Local structural changes during the disordered substitutional alloy transition in Bi2Te3 by high-pressure XAFS

A(2)B(3)-type 3D topological insulators, Bi2Te3 and Sb2Te3, have been reported to transform into disordered substitutional alloys under high pressure. However, gamma > delta phase transition and the local structure changes around Bi during the formation of the disordered Bi-Te binary alloy in Bi2Te3 still remain unclear. Here, high-pressure X-ray absorption fine structure (XAFS) combined with high-pressure X-ray diffraction has been used to explore the local structural transformations in the three structural phase transitions of Bi2Te3. The Bi L-3-X-ray absorbing near edge structure (XANES) spectra of delta-Bi2Te3 clearly showed that a new absorption feature at energy of about 13 465 eV would emerge during the gamma > delta phase transition. Through simulation of the XANES spectra by varying the cluster size, we confirmed that the new absorption peak arises from the medium-range order in bcc structure. The Bi L-3-EXAFS results in delta phase reveal that the Bi atoms in the third shell exhibit abnormal elongations with pressure until merging with the shrinking Te shell. Our findings indicate a two-step structural transition of the disordered substitutional alloys via an ordered bcc intermediate phase. Published by AIP Publishing.