N-14 NMR study of the glass transition in (NH4I)(0.44)(KI)(0.56)

The orientational glass transition in a (NH4I)(0.44)(KI)(0.56) single crystal has been studied by two-dimensional quadrupole perturbed N-14 NMR spectroscopy and N-14 spin-lattice relaxation time T-1 measurements. A N-14 T-1 minimum was found at 9 K. The N-14 inhomogeneous linewidth starts to increase strongly with decreasing T already below 50 K, i.e., on the high-temperature side of the T-1 minimum, demonstrating a breaking of the local cubic symmetry and the onset of the orientational glass transition in the fast motion regime. The N-14 magnetization recovery also changes from monoexponential to stretched-exponential below 50 K due to the development of spatial inhomogeneities in the sample as a result of a local glassy freeze-out. Both effects can be described by the presence of a local polarization distribution function W(p) with a nonzero second moment M(2) below 50 K. The temperature dependence of the N-14 M(2), which is proportional to the Edwards-Anderson order parameter, shows that the glass transition is of the random-bond-random-field type. The random bond contribution is about three times stronger than the random-field one.