The successive phase transitions of [Mg(H?0)6][SiF6] were studied by measuring 2 H NMR spectra. The quadrupole coupling constant e 2 Qq/h and asymmetry parameter 77 changed drastically at each transition temperature. 12 H and 19 F NMR T\ were measured for this compound to study the relation between the molecular motions and the successive phase transitions. The activation energy E a and the pre-exponential factor TO for the reorientation of [SiFö] 2~ were estimated as 28 kJmol" 1 and 6.0x 10~1 4 s, and those of the 180° flip of H 2 0 as 33 kJmol -1 and 4.0x 10" 14 s. These two motions occur rapidly even in phase V. For the reorientation of [Mg(H20)6] 2+ , E a = 62 kJmol -1 and TO = 1.1 x 10 -16 s were obtained from the simulation of : H NMR spectra. The jump rate of this motion is of the order of 10 4 -10 6 s _I in phase II. These results suggest that the successive phase transitions are closely related to the motion of [Mg(H 2 0) 6 ] 2+ .

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