We report D-2-NMR measurements on a single crystal of superconducting
alpha-(h(8)-BEDT-TTF)(2)ND4Hg(SCN)(4), where h(8)-BEDT-TTF is bis(ethy
lenedithiotetrathiafulvalene), C10S8H8. The temperature dependence of
the spectrum has revealed that two phase transitions occur on cooling;
first at T-A = 132 - 150 K, where a single resonance peak splits into
two subpeaks, and second at T-B = 20 K, where the subpeaks disappear
after the broadening at 25 K. The former associated by temperature hys
terisis is a first-order phase transition and the splitting width foll
ows (1-T/T-A)(1/2) dependence. This splitting, which is so sensitive t
o the field orientation due to I = 1, is found to be caused by the qua
drupole interaction. The spin-lattice relaxation rate T-1(-1) is stron
gly elongated by three orders of magnitude with decreasing temperature
from 200 K, followed by the maximum around 40 K. This dramatic effect
is interpreted by the Bloembergen-Purcell-Pound model applied to the
quadrupole interaction. Discussions are made from the viewpoint of the
dynamics of noncentrosymmetric tetrahedral molecule ND4+; at T-A, the
uniform rotation might switch to the thermally activated hopping moti
on between preferable sites bonded to nitrogens of (SCN)(-1) legands a
nd then the order-disorder transition might eventually occur at T(B )t
o form an anion ordering.