Chevrel Phases, M_xMo₆T₈ (M = Metals, T = S, Se, Te) as a structural chameleon: Changes in the rhombohedral framework and triclinic distortion

It is commonly accepted that the nature of triclinic distortion, responsible for the superconductivity loss in Chevrel phases, is different for the two main structural types: electronic instability of the Mo ₆-cluster in the compounds with large cations (type I) and "freezing" of the formerly mobile small cations in fixed positions (type II). Recently, for Chevrel phases with small cations, we proposed an alternative mechanism of the phase transition as one of the ways to attain the steric matching between the cations and the framework. It was shown that the triclinic structure is more flexible than the rhombohedral because it allows for additional deformations and a relative tilt of the main structural elements. Bond valence analysis performed for the first time for a variety of Chevrel phases in this work shows that the intrinsic instability of the binary compounds, Mo₆T₈, does not arise from the Mo ₆-cluster anisotropy, but rather from the extremely non-uniform distribution of the anion charge in their crystal structure. This distribution changes drastically with cation insertion or partial chalcogen-halogen substitution, resulting in compound stabilization. However, a steric mismatching between big cations such as Pb, Sr, or Ba and the framework of the large and rigid Mo₆T₈ blocks strongly destabilizes the structure. The mismatching is evident from the unreasonably high bond valence sums for these cations; it increases with the cation size. Triclinic distortion, occurring on cooling, is associated with minor atomic displacements, and consequently, only with partial relaxation of the bond strains. In contrast, application of pressure decreases fundamentally the constraints in the M-T bonding and prevents the material from the phase transition. Thus, this work presents a general mechanism of structural instability for all Chevrel phases and explains changes in the Mo₆-cluster shape in the framework of simple bond valence relations.