HOPPING AND DISSIPATIVE TUNNELING OF ORIENTATIONAL DEFECTS IN ICE

We have measured the dielectric relaxation time of orientational defec ts for H2O and D2O polycrysualine ice samples, in the temperature rang e 200-270 K, and over the frequency range 0.3-1000 kHz. Results are in good agreement with previous studies, and at T < 240 K departures fro m the familiar Arrhenius law have been observed. We show that these de viations from classical rate theory can be well described within the f ramework of dissipative quantum tunneling (DQT) theory, assuming impur ity-generated Bjerrum defects responsible for the observed dielectric relaxation process over the entire temperature range investigated. The temperature regions where quantum tunneling, crossover to thermal hop ping, and quantum corrections to classical laws, respectively prevail are here reviewed. Particularly significant is the perfect agreement, near the crossover temperature T(c), of all our different samples with a universal scaling law, as predicted by DQT theory. The crossover te mperature T(c), where quantum tunneling and thermal hopping merge, has been found close to 240 K and to 220 K for H2O and D2O ice respective ly, thus showing the higher relevance of quantum effects in H2O ice. I t is shown that the dielectric relaxation time of orientational defect s for both H2O and D2O ice samples never attains a fully classical beh aviour, even at their melting temperature.