Anisotropic Dielectric Relaxation in Single Crystal H₂O Ice Ih from 80-250 K

Three properties of the dielectric relaxation in ultra-pure single crystalline H$_{2}$O ice Ih were probed at temperatures between 80-250 K; the thermally stimulated depolarization current, static electrical conductivity, and dielectric relaxation time. The measurements were made with a guarded parallel-plate capacitor constructed of fused quartz with Au electrodes. The data agree with relaxation-based models and provide for the determination of activation energies, which suggest that relaxation in ice is dominated by Bjerrum defects below 140 K. Furthermore, anisotropy in the dielectric relaxation data reveals that molecular reorientations along the crystallographic $c$-axis are energetically favored over those along the $a$-axis between 80-140 K. These results lend support for the postulate of a shared origin between the dielectric relaxation dynamics and the thermodynamic partial proton-ordering in ice near 100 K, and suggest a preference for ordering along the $c$-axis.