Intergrowth between binary and ternary phases in Chevrel-phase compounds REMo6Se8 containing heavy rare-earth elements

A Chevrel-phase compound, based on a heavy rare-earth element (Er) was studied by nanodiffraction and high-resolution electron microscopy (HRTEM) in order to understand an intrinsic co-crystallization process, which develops between the ternary phase ErMo6Se8 (T) and the binary phase Mo6Se8 (B). These two phases crystallize as thin layers, alternating each other and creating extended domains, which grow along two specific directions. Inside one domain, successive B and T layers share common symmetry elements, such as one rhombohedral face (e.g. (100)B,T) and one rhombohedral axis (e.g. [001]B,T); the interphase boundary grows at 11° from the common plane, that is, parallel to the {105} crystallographic planes. In two adjacent domains, B (or T) keeps the same orientation and T (or B) layers shares the same symmetry elements but with different indexes (e.g. (100)B,T, [001]B,T in one domain and (001)B,T, [100]B,T in the other). Layers in two adjacent domains share one rhombohedral axis (e.g. [010]B), keeping an angle of about 70° (or 110°). In this work simple models are developed to explain the bi-directional growth and these characteristic angles. The orientation relationships allow a good lattice match between the binary and ternary phases. Single crystal data are analyzed in order to understand why the intergrowth does not occur when light rare-earth elements (i.e. from La to Sm) are inserted into the structure. The elongation of the rhombohedral lattice, associated to a contraction of the Mo6Se8 building block, appear to be the most probable mechanism to generate large microstructural stresses and increase the interface energy, precluding an epitaxial growth for the first half of the series. At the heavy rare-earth side (i.e. from Gd to Lu), lattice parameters, interatomic distances and atomic positions get close enough to those of the binary phase, thus favoring the intergrowth.