Synthesis and insights into crystal structure, lattice dynamics and dielectric function of a new 9P–van der Waals compound SnBi4Te4

The single crystal of SnBi₄Te₄—a member of n(Bi₂)∙m(SnBi₂Te₄) homological series and potential candidate material with nontrivial topology was grown by the modified Bridgman method and characterized by X-ray diffraction (XRD), scanning electron microscopy with an energy dispersive X-ray spectrometer (SEM–EDX), and X-ray photoelectron spectroscopy (XPS). High single-crystalline quality of the grown crystal without any signs of surface contaminants detectable by SEM–EDX and XPS was confirmed by XRD. According to the results of Rietveld analysis of the XRD data, a trigonal structure with the P-3m1 space group symmetry and a stacking sequence of atomic layers similar to that of the 9P polytype crystal structure of GeSb₄Te₄ is inherent to SnBi₄Te₄. The crystal structure is built up from 7-layer rocksalt-type slabs, alternated with bismuth bilayers, with van der Waals (vdW) bonding between the two. Raman spectroscopy and spectroscopic ellipsometry (SE), complemented and supported by ab-initio calculations, provided the core information regarding the lattice dynamics and dielectric function of SnBi₄Te₄, as well as preliminary insight into possible functionalities of the grown material.