  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.
