NMR

We report (Formula presented) measurements on a single crystal of superconducting (Formula presented) where (Formula presented) is bis(ethylenedithiotetrathiafulvalene), (Formula presented) The temperature dependence of the spectrum has revealed that two phase transitions occur on cooling; first at (Formula presented) where a single resonance peak splits into two subpeaks, and second at (Formula presented) where the subpeaks disappear after the broadening at 25 K. The former associated by temperature hysterisis is a first-order phase transition and the splitting width follows (Formula presented) dependence. This splitting, which is so sensitive to the field orientation due to (Formula presented) is found to be caused by the quadrupole interaction. The spin-lattice relaxation rate (Formula presented) is strongly 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 (Formula presented) at (Formula presented) the uniform rotation might switch to the thermally activated hopping motion between preferable sites bonded to nitrogens of (Formula presented) legands and then the order-disorder transition might eventually occur at (Formula presented) to form an anion ordering.