Antimony NQR and Nuclear Magnetic Relaxation

Antimony nuclear quadrupole resonance frequencies in (NH 4 ) 6 Sb 4 (S0 4 ) 3 F 1 2 crystals were measured as a function of temperature, and no evidence of phase transition was detected between 77 K and room temperature. However, the resonance frequencies do not follow the Bayer type exactly. Such an anomaly is attributed to the effect of rotation of ammonium ions which form hydrogen bonds of types N —H • • • F and N —H • • O. In fact, the spin-lattice relaxation time of H and 1 9 F has a minimum which is ascribed to the reorientation of N H 4 with an activation energy of 7.14 kJ m o l 1 . The switch model for the electric field gradient at Sb nuclei was examined.