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.