neutronium theory when invalid?

Answer 1

Neutronium is a state of matter consisting of close-packed neutrons at nuclear density. It is only stable under the extremely high pressures of a neutron star. The pressure is so high, heavier stars collapse further into a black hole.

The term was popularized by Star Trek, where it was assumed to be stable at low pressures. You could then use it for the impenetrable hull of a planet killer or such.