MOLECULAR-REORIENTATION AND PHASE-TRANSITION IN PYRIDINIUM HEXAFLUOROANTIMONATE

The temperature dependence of H-1 and F-19 NMR second moment and spin lattice relaxation times in high (upsilon(L)=60 MHz and low (B-1=2 mT) magnetic fields allow one to determine both ion dynamics in polycryst alline pyridinium hexafluoroantimonate. The solid-solid phase transiti on discovered at 268 K appears to be connected with symmetrization of energy barriers for pseudohexagonal cation reorientation. The energy d ifference Delta characterizing the inequivalence of the potential well s can be treated as an order parameter. The effect of coupling of the rotational modes of cations and anions is found at the phase transitio n.