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.