Pulsed NMR studies of fluorine relaxation in MPF6

The fluorine spin‐lattice relaxation time T₁ was studied in polycrystalline Na, K, Rb, and Cs hexafluorophosphates at temperatures between 77 and 476 °K by pulsed NMR methods. In each case a discontinuity occurs in the temperature dependence, at 276, 275, 210, and 82 °K, respectively; those for the K and Rb salts corresponding to previously reported phase transitions. The temperature dependence of T₁ observed in the low‐temperature phase of the Na, K, and Rb salts has a minimum and an asymmetric shape which are attributed to dipolar relaxation by thermally activated, random reorientations of the ${\mathrm{PF}}_6^{-}$ ions, the activation energy E_a being smaller on the low‐temperature side of the T₁ minimum than on the high side. The temperature of the T₁ minimum and E_a decrease with increasing size of the cation, as does the transition temperature. The T₁ 's are very sensitive to the physical state of KPF₆ and RbPF₆ and to thermal cycling in RbPF₆. Powering the sample broadens the T₁ minimum, shifts it to lower temperatures by 2 to 8 °K, reduces its value by about 25%, and decreases the low‐temperature E_a. These effects are analogous to those reported in earlier, line shape studies of MPF₆ and are consistent with the two‐phase model proposed in that connection. One phase consists of relatively perfect crystalline domains, with a high E_a, and the other of domains with defects which lower E_a. Spin exchange between the two types of domains is fast enough to average out the different T₁ 's in the two phases and produce exponential relaxation. However, in finely powdered RbPF₆ at 77 °K, nonexponential relaxation was observed and is attributed to paramagnetic impurities. In the high‐temperature phase of all four salts, T₁ exhibits a broad frequency‐independent maximum of 4–7 sec which is attributed to relaxation by spin‐rotation interactions within the rapidly reorienting ${\mathrm{PF}}_6^{-}$ ions.