O-D . . .O deuteron intra- and interbond exchange and frequency-dependent order parameter in deuteron glasses studied by two-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 10³--10⁸ 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 τexch in Rb0.68(ND₄{)}0.32D₂{AsO}₄$ 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{}τexch, whereas for times long compared to τexch, ergodicity is restored.