Possible lattice and charge order in Cu_xBi₂Te₂Se

Abstract Metal intercalation into layered topological insulator materials such as the binary chalcogenide Bi₂X₃ (X = Te or Se) has yielded novel two-dimensional (2D) electron-gas physics, phase transitions to superconductivity, as well as interesting magnetic ground states. Of recent interest is the intercalation-driven interplay between lattice distortions, density wave ordering, and the emergence of new phenomena in the vicinity of instabilities induced by intercalation. Here, we examine the effects of Cu-intercalation on the ternary chalcogenide Bi₂Te₂Se. We report the discovery, in Cu_0.3Bi₂Te₂Se, of a periodic lattice distortion (PLD) at room temperature, together with a charge density wave (CDW) transition around T _d = 220 K. We also report, for the first time, a complete study of the Cu _x Bi₂Te₂Se system, and the effect of Cu-intercalation on crystal structure, phonon structure, and electronic properties for 0.0 ≤ x ≤ 0.5. Our electron diffraction studies reveal strong Bragg spots at reciprocal lattice positions forbidden by ABC stacking, possibly resulting from stacking faults, or a superlattice. The c-axis lattice parameter varies monotonically with x for 0 < x < 0.2, but drops precipitously for higher x. Similarly, Raman phonon modes {A}_1g² and {E}_g² soften monotonically for 0 < x < 0.2 but harden sharply for x > 0.2. This indicates that Cu likely intercalates up to x ∼ 0.2, followed by partial site-substitutions at higher values. The resulting strain makes the 0.2 ≤ x ≤ 0.3 region susceptible to instabilities and distortions. Our results point toward the presence of an incommensurate CDW above T _d = 220 K. This work strengthens prevalent thought that intercalation contributes significantly to instabilities in the lattice and charge degrees of freedom in layered chalcogenides. Further work is required to uncover additional density wave transitions, and possible ground states such as superconductivity.