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

N-14 spin-lattice relaxation-time measurements in Rb-0.28(ND4)(0.72)D2PO4 p rovide direct evidence for the fast exchange averaging of the "long" and "s hort" N-D...O hydrogen bonds between the ND4 group and the four surrounding PO4 groups. This exchange and the associated distortion of the ND4 tetrahe dron 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 ref lects the gradual freeze-out of the O-D...O deuterons between the two equil ibrium sites in the O-D O bonds. [S0163-1829(99)10025-0].