On the heterogeneous character of water's amorphous polymorphism

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 ${\lambda }_{\mathrm{i}}$ 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.