Neutronium was a material composed of highly compressed neutrons.
On Skaro, the First Doctor helped the Daleks stop the machine brain from producing enough neutronium to destroy the planet and potentially Earth as well. According to this account, neutronium was a nuclear substance. (PROSE: Doctor Who and the Daleks)
The Fourth Doctor speculated that Deepcity's artificial gravity was created by a layer of stabilized neutronium on the floor of the cavern inside the asteroid where it was hidden. (PROSE: A Device of Death)
According to the Seventh Doctor, the original homeworld of the Quoth was a cylinder of neutronium that was created when three neutron stars collided "in exactly the right way." (PROSE: The Death of Art)