NMR study of the order-disorder phase transitions of KHCO3 and KDCO3 single crystals

摘要

KHCO3 and its deuterated analogue KDCO3 are typical materials that undergo order-disorder phase transitions at 318 and 353 K, and respectively. The spin-lattice relaxation times, T-1, spin-spin relaxation times, T-2,T- and the number of resonance lines for the H-1 D-2, and K-39 nuclei of these crystals were investigated using NMR spectrometer. These materials are known to exhibit anomalous decreases in T-1 near T-C, which have been attributed to a structural phase transition. Additionally, changes in the symmetry of the (HCO3)(2)(-)(2) dimers in these materials are associated with large changes in T-1,T-2, and the number of resonance lines. Here we found that (DCO3)(2)(-)(2)) the resonance lines for H-1, D-2, and K-39 nuclei decrease in number as the temperature is increased up to T-C, indicating that the orientations of the (HCO3)(2)(-)(2)(or (DCO3)(2)(-)(2)) dimers and the environments of the K ions change at T-C. Moreover, based on number of orientations of the resonance lines, the results further indicate that the (HCO3)(2)(-)(2) 2 or (DCO3)(2)(-)(2) dimers reorientate to approximately parallel to the directions of the hydrogen bonds (or deuteron bonds) and the direction of the a-axis. The transitions at 318 and 345 K of the two crystals)2 -)2 - dimers and the are of the order-disorder type. The present results therefore indicate that the orientations of the (HCO3)22- and (DCO3)(2)(-)(2) 2 environment of the K ion play a significant role in these phase transitions. (c) 2005 Elsevier Inc. All rights reserved.