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

Abstract

The orientational glass transition in a (${\mathrm{NH}}_{4}$I${)}_{0.44}$(KI${)}_{0.56}$ single crystal has been studied by two-dimensional quadrupole perturbed $^{14}\mathrm{N}$ NMR spectroscopy and $^{14}\mathrm{N}$ spin-lattice relaxation time ${\mathit{T}}_{1}$ measurements. A $^{14}\mathrm{N}$ ${\mathit{T}}_{1}$ minimum was found at 9 K. The $^{14}\mathrm{N}$ inhomogeneous linewidth starts to increase strongly with decreasing T already below 50 K, i.e., on the high-temperature side of the ${\mathit{T}}_{1}$ minimum, demonstrating a breaking of the local cubic symmetry and the onset of the orientational glass transition in the fast motion regime. The $^{14}\mathrm{N}$ 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 ${\mathit{M}}_{2}$ below 50 K. The temperature dependence of the $^{14}\mathrm{N}$ ${\mathit{M}}_{2}$, 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.