Page "Neutronium" Paragraph 6

Although the term is not used in the scientific literature either for a condensed form of matter, or as an element, there have been reports that, besides the free neutron, there may exist two bound forms of neutrons without protons.

If neutronium were considered to be an element, then these neutron clusters could be considered to be the isotopes of that element.

However, these reports have not been further substantiated.

Further information can be found in the following articles: