On the heterogeneous character of water's amorphous polymorphism

In this paper we report in situ small-angle neutron scattering results on the high-density 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 = (4 pi / lambda(i)) sin(theta) with theta being half the scattering angle and lambda(i) the incident neutron wavelength]. The very low Q regime (< similar to 5 x 10(-2) angstrom(-1)) 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 x 10(-2) < Q < 0.5 angstrom(-1) 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 HDA-to-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.