Se8

Chevrel phases (CPs), MxMo6T8(M metal, T S, Se), may be used as unique cathode materials for rechargeable Mg batteries because they ensure the high mobility of multivalent cations. However, the electrochemical behavior is strongly affected by the host composition. For the selenide, the intercalation process is completely reversible, while partial Mg trapping occurs upon its extraction from the sulfide at room temperature. A combination of powder X-ray and high-resolution neutron diffraction was used to study the crystal structure of triclinic MgMo6Se8, especially for determining the precise location of the Mg2cations within the host lattice. It was shown that the crystal structure of the selenide is similar to that of triclinic Fe2Mo6S8:  The Mg2cations are distributed between two sites (per formula unit) with a square-pyramidal anion coordination. The environment analysis of all the cation sites based on the bond valence sum theory led us to propose the most favorable routes for Mg2ion transport, as well as to explain the peculiarities of the electrochemical behavior of the CPs as intercalation materials for Mg batteries.