NQR Studies of Phase Transitions and Intermolecular Interactions in Crystals*

Simple methods to analyze the anomalous behavior o f NQ R parameters are presented. Unusual positive temperature coefficients o f antimony resonance frequencies in (N H 4)2SbF5 below the phase transition at 169 K were interpreted in terms o f ordering processes o f the ammonium ions. Calculation o f the EFG in LaF3 revealed that the translational diffusion of Li ions brings about an anomalous decrease in // o f the 139La NQ R at high temperatures. The same kind o f an EFG calculation was made to predict the 14N NQ R frequency in Li3N. The calculation o f the intermolecular potential energy was used to evaluate the potential barrier to molecular rotation in Q C lh and 5>w-C6C13F3. 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.