Atomic ordering in cubic bismuth telluride alloy phases at high pressure

Abstract

Pressure-induced transitions from ordered intermetallic phases to substitutional alloys to semi-ordered phases were studied in a series of bismuth tellurides. By using angle-dispersive x-ray diffraction, the compounds ${\mathrm{Bi}}_{4}{\mathrm{Te}}_{5}$, BiTe, and ${\mathrm{Bi}}_{2}\mathrm{Te}$ 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 ${\mathrm{Bi}}_{2}\mathrm{Te}$ alloys and the previously discovered high-pressure alloys of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ and ${\mathrm{Bi}}_{4}{\mathrm{Te}}_{3}$ were all found to show atomic ordering after gentle annealing at very moderate temperatures of $\ensuremath{\sim}100{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. Upon annealing, BiTe transforms from 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 ${\mathrm{Bi}}_{m}{\mathrm{Te}}_{n}$ 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.