DISSIPATIVE QUANTUM TUNNELING OF ORIENTATIONAL DEFECTS IN POLYCRYSTALLINE ICE

Ice is the only crystalline solid in which a non-Arrhenius temperature dependence of the dielectric relaxation time has been observed. We have measured the dielectric relaxation time of orientational defects in polycrystalline H2O and D2O ices in the temperature range (200 divided-by 270) K, and over the frequency range 500 Hz to 1 MHz. Results are in good agreement with previous studies. We show that large departures from classical rate theory at T < 240 K can be well described by the theory of dissipative quantum tunnelling by Grabert, Weiss and Hanggy. The relevance of quantum effects is noticeably smaller in D2O than in H2O ices. Quantum tunnelling of orientational defects in H2O ice is still observable at temperatures not far from its melting point.