  We have investigated sodium containing fullerene superconductors Na2AC60, A =
Cs, Rb, and K, by Na-23 nuclear magnetic resonance (NMR) spectroscopy at 7.5 T
in the temperature range of 10 to 400 K. Despite the structural differences
from the Rb3C60 class of fullerene superconductors, in these compounds the NMR
line of the tetrahedrally coordinated alkali nuclei also splits into two lines
(T and T') at low temperature. In Na2CsC60 the splitting occurs at 170 K; in
the quenched cubic phase of Na2RbC60 and Na2KC60 we observe split lines at 80
K. Detailed investigations of the spectrum, spin-spin and spin-lattice
relaxation as well as spin-echo double resonance (SEDOR) in Na2CsC60 we show
that these two different tetrahedral sites are mixed on a microscopic scale.
The T and T' sites differ in the orientation of first-neighbor C60 molecules.
We present evidence that the orientations of neighboring molecules are
uncorrelated. Thermally activated molecular reorientations cause an exchange
between the T and T' sites and motional narrowing at high temperature. We infer
the same activation energy, 3300 K, in the temperature range 125 to 300 K. The
spin lattice relaxation rate is the same for T and T' down to 125 K but
different below. Both the spin-lattice relaxation rate and Knight shift are
strongly temperature dependent in the whole range investigated. We interpret
this temperature variation by the effect of phonon excitations involving the
rigid librational motion of the C60 molecules. By extending the understanding
of the structure and molecular dynamics of C60 superconductors, these results
may help in clarifying the effects of the structure on the superconducting
properties.
