Phase diagram of Mg insertion into chevrel phases, Mg_xMo ₆T₈ (T = S, Se). 2. THe crystal structure of triclinic MgMo₆Se₈

Chevrel phases (CPs), M_xMo₆T₈ (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 Mg²⁺ cations within the host lattice. It was shown that the crystal structure of the selenide is similar to that of triclinic Fe₂Mo₆S₈: The Mg²⁺ cations 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 Mg²⁺ ion transport, as well as to explain the peculiarities of the electrochemical behavior of the CPs as intercalation materials for Mg batteries.