Proton-spin—lattice relaxation in ammonium chloride at high pressure: III. Experimental results outside the λ-transition region

Abstract

The proton-spin—lattice relaxation time T 1 has been measured in NH 4Cl over a wide range of temperature and at pressures up to 2000 atm, both on polycrystalline samples and on a single crystal. In this paper the regions away from the lambda point are being discussed. The variation of T 1 with pressure appears to be rather small in contrast to the remarkably large effects of a pressure change on the temperature dependence of the relaxation time around the transition point (paper IV). The results can be interpreted in terms of a BPP model for the relaxation with one single correlation time τ c for the reorientational motions of the ammonium ions. The activation energies for τ c far below and above T λ differ by about 840 cal/mol. This can be accounted for quantitatively from the order-disorder character of the phase transition. At low temperatures the T 1 value in the single crystal turns out to be dependent on the orientation of the crystal with respect to the magnetic field. The angular dependence is in favour of a model where reorientations of the ammonium ions to all possible equilibrium positions are directly allowed. A nonexponential relaxation of the magnetization has been observed at temperatures below - 170°C.