Phase diagram of Mg insertion into Chevrel phases, MgxMo6T8 (T = S, Se). 3. The crystal structure of triclinic Mg2Mo6Se8.

This series of papers is devoted to unique cathode materials for Mg batteries, MgxMo6T8 (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 A, b = 6.921 A, c = 6.880 A, alpha = 93.00 degrees , beta = 94.40 degrees , gamma = 96.22 degrees ). In contrast to other members of the MgxMo6T8 family, this compound does not follow the classic scheme of successive cation insertion into so-called inner and outer sites: Both the Mg(2+) 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 Mg(2+) ions in Mg2Mo6Se8.

On the mechanism of triclinic distortion in Chevrel Phase as probed by in-situ neutron diffraction.

This work presents, for the first time, a general mechanism of a rhombohedral (R)-triclinic (T) phase transition in Chevrel Phases (CPs) with small cations (radius<1 A), which was unclear in spite of intensive studies of these important materials in the past. In contrast to previous interpretation of the R<-->T transition in some CPs as cation ordering, T-distortion is regarded here as a particular case of general adaptation of the framework to cation insertion, which includes the deformations of the coordination polyhedra and their tilting. The research is based on a combination of experimental studies (in-situ neutron diffraction at different temperatures) for one model compound, MgMo6Se8, and structural analysis for a variety of known CPs. This analysis shows that the structure flexibility is fundamentally different for the R and T forms. As a result of the lower flexibility, in the R form, a strict correlation exists between the compression of the framework along the -3 symmetry axis and the cation position in the structure (the so-called 'delocalization'). The decreasing delocalization in the R-CPs, which occurs on cooling, leads to excessive repulsion within the cations pairs (R-Cu1.8Mo6S8 case) or undesirable asymmetry in the cation polyhedra (R-MgMo6Se8 case). The higher flexibility of the T framework allows for relaxation of these structural strains by increasing the cation-cation distances and forming a more symmetric cation environment, sometimes with higher coordination number (CN), like CN=5 in the T-Fe2Mo6S8 type. Thus, this work also proposes possible driving forces for T-distortion in CPs.

New cathode materials for rechargeable Mg batteries: fast Mg ion transport and reversible copper extrusion in CuyMo6S8 compounds.

We report on a discovery of fast cathode materials, ternary Chevrel phases (CPs), CuyMo6S8, for rechargeable magnesium batteries; the related electrochemical process displays a unique coupling between reversible Mg insertion, and Cu extrusion/ reinsertion; this coupling results in an entirely new intercalation mechanism which combines the total chemical reversibility of the electrochemical reaction of MgxCuyMo6S8 with irreversibility of its separate stages once Cu extrusion stage is reached (in MgxCuyMo6S8: 0.5x + y > 4).