Deuterium NMR Study of Unstable Phenomena and Water Molecular Dynamics in Samarium Nitrate Hexahydrate Crystal

Abstract

Deuterium NMR spectra, spin−lattice relaxation time T 1, and differential thermal analysis (DTA) were measured for Sm(NO 3 ) 3 ·6D 2 O. The magnetization 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 dominated by the 180° flip of the crystallization and the coordinated water molecules, respectively. For the nonannealed sample, the reproducibility 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 the activation energy E a = 20 kJ mol -1 and the correlation time at infinite temperature τ c0 = 7.8 × 10 -12 s for the 180° flip of the coordinated water molecule. The motion of the crystallization water at low temperatures can be explained by the 180° flip in the asymmetric double minimum potential. τ c0 = 1.2 × 10 -12 s, E a = 16 kJ mol -1, and the energy difference in the potential wells Δ E = 2.0 kJ mol -1 were obtained for the 180° flip of the crystallization water from the short component of T 1 . The potential wells for the 180° flip of the crystallization water changed gradually in phase II and approached the symmetric wells with increasing temperature. For the annealed sample, the drastic change 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 phenomena are interpreted in terms of the metastable state due to the change of the hydrogen bond between the crystallization and the coordinated waters by annealing.