ATOMIC MOTIONS IN AN UNUSUAL MOLECULAR SEMICONDUCTOR - NASN

Recent investigations of the compound NaSn (1:1) have indicated that b oth fast conduction of Na+ cations and rapid reorientations of (Sn-4)( 4-) anions are present in the high-temperature solid phase, alpha-NaSn . We have used Na-23 and Sn-119 nuclear magnetic resonance to separate ly monitor the motions of Na and Sn in the low-temperature ordered sol id phase, beta-NaSn, where the correlation times of the motions are mu ch longer. The Na+ motions are evident in T-1, T-1 rho, T-2, and T-2 (linewidth) Na-23 data at magnetic fields of 8.0 and 2.0 T. The Na mot ions are described by a single time constant with thermal activation p arameters E(a)/k(B) = 9700 K and omega(D)(infinity) = 1.7 x 10(14) s(- 1). Sn motions are evident in T-2, line shape, and stimulated echo dat a taken at 8.0 T. Sn-119 stimulated echo data unequivocally show the S n motion to be reorientations of Snr tetrahedra; any diffusion of Sn b etween tetrahedra is much slower. The combined Ta and stimulated echo data for Sn-119 demonstrate that the Sn-4 reorientations are thermally activated with E(a)/k(B) = 13 800 K and omega(R)(infinity) = 2.3 x 10 (15) s(-1). The temperature dependence of the Sn-119 T-1 fits an activ ation energy of 7000 K; the Sn T-1 is believed to be due to thermally- activated charge carriers.