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

14N spin-lattice relaxation-time measurements in Rb0.28(ND4)0.72D2PO4 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 14N 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.