(NH4)(3)C-60 - A NEW C-60 SUPERCONDUCTOR

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 so lid 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 secon d highest superconducting transition temperature, with T-c = 28 K, whi ch suggests that the T-c of a superconducting (NH4)(3)C-60 could be hi gher than yet achieved for C-60 superconductors, of which Cs3C60(s) ha s the highest T-c of 40 K. There is a 28% relative mass change when th e NH4+ countercation is replaced by (ND4+)-N-15, which is a much large r relative change than can be achieved with the alkali metal atoms, wh ich is important for study of the isotopic substitution effect on T-c. There is also the possibility of unique dynamics in which the ammoniu m ion rotates in the lattice; the presence of a molecular ion, rather than an atomic ion, could play a role in the mechanism of superconduct ivity, if the solid is superconducting. Finally, alternative methods t o produce such an ammonium salt of C-60, such as electrosynthesis or d irect synthesis in liquid ammonia, would be required in contrast to th e method of the production of M(3)C(60) (M = alkali atom) based on vap or phase transport of M via sublimation in sealed tubes.