Study of the structural phase transitions of solid NH 4Br by the measurements of T1 for both 81Br and 79Br

Abstract

The spin-lattice relaxation times ( T 1) were measured at 12 MHz for both 81Br and 79Br in polycrystalline NH 4Br as a function of temperature in the range 77 to 300 K. Two structural phase transitions were observed. An order-disorder phase transition occurs at 235 K and a displacive phase transition occurs at ∼95 K. The experimental results are as follows: (1) Critical effects are observed in the S phase at temperatures close to the displacive phase transition. The softening occurs at temperatures less than 8 K from the displacive phase transition temperature. (2) A simple Arrhenius law of relaxation rates was observed in the high-temperature range of the disordered β phase with an activation energy of 3.65 kcal/mol. (3) In the δ phase, the ratio of the spin-lattice relaxation times for the isotopes 81Br and 79Br is 1.389, which is just the inverse of the ratio of their static quadrupole moments. This indicates that the relaxation mechanism in the δ phase is purely quadrupolar. (4) A large hysteresis was observed in the displacive phase transition indicating the transition is first order. A theoretical analysis of the results is also presented.