Study of ionic motion in salts of the type (NH4)2MX6 by NMR relaxation

The proton spin-lattice relaxation time in the laboratory frame, T₁, and rotating frame T_1ρ for polycrystalline cubic (NH₄)₂SiF₆, (NH₄)₂SnBr₆ and (NH₄)₂SnCl₆ have been measured over a temperature range 60–500°K. Reorientation of the ammonium ion is generally the dominant relaxation mechanism and T₁ minima are observed in all samples. Activation energies are low in each case, being 2·2 Kcal/mole for the fluosilicate, 1·44 and 1·24 Kcal/mole for the bromo- and chloro-stannate respectively. For the bromostannate a λ-point occurs at 145°K above which the activation energy apparently decreases to 0·26 Kcal/mole. Anion reorientation is detected in the fluosilicate at high temperatures, the correlation time for this motion being obtained from T_1ρ measurements. There is also some evidence to suggest anion reorientation is becoming important in the stannihalides at high temperatures. The proton T_1ρ in the stannibromide is largely determined by the rapid quadrupolar controlled relaxation of the bromine nuclei. Values for the bromine T₁ are deduced and the quadrupolar relaxation mechanism discussed.