O-D...O DEUTERON INTRABOND AND INTERBOND EXCHANGE AND FREQUENCY-DEPENDENT ORDER-PARAMETER IN DEUTERON GLASSES STUDIED BY 2-DIMENSIONAL EXCHANGE NMR

The question of whether the deuteron glass transition is an equilibrium phase transition or a metastable kinetic phenomenon, observed because of the finite experimental observation time, is addressed. Two-dimensional (2D) exchange NMR of O-D...O deuterons in deuteron glasses provides a unique possibility of a direct determination of the O-D...O intra- and interbond exchange times as well as for a determination of the Edwards-Anderson order parameter deeply in the glassy phase where 1D NMR methods for order parameter determination fail. On the frequency observation scales of spin-lattice relaxation and line-shape studies 10(3)-10(8) Hz, the deuteron glass phase appears static. 2D exchange NMR extends the frequency observation window into the mHz region. The O-D...O deuteron intrabond exchange time tau(exch) in Rb0.68(ND4)0.32D2AsO4 has been determined as a function of temperature below 45 K. A very slow fluctuation in the H-bond double minimum potential is detected which averages out the H-bond asymmetry and the glass order parameter to zero at long-enough times. The deuteron glass phase is nonergodic, showing a frozen-in disorder at times t << tau(exch), whereas for times long compared to tau(exch), ergodicity is restored.