Pulsed NMR studies of fluorine relaxation in MPF6

Abstract

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 PF6− ions, the activation energy Ea being smaller on the low-temperature side of the T1 minimum than on the high side. The temperature of the T1 minimum and Ea 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 Ea. 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 Ea, and the other of domains with defects which lower Ea. 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 PF6− ions.