Deuterium NMR study of unstable phenomena and water molecular dynamics in samarium nitrate hexahydrate crystal

Deuterium NMR spectra, spin-lattice relaxation time T-1, and differential t hermal analysis (DTA) were measured for Sm(NO3)(3). 6D(2)O. The magnetizati on recovery can be divided into two components. From deuterium NMR spectra and T-1, the short and the long components of T-1 were found to be mainly d ominated by the 180 degrees flip of the crystallization and the coordinated water molecules, respectively. For the nonannealed sample, the reproducibi lity of T-1 was observed. The influence of the instability was not seen in the long component of T-1. The results of the long component of T-1 gave th e activation energy E-a = 20 kJ mol(-1) and the correlation time at infinit e temperature tau(c0) = 7.8 x 10(-12) s for the 180 degrees flip of the coo rdinated water molecule. The motion of the crystallization water at low tem peratures can be explained by the 180 degrees flip in the asymmetric double minimum potential. tau(c0) = 1.2 x 10(-12) s, E-a = 16 kJ mol(-1), and the energy difference in the potential wells Delta E = 2.0 kJ mol(-1) were obt ained for the 180 degrees flip of the crystallization water from the short component of T-1. The potential wells for the 180 degrees flip of the cryst allization water changed gradually in phase II and approached the symmetric wells with increasing temperature. For the annealed sample, the drastic ch ange of T-1 was observed in phases II and III. Both the long and the short components of T-1 did not show the reproducibility. These unstable phenomen a are interpreted in terms of the metastable state due to the change of the hydrogen bond between the crystallization and the coordinated waters by an nealing.