Abstract
We report an experimental and theoretical lattice dynamics study of bismuth telluride (Bi2Te 3 )u p to 23 GPa together with an experimental and theoretical study of the optical absorption and reflection up to 10 GPa. The indirect bandgap of the low-pressure rhombohedral (R-3m) phase (α-Bi2Te 3) was observed to decrease with pressure at a rate of − 6m eV/GPa. In regard to lattice dynamics, Raman-active modes of α-Bi2Te 3 were observed up to 7.4 GPa. The pressure dependence of their frequency and width provides evidence of the presence of an electronic-topological transition around 4.0 GPa. Above 7.4 GPa a phase transition is detected to the C2/m structure. On further increasing pressure two additional phase transitions, attributed to the C2/c and disordered bcc (Im-3m) phases, have been observed near 15.5 and 21.6 GPa in good agreement with the structures recently observed by means of x-ray diffraction at high pressures in Bi2Te 3. After release of pressure the sample reverts back to the original rhombohedral phase after considerable hysteresis. Raman- and IR-mode symmetries, frequencies, and pressure coefficients in the different phases are reported and discussed.
Highlights
Bismuth telluride (Bi2Te3) is a layered chalcogenide with a tremendous impact for thermoelectric applications.[1]
The large amplitude of the interference fringe pattern in the transparent region is a result of the high value of the refractive index, that is larger than 9.42,65,66 The sample transmittance and the interference-fringe amplitude decreases at low-photon energy due to the onset of free-carrier absorption and to high energies due to the fundamental absorption edge caused by band-to-band absorption
Since A1g modes are polarized in the direction perpendicular to the layers while the Eg modes are polarized along the layers, our observation of a less positive pressure coefficient at 4.0 GPa of both modes in α-Bi2Te3 suggests that the electronic topological transition (ETT) in Bi2Te3 is related to a change of the structural compressibility of both the direction perpendicular to the layers and the direction along the layers
Summary
Bismuth telluride (Bi2Te3) is a layered chalcogenide with a tremendous impact for thermoelectric applications.[1]. It has been verified that the thermoelectric properties of semiconductor chalcogenides improve with increasing pressure, and that the study of the properties of these materials could help in the design of better thermoelectric materials by substituting external pressure by chemical pressure.[14,15,16,17,18] the electrical and thermoelectric properties of Sb2Te3, Bi2Te3, and Bi2Se3, as well as their electronic-band structure, have been studied at high pressures.[19–27] a decrease of the bandgap energy with increasing pressure was found in Bi2Te3.19,20 recent high-pressure studies in these compounds have shown a pressure-induced superconductivity[28,29] that has further stimulated high-pressure studies.[30]. Recent high-pressure powder x-ray diffraction measurements have evidenced a pressure-induced electronic topological transition (ETT) in Bi2Te3 around 3.2 GPa as a change in compressibility.[29,31,32,35,36]. As a part of our systematic study of the structural stability and the vibrational properties of the semiconductor chalcogenide family, we report in this work room-temperature Raman-scattering measurements in Bi2Te3 up to 23 GPa together with total-energy and lattice-dynamical ab initio calculations at different pressures. We discuss the recent observation of a pressure-induced ETT in the rhombohedral phase of α-Bi2Te3 and study whether the Raman-scattering signal of the Bi2Te3 at pressures above 7.4 GPa match with the proposed high-pressure phases recently reported for this compound[33,34] and which have been found in Sb2Te3 at high pressures.[51]
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