On the heterogeneous character of water's amorphous polymorphism

TL;DR

This study uses in situ small-angle neutron scattering to reveal the heterogeneous nature of amorphous ice structures and their transformation, showing distinct scattering regimes and transient heterogeneity during phase change.

Contribution

It provides new insights into the heterogeneity and transformation kinetics of amorphous ice using in situ small-angle neutron scattering.

Findings

HDA and LDA exhibit different small-angle scattering regimes.

HDA shows heterogeneity marked by increased scattering at low Q.

Intermediate structures during HDA to LDA transition are strongly heterogeneous.

Abstract

In this letter we report {\it in situ} small--angle neutron scattering results on the high--density (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 characterised by two $Q$ regimes featuring different properties ($Q$ is the modulus of the scattering vector defined as $Q = 4\pi\sin{(\Theta)}/\lambda_{\rm i}$ with $\Theta$ being half the scattering angle and $\lambda_{\rm i}$ the incident neutron wavelength). The very low--$Q$ regime ($< 5\times 10^{-2}$ \AA $^{-1}$) is dominated by a Porod--limit scattering. Its intensity reduces in the course of the HDA to LDA transformation following a kinetics reminiscent of that observed in wide--angle diffraction experiments. The small--angle neutron scattering formfactor in the intermediate regime of $5 \times 10^{-2} < Q < 0.5$ \AA$^{-1}$ 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 HDA to LDA transition the formfactor shows a pronounced transient excess in intensity marking all intermediate structures as strongly heterogeneous on a length scale of some nano--meters.