Experimental and theoretical study of beta-As2Te3 under hydrostatic pressure

We report a joint experimental and theoretical high-pressure study of the structural and vibrational properties of tetradymite-like (R (3) over barm) beta-As2Te3. Two samples have been characterized by angle-dispersive synchrotron powder X-ray diffraction and Raman scattering measurements under hydrostatic conditions with the help of ab initio calculations. One sample was synthesized at high pressure and high-temperature conditions with a Paris-Edinburg cell and the other by the melt-quenching technique. Both beta-As2Te3 samples show the same properties and exhibit two isostructural phase transitions of order higher than 2, i.e. of electronic origin, near 2.0(2) and 6.0(5) GPa that are compatible with the changes predicted by recent electronic band structure calculations. The first isostructural phase transition can be attributed to the topological quantum phase transition from a trivial insulator to a topological insulator, passing through a 3D Dirac topological semimetal. This topological transition, specific to beta-As2Te3, is not observed in isostructural Te-based sesquichalcogenides alpha-Sb2Te3 and alpha-Bi2Te3 that are topological insulators at room conditions. The second isostructural phase transition is likely re lated to an insulator-metal transition. Additionally, we have observed two partially reversible first-order phase transitions in beta-As2Te3 above 10 and 17 GPa. We have found a high anharmonic behavior of the two Raman-active modes with the lowest frequencies in beta-As2Te3 that explains the already reported ultra-low lattice thermal conductivity of beta-As2Te3. Moreover, we have studied the similarities of beta-As2Te3 with alpha-Sb2Te3 and alpha-Bi2Te3 (two of the best thermoelectric materials), thus providing insights into the origin of the ultra-low lattice thermal conductivity values in these compounds related to unconventional chemical bonds present in these isostructural materials.