Phase diagram of Mg insertion into chevrel phases, Mg_xMo ₆T₈ (T = S, Se). 3. The crystal structure of triclinic Mg₂Mo₆Se₈

This series of papers is devoted to unique cathode materials for Mg batteries, Mg_xMo₆T₈ (T = S, Se, x = 1 and 2) Chevrel phases (CPs). In this part, a combination of neutron and high-resolution synchrotron X-ray diffractions was used to study the crystal structure of Mg₂Mo₆Se₈, which is triclinic at room temperature (space group P1, a = 6.868 Å, b = 6.921 Å, c = 6.880 Å, α = 93.00°, β = 94.40°, γ = 96.22°). In contrast to other members of the Mg_xMo₆T₈ family, this compound does not follow the classic scheme of successive cation insertion into so-called inner and outer sites: Both the Mg²⁺ ions per formula are located in the tetrahedral sites of the outer ring. This surprising cation location, predicted previously for Mg-containing CPs by ab initio calculations, provides the uniform distribution of the cation charge in the triclinic structure, which is similar to that of rhombohedral CPs. A mapping of the cation sites was widely used to demonstrate the variety of cation arrangement in CPs and the factors affecting this arrangement, as well as to clarify the origin of the exceptionally high mobility of the Mg²⁺ ions in Mg₂Mo₆Se₈.