Crystal structure and Raman-active lattice vibrations of magnetic topological insulators MnBi2Te4·n(Bi2Te3) ( n=0 , 1,...,6)

Abstract

Further to the structure of the intrinsic magnetic topological insulators $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}\ifmmode\cdot\else\textperiodcentered\fi{}n({\mathrm{Bi}}_{2}{\mathrm{Te}}_{3})$ with $n<4$, where index $n$ is the number of quintuple Te-Bi-Te-Bi-Te building blocks inserted between the neighboring septuple Te-Bi-Te-Mn-Te-Bi-Te building blocks, the structure of the members with $n=4$, 5, and 6 was studied using x-ray powder diffraction. The unit-cell parameters and atomic positions were obtained. The obtained and available structural data were summarized to show that the crystal structure of all members of $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}\ifmmode\cdot\else\textperiodcentered\fi{}n({\mathrm{Bi}}_{2}{\mathrm{Te}}_{3})$ follows the cubic close-packing principle, independently of the space group of the given member. Confocal Raman spectroscopy was then applied. Comparative analysis of the number, frequency, symmetry, and broadening of the vibration modes responsible for the lines in the Raman spectra of the systems with $n=1$,\dots{},6, as well as $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}\phantom{\rule{0.16em}{0ex}}(n=0)$ and ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ ($n=\ensuremath{\infty}$) has shown that lattice dynamics of $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}\ifmmode\cdot\else\textperiodcentered\fi{}n({\mathrm{Bi}}_{2}{\mathrm{Te}}_{3})$ with $n>0$ overwhelmingly dominates by the cooperative atomic displacements in the quintuple building blocks.