Bi‐centric view of the isostructural phase transitions in α‐Bi2Se3 and α‐Bi2Te3

Abstract

Most of A2B3‐type 3D topological insulators (e.g., Bi2Se3, Bi2Te3, Sb2Te3) have been reported to exhibit a pressure‐induced isostructural phase transition (IPT) in the low‐pressure region (<5 GPa); however, the underlying mechanism is not fully understood yet. Here, we comparatively investigated the IPTs in α‐Bi2Se3 and α‐Bi2Te3 using high‐pressure Bi L3‐edge X‐ray absorption fine structure spectroscopy, complemented with density functional theory (DFT) calculation. Near the IPT, the slope of the Bi L3‐edge absorption energy shows a sudden decrease. In contrast to the monotonous shortening of the Bi–Bi bond‐distance under compression, it is moreover rather surprising that the Bi–Se2 (Te2) and Bi–Se1 (Te1) bond‐distances exhibit abnormal elongations around the IPT, which is synchronized with abrupt increases for their Debye–Waller factor. DFT calculation results show that near the IPT a structural distortion may appear for extremely small energy barrier. Those results indicate that each quintuple layer unit shrinks along the layer direction but elongates perpendicular to the layers at the onset of the IPT, which would be associated with a remarkable increase for the local structural disorder. Therefore, our findings suggest that the IPTs in α‐Bi2Se3 and α‐Bi2Te3 are coupled with a simultaneous charge redistribution and structural distortion accompanied with a static structural disorder increase.Sketch of the changes of local structure of centric Bi and lattice structure around the IPT.