Preferred selenium incorporation and unexpected interlayer bonding in the layered structure of Sb2Te3-xSex

We have performed a detailed structural analysis of several crystals from the Sb2Te3-xSex solid solution (with x=0-1.55) using a combination of single-crystal and powder X-ray diffraction and high-resolution scanning transmission electron microscope imaging combined with energy-dispersive X-ray spectroscopic mapping. The experimental study was supported by quantum-chemical calculations. All compounds crystallize in the rhombohedral tetradymite structure type in which the atoms occupy three symmetrically independent crystallographic sites; the two chalcogenide sites A1 and A2, and the antimony site Sb. They form quintuple A2-Sb-A1-Sb-A2 layers perpendicular to the [001] direction, held together via A2-A2 chalcogenide-chalcogenide interactions. The results of our experimental study show unambiguously, and in agreement with quantum-chemical calculations, a preferred incorporation of selenium into the A2 position inside the layers. An unexpected increase of the van-der-Waals inter-layer distances is observed for low Se contents (x <1) and discussed with the help of DFT calculations.