Polymer-monomer phase transition in ${\mathrm{Na}}_4{\mathrm{C}}_{60}$

We have shown that the two-dimensional polymer of ${\mathrm{Na}}_4{\mathrm{C}}_{60}$ transforms into a monomeric phase above $\sim 500\mathrm{K}.$ The $\mathrm{polyme}\overrightarrow{r}\mathrm{monomer}$ phase transition is reversible and the magnitude of the formation enthalpy corroborates the polymeric structure. The high-temperature tetragonal phase is isometric to other $A_4{\mathrm{C}}_{60}$ ($A=\mathrm{K},$ Rb, Cs) compounds but with an extremely short interfullerene distance. ${\mathrm{Na}}_4{\mathrm{C}}_{60}$ is the only metal among its $A_4{\mathrm{C}}_{60}$ analogs and this uniqueness can be rationalized in the Mott-Hubbard picture.