Materials intermediate in density between osmium and neutronium The densest elements on Earth are metals. Osmium is the most dense. The densest matter in the Universe is Neutronium which is not an element but degenerate matter. If we put Osmium and Neutronium on the same scale what happens in between? I would like to know how we get from Osmium to the next densest material and all the way to Neutronium. I realize that it may be more complicated than a simple range because elements might behave differently under different conditions but could anyone give this a bit of a go or give me a few pointers?
 A: "Neutronium" is a science fiction term. Pure neutron matter does not exist, since neutrons decay. But something like it exists in neutron star interiors in the form of a n, p, e fluid (mainly n, but a small fraction of p, e must be present - see https://physics.stackexchange.com/a/149656/43351) . This fluid is stable at densities from about $3\times 10^{16}$ kg/m$^3$ to around $10^{18}$ kg/m$^3$.
At lower densities, the next phase is "nuclear pasta" - very neutron-rich nuclear matter bound into planar or spaghetti-like forms surrounded by a (degenerate) free neutron plus electron fluid.
Below a few $10^{15}$ kg/m$^3$, more "normal" nuclei form with pseudo-spherical shapes. These nuclei are still very neutron-rich (n/p ratios of 3 or more and masses of several hundred amu) locked into some sort of solid lattice by Coulomb forces and bathed in a fluid of neutrons and relativistic degenerate electrons.
At densities below $4\times 10^{14}$ kg/m$^3$ the lowest energy configuration sees the free neutrons absorbed into the neutron-rich nuclei. The equilibrium nuclei are still very heavy, but not so heavy as at higher densities.
At lower densities, we gradually get back to material which has the n/p ratios of the stable elements we are familiar with.
Thus there aren't really too many abrupt phase changes. The surfaces of neutron stars are made of familiar elements, probably even a bit of osmium, and are already at densities of $\sim 10^9$ kg/m$^3$, compressed by the enormous gravity.
