Spin-Lattice Relaxation in Zinc Silicofluoride Containing Manganese and Iron

The spin-lattice relaxation of the ${\mathrm{H}}^1$ and ${\mathrm{F}}^{19}$ nuclei at 29.4 MHz is studied between 4 and 300°K in single crystals of ZnSi${\mathrm{F}}_6$·6${\mathrm{H}}_2$O containing iron at a concentration of approximately 3×${10}^{17}$ ${\mathrm{cm}}^{-3\mathrm{}}$, and also containing controlled concentrations of ${\mathrm{Mn}}^{2+}$ of 2.9×${10}^{17}$ and 4.9×${10}^{19}$ ${\mathrm{cm}}^{-3\mathrm{}}$. Motional relaxation attributed to tumbling of the complexes is observed in an interval extending about 100°K below room temperature, with an energy of activation of 0.21 eV for ${\mathrm{H}}^1$ and 0.26 eV for ${\mathrm{F}}^{19}$. In the more concentrated sample, relaxation times for both nuclear types at temperatures below the motional region behave as expected for the case of nuclear transitions controlled by the mutual spin flipping in the ${\mathrm{Mn}}^{2+}$ impurity system. The nuclear relaxation observed in the low-concentration sample at the low end of the temperature range is in agreement with an approximate calculation in which it is assumed that the iron is ${\mathrm{Fe}}^{2+}$. Measurements of EPR relaxation of ${\mathrm{Mn}}^{2+}$ which are reported for one transition reveal a relaxation rate varying as $T^5$ in the Raman region. From this result, it shown that nuclear relaxation controlled by the spin-lattice relaxation time for ${\mathrm{Mn}}^{2+}$ is largely obscured by the motional relaxation. Several EPR lines are studied in the direct region below 20°K.