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

KHCO3 and its deuterated analogue KDCO3 are typical materials that undergo order–disorder phase transitions at 318 and 353 K, respectively. The spin–lattice relaxation times, T1, spin–spin relaxation times, T2, and the number of resonance lines for the 1H, 2D, and 39K nuclei of these crystals were investigated using NMR spectrometer. These materials are known to exhibit anomalous decreases in T1 near TC, which have been attributed to a structural phase transition. Additionally, changes in the symmetry of the (HCO3)22− (or (DCO3)22−) dimers in these materials are associated with large changes in T1, T2, and the number of resonance lines. Here we found that the resonance lines for 1H, 2D, and 39K nuclei decrease in number as the temperature is increased up to TC, indicating that the orientations of the (HCO3)22− (or (DCO3)22−) dimers and the environments of the K ions change at TC. Moreover, based on number of resonance lines, the results further indicate that the (HCO3)22− (or (DCO3)22−) 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 are of the order–disorder type. The present results therefore indicate that the orientations of the (HCO3)22− and (DCO3)22− dimers and the environment of the K ion play a significant role in these phase transitions.