Abstract
Recently, it became clear that relaxation effects in amorphous ices play a very important role that has previously been overlooked. The thermodynamic history of amorphous samples strongly affects their transition behavior. In particular, well-relaxed samples show higher thermal stability, thereby providing a larger window to investigate their glass transitions. We here present neutron scattering experiments using fixed elastic window scans on relaxed forms of amorphous ice, namely expanded high density amorphous ice (eHDA), a variant of low density amorphous ice (LDA-II) and hyperquenched glassy water (HGW). These amorphous ices are expected to be true glassy counterparts of deeply supercooled liquid water, therefore fast precursor dynamics of structural relaxation are expected to appear below the calorimetric glass transition temperature. The Debye-Waller factor shows a very weak sub-T(g) anomaly in some of the samples, which might be the signature of such fast precursor dynamics. However, we cannot find this behavior consistently in all samples at all reciprocal length scales of momentum transfer.
Highlights
In spite of this research effort, the question of whether the amorphous forms are linked to deeply supercooled, ultraviscous liquids has not been resolved entirely to date
The first diffraction peak for expanded high density amorphous ice (eHDA) appears at Q = 2.1 Æ 0.1 A À1,18 for low-density amorphous (LDA)-II and hyperquenched glassy water (HGW) at Q = 1.7 Æ 0.1 A À1.40 This corresponds to a d-spacing of 3.0 Aand 3.7 A, respectively
The transformation behavior of eHDA, monitored by the intensity change with temperature at Q = 1.7, 2.1 A À1 clearly shows that the transition to LDA-II appears at 123 K, which is about 20 K higher than the transition from uHDA to LDA-I
Summary
In spite of this research effort, the question of whether the amorphous forms are linked to deeply supercooled, ultraviscous liquids has not been resolved entirely to date. The onset of the glass-to-liquid transition for HGW as well as LDA was determined to be at B136 K, at a heating rate of 30 K minÀ1.10,28 Another less common method is inelastic neutron scattering, which gives direct access to the time dependence of correlation functions and allows to study dynamic or relaxation processes on the nanosecond time scale. Additional dynamic behavior, e.g. precursor effects below the glass transition temperature, causes a deviation from harmonic W p T behavior to lower values Such an experiment, which looks for an anomaly in the Debye–Waller factor according to eqn (1), has already been performed previously using the high-Q backscattering spectrometer IN13 on uHDA and LDA-I.35,36. In view of the above-mentioned recent developments in our understanding of amorphous ices we report here a complementary study on LDA-II and eHDA as well as HGW with the focus on assessing possible precursor dynamics and crystallization stability of these materials
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