Structural ordering and IR-active vibrations within Bi₂(Te_(1-x)Se_x)₃

Bismuth selenide (Bi₂Se₃), bismuth telluride (Bi₂Te₃), and their ternary alloy Bi₂(Te_(1-x)Se_x)₃ are 3D topological insulators, in addition to being room temperature thermoelectrics. Phonon modes play a key role in both these applications, and as such, an understanding of the role that these phonon modes play in the electronic and thermal transport for different compositions of the ternary alloy Bi₂(Te_(1-x)Se_x)_{3 ­}is of experimental interest.

We have performed an investigation into the THz conductivity spectra in the ternary alloy Bi₂(Te_(1-x)Se_x)₃ as a function of selenium fraction, x, and temperature. The crystal structure of these alloys is built from layers of atoms ordered in the sequence C₁-Bi-C₂-Bi-C₁ (where C_x can be occupied by the chalcogenides). Selenium is more electronegative than tellurium, and so for small x, selenium is found almost exclusively on the C₂ sites. This creates an ordered phase of the topological insulator when x=1/3 (i.e most C₂ sites are occupied by selenium, and nearly all C₁ sites are occupied by tellurium). We find that the reduction in crystalline anharmonicity due to the preferential ordering of the x=1/3 phase of Bi₂(Te_(1-x)Se_x)₃ results in the prominent E¹_u phonon red-shifting on cooling less than the binary Bi₂Te₃ and Bi₂Se₃ samples. We also find that the E¹_u phonon couples to an electronic continuum at low temperatures (T ≤ 40 K), regardless of x in Bi₂(Te_(1-x)Se_x)₃ or the mobility of the topological insulator, possibly indicating the presence of protected surface states