Anomalous temperature dependence of the experimental x-ray structure factor of supercooled water

The structural changes of water upon deep supercooling were studied through wide-angle x-ray scattering at the Bernina Instrument of SwissFEL. The scattering signals covered the principal doublet of the x-ray structure factor of water, obtained for temperatures down to 228.5 ± 0.6 K by evaporative cooling in liquid phase. Upon supercooling towards a local tetrahedral structure, the second diffraction peak increased strongly in amplitude.  Additionally, an anomalous trend for the second peak position of the oxygen–oxygen structure factor was found. It exhibits an unprecedented positive partial derivative with respect to temperature for temperatures below 236 K. Based on Fourier inversion of the experimental data combined with reference data, the experimenters propose that a repeat spacing in the tetrahedral network, associated with all peaks in the oxygen–oxygen pair-correlation function, gives rise to a less dense local ordering that resembles that of low-density amorphous ice. These findings are consistent with that liquid water consists of a pentamer-based hydrogen-bonded network with low density upon deep supercooling.