14N NMR study of the role of the N-D•••O bonds in the deuteron glass transition

N-14 spin-lattice relaxation-time measurements in Rb-0.28(ND4)(0.72)D2PO4 provide direct evidence for the fast exchange averaging of the "long" and "short" N-D...O hydrogen bonds between the ND4 group and the four surrounding PO4 groups. This exchange and the associated distortion of the ND4 tetrahedron induced by the "acid" deuteron intra-O-D...O motion determine the N-14 spin-lattice relaxation-time minimum around 78 K. The glass transition is not affected by the slowing down of the rotations of the ND4 groups but reflects the gradual freeze-out of the O-D...O deuterons between the two equilibrium sites in the O-D O bonds. [S0163-1829(99)10025-0].