On the heterogeneous character of water's amorphous polymorphism

Abstract

In this paper we report in situ small-angle neutron scattering results on the highdensity amorphous (HDA) and low-density amorphous (LDA) ice structures and on intermediate structures as found during the temperature-induced transformation of HDA into LDA. We show that the small-angle signal is characterized by two Q regimes featuring different properties [Q is the modulus of the scattering vector defined as Q ¼ ð4 = i Þ sinð Þ with being half the scattering angle and i the incident neutron wavelength]. The very low Q regime (< $ 5 Â 10 À2 Å À1 ) is dominated by a Porod-limit scattering. Its intensity reduces during the course of the HDA-to-LDA transformation following kinetics reminiscent of those observed in wide-angle diffraction experiments. The small-angle neutron scattering form factor in the intermediate regime of 5 Â 10 À2 < Q < 0:5 Å À1 for HDA and LDA features a rather flat plateau. However, the HDA signal shows an ascending intensity towards smaller Q marking this amorphous structure as heterogeneous. When following the HDAto-LDA transition, the form factor shows a pronounced transient excess in intensity marking all intermediate structures as strongly heterogeneous on a length scale of some nanometres.

Structural transformations in amorphous ice and supercooled water and their relevance to the phase diagram of water

“…We note from the structure factors that there is no sign of increased scattering at low Q in the "mixture" samples, (b), (c) and (d). Hence, at least within the limitations and range of the data and computer simulation there was no sign of the structural heterogeneity that was seen experimentallly for the transformation from HDA to LDA [134]. In r-space the first peak is seen to be almost stationary with density, while the second peak, which is at ∼ 4.5Å in LDL moves to, or is replaced by a peak at ∼ 3.5Å.…”

“…Clearly any transform from one state to another might involve significant heterogeneities during the transition if it were discontinuous. Here the work of M. Koza and colleagues has yielded some insight, [133,134]. Using small and wide angle neutron scattering they watch the in situ transformation of In particular it appears that the main diffraction peak evolves continuously through the transition, becoming markedly broader in a state labelled SSH, which corresponds to the "structure of strongest heterogeneity".…”

Evidence for liquid water during the high-density to low-density amorphous ice transition

“…It has been reported that the HDA ice prepared by the conventional method [compressing hexagonal ice Ͼ1 GPa at 77 K (9)] exhibits a relaxation process in the transition to LDA ice (15)(16)(17)(18). Using high-pressure cryocooling, HDA ice is induced by pressurizing samples at 283 K, which are then cooled to 77 K while still under pressure (23).…”

“…However, experimental study of the HDL-LDL phase transition is challenging as supercooled water spontaneously converts to crystalline forms below the homogeneous nucleation temperature (Ϸ235 K at 0.1 MPa). The transition between 2 glassy forms of water, high-density amorphous (HDA) and low-density amorphous (LDA) ice, has been extensively studied (9)(10)(11)(12)(13)(14)(15)(16)(17)(18), as an analogue of the HDL-LDL transition. Controversy still exists as to whether the HDA-LDA ice transition is truly a first-order phase transition (10)(11)(12)(13) or if it occurs because of a relaxation process of an unstable amorphous structure (15)(16)(17)(18).…”

“…The transition between 2 glassy forms of water, high-density amorphous (HDA) and low-density amorphous (LDA) ice, has been extensively studied (9)(10)(11)(12)(13)(14)(15)(16)(17)(18), as an analogue of the HDL-LDL transition. Controversy still exists as to whether the HDA-LDA ice transition is truly a first-order phase transition (10)(11)(12)(13) or if it occurs because of a relaxation process of an unstable amorphous structure (15)(16)(17)(18). More importantly, the connection between the HDA-LDA ice transition and the HDL-LDL phase transition, implied by thermodynamic and structural studies on water (19)(20)(21)(22), remains challenging to prove experimentally (8).…”