NQR Studies of Phase Transitions and Intermolecular Interactions in Crystals

Abstract

Simple methods to analyze the anomalous behavior of NQR parameters are presented. Unusual positive temperature coefficients of antimony resonance frequencies in (NH4)2SbF5 below the phase transition at 169 K were interpreted in terms of ordering processes of the ammonium ions. Calculation of the EFG in LaF3 revealed that the translational diffusion of Li ions brings about an anomalous decrease in η of the 139La NQR at high temperatures. The same kind of an EFG calculation was made to predict the 14N NQR frequency in Li3N . The calculation of the intermolecular potential energy was used to evaluate the potential barrier to molecular rotation in C6Cl6 and sym-C6Cl3F3. It was shown in the latter that the quadrupolar relaxation, due to a reorientational jump to a metastable site, works in the low temperature phase below 296 K. while the molecular three-fold reorientation is the dominant relaxation mechanism in the high temperature phase.