Anomalous Behavior of the Elastic and Optical Properties in Bi1.5Sb0.5Te1.8Se1.2 Topological Insulator Induced by Point Defects

The experimental evidence of the defects influence on the optical and elastic properties of single crystal Bi1.5Sb0.5Te1.8Se1.2 has been reported. The extra vibrational excitation is observed in the Raman spectrum near 125 cm−1. Its line width has increased substantially with the increasing temperature, and its frequency shows a sharp softening in the temperature range of 200–250 K. In the same temperature range, a large peak in attenuation of longitudinal ultrasound wave propagating along the trigonal axis and the anomalous behavior of elastic modulus C33 are found. These results indicate the presence of disorder in Bi1.5Sb0.5Te1.8Se1.2 induced by the bulk defects. The antisite and interstitial defects on Bi/Sb and Te/Se(1) sublattices have been considered as the most probable ones. The results of ultrasound, Raman and X‐ray powder diffraction studies on the Bi1.5Sb0.5Te1.8Se1.2 topological insulator are presented here. The correlation between the anomalous behavior of the elastic and optical properties near temperature 200 K has been revealed for the first time, and it is suggested that they are of a common nature caused by the point defects with a low symmetry. The antisite and interstitial defects on Bi/Sb and Te/Se(1) sublattices have been considered as the most probable ones.