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This question is basically a modification of this.

It's well known that Osmium is the densest material you can find at room temperature and pressure. I am curious at higher pressures (and lower temperature?) if any denser materials are known. Obviously if the pressure approaches that of a Neutron star's gravitational environment we would expect pure neutron star fluid to be of higher density. I am interested in pressures less than this. Ideally, pressure that can be achieved in a lab such as up to 770 gigapascals.

Have we found any materials in a lab which have a higher density than osmium at colder and higher pressure environment than ambient conditions?

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Iridium appears to become denser than osmium under high pressure (2.98 GPa) and normal temperatures. The reason is that osmium is less compressible than iridium, so at high pressures iridium densities go beyond osmium densities.

Right now Jupiter's core has a density of about 25 g/cm$^3$ and the sun's core about 130 g/cm$^3$. They are of course not at low temperature, but in the very long run they can be expected to cool down (and increase in density). In Jupiter's case the substance is likely iron from the initial planetesimals that started the accretion, while the sun will eventually become a carbon-oxygen white dwarf with much denser core.

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Osmium may be the least compressible substance known, so various substances under pressure become more dense than room temperature osmium.

As @Anders-Sandberg notes, according to "Is Osmium Always the Densest Metal?", Iridium is denser than osmium at pressures above about 3Gpa.

I don't know what the substance with the record density is, but Uranium has a density $>27\,\mathrm{g/cm^3}$ at $100\,\mathrm{Gpa}$ in a diamond anvil, significantly greater than osimium's density of $22.59\,\mathrm{g/cm^3}$ at normal room temperature and pressure. Gold has a density $>30\,\mathrm{g/cm^3}$ at $355\,\mathrm{Gpa}$, but I believe that has only been measured under shock-wave compression.

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Osmium’s density at 500 Gpa is about 35~36 g/ml according to osmium unit cell volume from dividing the room temp density by the percent volume

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    $\begingroup$ Please provide a link and/or citation to the source of the diagram. $\endgroup$ Nov 27, 2023 at 21:53
  • $\begingroup$ from nature.com/articles/nature14681 is where the diagram is from $\endgroup$
    – user388708
    Dec 15, 2023 at 6:23
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    $\begingroup$ Thanks user388708, but please edit your answer to include the link. Comments are in principle temporary, so should not be used to provide information that should be part of the answer. $\endgroup$ Dec 15, 2023 at 6:54

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