Pressure-induced topological and structural phase transitions in natural van der Waals heterostructures from the (SnTe)m(Bi2Te3)n

A new class of van der Waals heterostructures of ${(\mathrm{SnTe})}_{\mathrm{m}}{({\mathrm{Bi}}_{2}{\mathrm{Te}}_{3})}_{\mathrm{n}}$ (with $m=1,2,.\phantom{\rule{0.16em}{0ex}}.$ and $n=1,2,.\phantom{\rule{0.16em}{0ex}}.$), consisting of a topological crystalline insulator SnTe and a topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ are emerging with exciting properties and applications, such as in thermoelectrics. Our study examines the stability of these heterostructures ($m$ = 1 and $n$ = 1,2) under pressure using Raman scattering, synchrotron x-ray diffraction, and density functional theory. Raman studies as a function of pressure carried out at room temperature reveal a phase transition in the pressure regime of 3--5 GPa for both the compounds, which is shown to be associated with an electronic topological transition involving change in the ${\mathrm{Z}}_{2}$ topological invariant. In addition to the electronic changes, our Raman experiments indicate that rhombohedral ($R\overline{3}m$) ${\mathrm{SnBi}}_{2}{\mathrm{Te}}_{4}$ undergoes structural transition at $\ensuremath{\sim}6.0$ to a possible monoclinic phase and another transition at $\ensuremath{\sim}12.0$ GPa. Raman and x-ray diffraction experiments on trigonal ($P\overline{3}m1$) ${\mathrm{SnBi}}_{4}{\mathrm{Te}}_{7}$ show two structural transitions at $\ensuremath{\sim}9.5$ GPa to a monoclinic phase followed by one to cubic phase at $\ensuremath{\sim}14.1$ GPa. Our analysis of electronic structure reveals that the phase transition at 9.5 GPa in ${\mathrm{SnBi}}_{4}{\mathrm{Te}}_{7}$ is accompanied by an insulator to semimetal transition.