The Wikipedia entry for density states

Osmium and iridium are the densest known elements at standard conditions for temperature and pressure but certain chemical compounds may be denser.

I have not been able to turn up what these chemical compounds are. I can imagine a salt or some molecule in which dense atoms (e.g. of mercury) are packed in a conformation such that these atoms are held closer together than in the pure metal, so giving the molecule a higher density than the pure metal or of higher molecular weight metals like osmium.

I know there are elements with higher molecular weight than osmium and I have read about superdense exotic states of matter. I am curious about this chemical (presumably comprised of atoms with lower MW that osmium) which is more dense than osmium - presumably at earthly temperatures and pressures. Or an osmium-containing molecule more dense than metallic osmium would qualify.

If someone can answer this question he or she might also consider editing the Wikipedia article to clarify.

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    $\begingroup$ I read once that some alloys of osmium and iridium may be denser than either pure element but when I tried to find it again later, I failed. At least, there are non-exotic possibilities other than compounds. $\endgroup$ – badjohn Oct 31 '17 at 13:37
  • $\begingroup$ The core of the Sun is denser than osmium, but that is "cheating" because we use high pressure. $\endgroup$ – Oscar Lanzi Oct 31 '17 at 19:46
  • $\begingroup$ I edited the wikipedia entry. It now reads "Osmium and iridium are the densest known elements at standard conditions for temperature and pressure but certain chemical compounds[citation needed] may be denser. " $\endgroup$ – Karsten Theis Sep 19 '19 at 13:27
  • $\begingroup$ As the set of chemical elements is just a subset of the set of all chemical substances, therefore maximal density within the elements is less or equal to maximal density within all substances. It was said "may", not "are". $\endgroup$ – Poutnik Jan 17 at 18:20

The wiki entry for density of "certain chemical compounds" is maybe incorrect. Thorium dioxide has a density of $\pu{10 g/cm^3},$ while lutetium tantalate $(\ce{LuTaO4})$ is $\pu{9.81 g/cm^3}.$ These are less than half the densities of $\ce{Os}$ and $\ce{Ir}.$

Tungsten carbide is $\pu{15.63 g/cm^3},$ so time to check all the carbides!

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    $\begingroup$ No luck finding a carbide or boride denser than tungsten carbide, so maybe the Os-Ir alloys might (as a commenter indicated) be the answer. Back to searching ... $\endgroup$ – Ed V Aug 20 '19 at 13:17
  • $\begingroup$ Apparently, thorium dioxide is the densest white compound and more dense compounds are non-white. Seems rather odd. $\endgroup$ – Ed V Sep 19 '19 at 13:25

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