Absence of molecular mobility on nano-second time scales in amorphous ice phases

Abstract

High-resolution neutron backscattering techniques are exploited to study the elastic and quasi-elastic response of the high-density amorphous (HDA), the low-density amorphous (LDA) and the crystalline ice I_c upon temperature changes. Within the temperature ranges of their structural stability (HDA at T ≤ 80 K, LDA at T ≤ 135 K, ice I_c at T ≤ 200 K) the Debye–Waller factors and mean-square displacements characterise all states as harmonic solids. During the transformations HDA→LDA (T ≈ 100 K), LDA→I_c (T ≈ 150 K) and the supposed glass transition with T_g ≈ 135 K no relaxation processes can be detected on a time scale t < 4 ns. It can be concluded from coherent scattering measurements (D₂O) that LDA starts to recrystallise into ice I_c at T ≈ 135 K, i.e. at the supposed T_g. In the framework of the Debye model of harmonic solids HDA reveals the highest Debye temperature among the studied ice phases, which is in full agreement with the lowest Debye level in the generalised density of states derived from time-of-flight neutron scattering experiments. The elastic results at low T indicate the presence of an excess of modes in HDA, which do not obey the Bose statistics.