Neutronium is a common substance in science fiction. It is a real substance of incredible density, so a lot of authors have decided that it would make a neato structural element. The problem: it would explode.
Neutronium is, as the name suggests, a substance made out of pure neutrons. It exists in neutron stars… but really nowhere else. It *can’t* really exist anywhere else. Neutrons exists in close proximity in the nuclei of atoms, but there they are held in place with the strong nuclear force; eliminate the protons, and the neutrons will go flying apart. In neutron stars, the neutrons are held together solely by gravity. If you were to somehow teleport a chunk of neutronium off a neutron star, it would promptly explode.
Here’s an old video from Thunderf00t explaining another problem with neutronium: outside of the gravitational field of neutron stars and the strong force of nuclei, neutrons decay with a half life of about 10 minutes. And the results of that are pretty energetic.
So if you want to use neutronium in your sci-fi story, you got some ‘splainin’ to do.