N${\mathrm{a}}_4$${\mathrm{C}}_{60}$: An Alkali Intercalated Two-Dimensional Polymer

The long-missing stoichiometric phase of ${\mathrm{Na}}_4{\mathrm{C}}_{60}$ has been identified by Rietveld analysis of synchrotron powder diffraction data. Its monoclinic structure is based on polymer planes of ${\mathrm{C}}_{60}$ where each molecule forms four “single” bonds within the plane. This compound is not only the first fulleride polymer with such bonds, but also the first two-dimensional polymer which is naturally intercalated with alkali ions and can be synthesized at ambient pressure. ${\mathrm{Na}}_4{\mathrm{C}}_{60}$ is a metal and is expected to be a prototype structure where electronic overlap between the planes can be tuned by the alkali cation size.