(NH4)3C60: A New C60 Superconductor?

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The enthalpy of formation (ΔHf) of the ionic solid (NH4+)3C603- is assessed. The solid is found to be stable with respect to the standard state reactants (N2(g), H2(g), and C60(s)), with a ΔHf of −1.82 eV/mol. For comparison, this enthalpy of formation is less than the enthalpy of formation of, e.g., K3C60 (−6.27 eV/mol). There are several attractive features of (NH4+)3C603- as a new ionic solid and potential superconductor, if it can be synthesized. It is well-known that the size of the NH4+ cation is almost exactly the same as that of Rb+. Among the M3C60 superconductors, Rb3C60 has the second highest superconducting transition temperature, with Tc = 28 K, which suggests that the Tc of a superconducting (NH4)3C60 could be higher than yet achieved for C60 superconductors, of which Cs3C60(s) has the highest Tc of 40 K. There is a 28% relative mass change when the NH4+ countercation is replaced by 15ND4+, which is a much larger relative change than can be achieved with the alkali metal atoms, which is important for study of the isotopic substitution effect on Tc. There is also the possibility of unique dynamics in which the ammonium ion rotates in the lattice; the presence of a molecular ion, rather than an atomic ion, could play a role in the mechanism of superconductivity, if the solid is superconducting. Finally, alternative methods to produce such an ammonium salt of C60, such as electrosynthesis or direct synthesis in liquid ammonia, would be required in contrast to the method of the production of M3C60 (M = alkali atom) based on vapor phase transport of M via sublimation in sealed tubes.