Unusual Temperature Dependence of 35Cl NQR Spin-Lattice Relaxation Time in [(CH3)4N]2[MCl6] (M = Pb, Sn, Te)

Abstract

Abstract The temperature dependence of the spin-lattice relaxation time (T1H) of 1H NMR measured in tetramethylammonium hexachloroplumbate(IV), (Me4N)2[PbCl6], showed a deep and a shallow minimum near 190 and 115 K, respectively. Since the presence of two kinds of crystallographically nonequivalent cations in the room-temperature Fd 3c unit cell has been reported, the deep T1H minimum was assigned to the overall reorientation of three quarters of the Me4N+ ions and the shallow minimum to that of the remaining cations. Two different temperature dependences of the chlorine NQR spin-lattice relaxation time (T1Q), attributable to a modulated electric-field-gradient by the protonic motion, were observed in (Me4N)2[MCl6] (M = Pb, Sn, Te). One is found in the Pb complex whose T1Q stems from the cationic motion responsible for the deep T1H minimum, and the other one is determined by the cationic motion giving the shallow T1H minimum. Although all room-temperature phases of these complexes are well described by the Fd 3c unit cell, the presence of different temperature dependences of T1Q suggests that the CH3 groups in the respective complexes take different orientations in the crystals