Phase diagram of Mg insertion into chevrel phases, Mg _xMo ₆T ₈ (T = S, Se). 1. Crystal structure of the sulfides

A combination of ab initio calculations and experimental methods (high-resolution neutron and powder X-ray diffractions) was used to solve the crystal structure of Mg _xMo ₆S ₈ (x = 1 and 2). It was shown that at room temperature, the latter are similar to the crystal structure of classic Chevrel phases (CPs) such as Cu _xMo ₆S ₈: space group R3̄, a _r = 6.494 Å, α = 93.43° for MgMo ₆S ₈ and a _r = 6.615 Å, α = 95.16° for Mg ₂Mo ₆S ₈. For x = 1, one Mg ²⁺ cation per formula unit is distributed statistically between inner sites. For x = 2, the second Mg ²⁺ cation per formula unit is located in the outer sites. Peculiarities of the electrochemical behavior of the CPs as electrode materials for Mg batteries were understood on the basis of the analysis of the interatomic distances. It was shown that the circular motion of the Mg ²⁺ ions between the inner sites in MgMo ₆S ₈ is more favorable than their progressive diffusion in the bulk of the material, resulting in relatively slow diffusion and Mg trapping in this phase. In contrast, in Mg ₃Mo ₆S ₈, the repulsion between the Mg ²⁺ ions located in the inner and outer sites facilitates their transport through the material bulk.