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$^{o}$. Originally, Po was assigned Strukturbericht designation: A19, which is now considered to be incorrect. (Donohue, 1982, 390) Hexagonal settings of this structure can be obtained with the option ––hex.
W. H. Beamer and C. R. Maxwell, Physical Properties of Polonium. II. X–Ray Studies and Crystal Structure, J. Chem. Phys. 17, 1293–1298 (1949), doi:10.1063/1.1747155.
Found in
J. Donohue, The Structure of the Elements (Robert E. Krieger Publishing Company, Malabar, Florida, 1982)., pp. 392.