Pressure-induced topological and structural phase transitions in natural van der Waals heterostructures from the (SnTe)m(Bi2Te3)n homologous family: Raman spectroscopy, x-ray diffraction, and density functional theory

A new class of van der Waals heterostructures of (SnTe)m(Bi2Te3)n (with m=1,2,.. and n=1,2,..), consisting of a topological crystalline insulator SnTe and a topological insulator Bi2Te3 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 Z2 topological invariant. In addition to the electronic changes, our Raman experiments indicate that rhombohedral (R3¯m) SnBi2Te4 undergoes structural transition at ∼6.0 to a possible monoclinic phase and another transition at ∼12.0 GPa. Raman and x-ray diffraction experiments on trigonal (P3¯m1) SnBi4Te7 show two structural transitions at ∼9.5 GPa to a monoclinic phase followed by one to cubic phase at ∼14.1 GPa. Our analysis of electronic structure reveals that the phase transition at 9.5 GPa in SnBi4Te7 is accompanied by an insulator to semimetal transition.