M. J. Mehl, D. Hicks, C. Toher, O. Levy, R. M. Hanson, G. L. W. Hart, and S. Curtarolo, The AFLOW Library of Crystallographic Prototypes: Part 1, Comp. Mat. Sci. 136, S1-S828 (2017). (doi=10.1016/j.commatsci.2017.01.017)
D. Hicks, M. J. Mehl, E. Gossett, C. Toher, O. Levy, R. M. Hanson, G. L. W. Hart, and S. Curtarolo, The AFLOW Library of Crystallographic Prototypes: Part 2, Comp. Mat. Sci. 161, S1-S1011 (2019). (doi=10.1016/j.commatsci.2018.10.043)
D. Hicks, M.J. Mehl, M. Esters, C. Oses, O. Levy, G.L.W. Hart, C. Toher, and S. Curtarolo, The AFLOW Library of Crystallographic Prototypes: Part 3, Comp. Mat. Sci. 199, 110450 (2021). (doi=10.1016/j.commatsci.2021.110450)
This rhombohedral structure becomes cubic at various values of $c/a$ (or $\alpha$) to wit, \[ \begin{array}{ccc} \text{$\textbf{$c/a$}$} & \mathbf{\alpha} & \text{$\textbf{Cubic Lattice}$} \\ \sqrt{6} & 60^{o} & \href{A_cF4_225_a.html}{\text{Face-Centered Cubic}} \\ \sqrt{\frac{3}{2}} & 90^{o} & \href{A_cP1_221_a.html}{\text{Simple Cubic}} \\ \sqrt{\frac{3}{8}} & 109.47^{o} & \href{A_cI2_229_a.html}{\text{Body-Centered Cubic}} \\ \end{array} \]Note that $\beta$–Po (A_hR1_166_a, $\beta$–Po) and $\alpha$–Hg (A_hR1_166_a, $\alpha$–Hg) have the same AFLOW prototype label. They are generated by the same symmetry operations with different sets of parameters (––params) specified in their corresponding CIF files. Experimentally, $\beta$–Po (Ai) has c/a near 1, or $\alpha$ > 90$^{o}$, while $\alpha$–Hg (A10) has c/a near 2, or $\alpha$ < 90$^{o}$. Hexagonal settings of this structure can be obtained with the option ––hex.