What are these "certain chemical compounds" denser than osmium? 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.
This text was added as part of a refactoring/cleanup of the page in this edit in March 2013, where it replaced the text "Osmium and iridium are the densest known elements at standard conditions for temperature and pressure but not the densest materials." This older text was tagged with the note "Which are denser, at standard conditions?" It in turn was added in this edit in May 2011 by an anonymous editor (the IP address used is apparently registered to the Wyoming Community College Commission). The original wording, "Osmium is [...] not the densest material", is ambiguous, but could be read as referring to exotic forms of matter such as neutronium, in spite of the earlier "standard temperature and pressure" caveat.

In short, this likely derives from a well-meaning but misguided editor attempting to note the existence of much higher-density, non-chemical substances in an inappropriate place.