The ACS literature is on the mark. Polonium is a post-transition metal.
It is sometimes nevertheless shown as a metalloid because the author has not done sufficient research.
As with aluminium, adjacency to the traditional dividing line between metals and nonmetals does not necessarily warrant categorising polonium as a metalloid.
What do we know of polonium?
When the Curies announced in 1898 that they had synthesised polonium they wrote of it as a metal.
It has a silvery, metallic appearance; it conducts electricity like a metal; it has the electronic band structure of a metal; its enthalpy of fusion is near the average for close-packed metals; it is soluble in acids, forming the rose-coloured Po^++ cation and displacing hydrogen; many polonium salts are known; and the oxide (PoO2), which assumes the fluorite structure more typical of ionic compounds/metallic oxides, is predominately basic in nature.
Whether polonium is ductile or brittle is unclear. It is predicted to be ductile based on its calculated elastic constants.[1] It has a simple cubic crystalline structure. Such a structure has few slip systems and "leads to very low ductility and hence low fracture resistance".[2]
Polonium also has some intermediate or nonmetallic properties, as is normally the case for metals in, or in the vicinity of, the p-block. It has an intermediate coordination number, electronegativity, ionisation energy and metallicity ratio; and it can form anionic polonides; volatile and easily hydrolysed halides (which are soluble in organic solvents); and a volatile and unstable hydride (PoH2). Most of the latter properties are characteristic of the heavier noble metals or post-transition metals.
The elements commonly recognised as metalloids (B, Si, Ge, As, Sb, Te) are semiconductors (B, Si, Ge, Te) or exist in less stable semiconducting forms (As, Sb). Polonium is not known to have a semiconducting form. The commonly recognised metalloids have crystalline packing efficiencies of between 34% and 41%; cf polonium at 52%.
If the elements are categorised on the basis of whether they are judged to exhibit a preponderance of metallic or nonmetallic properties (or neither, in which case you may have a metalloid) then I suggest the weight of evidence, in the case of polonium, falls on the metal side of the line. A parallel may be drawn with gold, which exhibits several nonmetallic properties, including auride (Au^−) formation, yet is universally categorised as a metal on account of its distinctive metallic properties.
[1] Legit D, Friák M & Šob M 2010, ‘Phase stability, elasticity, and theoretical strength of polonium from first principles,’ Physical Review B, vol. 81, pp. 214118–1–19, doi:10.1103/PhysRevB.81.214118
[2] Manson SS & Halford GR 2006, Fatigue and Durability of Structural Materials, ASM International, Materials Park, OH, pp. 378; 410
