Deuteron NMR investigations of glass and phase transitions in (KI)1-x(ND4I)x mixed crystals

Abstract

Powdered potassium ammonium iodide mixed crystals with ammonium concentrations of 20% and 70% have been studied using deuteron NMR techniques. For the less doped sample the deuteron line starts to broaden and the spin-lattice relaxation time T1 becomes nonexponential below about 20 K. These observations indicate the breakdown of ergodicity and the onset of orientational glass freezing. At the lowest temperatures T1 reaches a plateau value. For the 70% sample a relatively abrupt onset of two-phase behavior at T<35 K is inferred from measurements of the spin-spin and spin-lattice relaxation times. These observations are taken to indicate an order-disorder transition to a crystalline phase which exhibits dynamically distinguishable lattice sites.