Possible Lattice and Charge Order in CuxBi2Te2Se

Metal intercalation into layered topological insulator materials such as the binary chalcogenide Bi2X3 (X=Te or Se) has yielded novel two-dimensional 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 Cu0.3Bi2Te2Se, of a periodic lattice distortion at room temperature, together with a charge density wave transition around Td = 220K. We also report, for the first time, a complete study of the CuxBi2Te2Se system, and the effect of Cu-intercalation on crystal structure, phonon structure, and electronic properties for 0.0 $\le$ x $\le$ 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 $\lt$ x $\lt$ 0.2, but drops precipitously for higher x. Similarly, Raman phonon modes $A^2_{1g}$ and $E_g$ soften monotonically for 0 $\lt$ x $\lt$ 0.2 but harden sharply for x $\gt$ 0.2. This indicates that Cu likely intercalates up to x $\sim$0.2, followed by partial site-substitutions at higher values. The resulting strain makes the 0.2 $\lt$ x $\lt$ 0.3 region susceptible to instabilities and distortions. Our results point toward the presence of an incommensurate CDW above Td = 220 K. This work strengthens prevalent thought that intercalation contributes significantly to instabilities in the lattice and charge degrees of freedom in layered chalcogenides.