Temperature-dependent structural studies of polycrystalline Bi₁−_xSb_x (0.10 ≤ x ≤ 0.20) alloys were carried out using synchrotron x-ray diffraction. In-depth powder diffraction analysis reveals that an iso-structural phase transition takes place in the rhombohedral Bi-Sb alloy around 200 K, which is accompanied by anomalous behavior in the temperature-dependent linear thermal expansion data. In addition, the thermal variation of refined isotropic thermal parameters or Debye-Waller factor (B_iso), ratio (a_H/c_H) of the lattice parameters and Debye temperature (θ_D) indicates that anisotropy, arising due to local structural disorder, plays a significant role in this material.