Atomic Ordering in Cubic Bismuth Telluride Alloy Phases at High Pressure

TL;DR

This study investigates how bismuth telluride alloys undergo atomic ordering and phase transitions under high pressure, revealing new ordered phases and their electronic properties through experiments and calculations.

Contribution

It provides new insights into pressure-induced atomic ordering and phase transitions in bismuth telluride alloys, combining experimental diffraction data with first-principles calculations.

Findings

Alloys form disordered bcc structures above 14-19 GPa.

Atomic ordering occurs after moderate annealing at ~100°C.

B2 structure forms from BiTe upon annealing.

Abstract

Pressure-induced transitions from ordered intermetallic phases to substitutional alloys to semi-ordered phases were studied in a series of bismuth tellurides. Using angle-dispersive x-ray diffraction, the compounds Bi4Te5, BiTe, and Bi2Te were observed to form alloys with the disordered body-centered cubic (bcc) crystal structure upon compression to above 14--19 GPa at room temperature. The BiTe and Bi2Te alloys and the previously discovered high-pressure alloys of Bi2Te3 and Bi4Te3 were all found to show atomic ordering after gentle annealing at very moderate temperatures of ~100{\deg}C. Upon annealing, BiTe transforms from the bcc to the B2 (CsCl) crystal structure type, and the other phases adopt semi-disordered variants thereof, featuring substitutional disorder on one of the two crystallographic sites. The transition pressures and atomic volumes of the alloy phases show systematic variations across the Bi_mTe_n series including the end members Bi and Te. First-principles calculations were performed to characterize the electronic structure and chemical bonding properties of B2-type BiTe and to identify the driving forces of the ordering transition. The calculated Fermi surface of B2-type BiTe has an intricate structure and is predicted to undergo three topological changes between 20 and 60 GPa.