(NH4)(3)C-60: A new C-60 superconductor?

Abstract

The enthalpy of formation (Delta H-f) of the ionic solid (NH4+)(3)C-60(3-) is assessed. The solid is found to be stable with respect to the standard state reactants (N-2(g), H-2(g), and C-60(s)), with a Delta H-f 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+)(3)C-60(3-) 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 M(3)C(60) superconductors, Rb3C60 has the second highest superconducting transition temperature, with T-c = 28 K, which suggests that the T-c of a superconducting (NH4)(3)C-60 could be higher than yet achieved for C-60 superconductors, of which Cs3C60(s) has the highest T-c of 40 K. There is a 28% relative mass change when the NH4+ countercation is replaced by (ND4+)-N-15, 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 T-c. 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 C-60, such as electrosynthesis or direct synthesis in liquid ammonia, would be required in contrast to the method of the production of M(3)C(60) (M = alkali atom) based on vapor phase transport of M via sublimation in sealed tubes.