A nuclear resonance investigation of molecular motion in solid PF₃

Abstract

The 19F nuclear resonance spectra and spin-lattice relaxation times have been observed in solid PFa over the temperature range 77 K to its melting point at 121"8 K. Within this temperature range PF8 exists in three crystallographic phases. Phase I immediately below the melting-point is a plastic crystalline phase in which the laF nuclear relaxation is dominated by translational diffusion with a correlation time vD satisfying ~'D= {(3 +__2) x 10 -ts s} exp { -(27"5 + 1) kJ mole-t/RT}.In phase I I, threefold reorientations of the fluorine triangle with an activation energy of (8"4_+ 0"2) kJ mole -~ govern the spectra and relaxation rates, whereas in phase III this activation energy is increased to (11.5_.+0"3)kJ mole -x, the reorientation rate is substantially reduced and it is suggested that rotational polarization effects cause a marked non-exponentiality in the recovery of the nuclear magnetization.