The two faces of water
Combining measurements at the Advanced Photon Source (APS) of Argonne National Laboratory (ANL) in the USA and the PETRA III synchrotron source at the DESY research centre in Germany, an international team led by the University of Stockholm in Sweden has unveiled a new, remarkable property of water. Most water ice in the solar system actually exists in an amorphous, glass-like form, which comes in two varieties, one with high density (HDA) and one with low density (LDA). Studying the phase change of HDA ice to LDA ice upon warming between temperatures of 115 K to 130 K, the team found evidence that the transition takes place via a liquid state: First, the HDA ice seems to turn into a liquid form of water of high density, then this high-density liquid (HDL) changes into a lower-density form (low-density liquid, LDL). The study thus demonstrated the existence of two varieties of liquid water, at least in the ultraviscous regime at very low temperatures – an important step towards further understanding this amazing liquid, whose special properties are indispensable to life on Earth.
The structural information for the experiment was obtained from wide-angle X-ray scattering (WAXS) data taken at APS. To uncover the details of the phase transition, however, the team had to gain dynamical information, which was accessible by X-ray photon correlation spectroscopy (XPCS) at PETRA III, performed using a LAMBDA detector. “The critical detector requirements for these measurements were a small pixel size, low noise and high efficiency – i.e. high signal-to-noise ratio – as the detected photon count rate was very low when investigating this radiation-sensitive sample,” says co-author Michael Sprung, scientist in charge of the P10 Coherence Applications beamline where the experiment was carried out. “Thanks to its small pixel size, the Lambda detector allowed us to spread the photon dose over a two times larger sample area. In addition, it enables effectively noise-free single-photon counting. The measurements would have been impossible without it.”
Diffusive dynamics during the high-to-low density transition in amorphous ice
Proceedings of the National Academy of Sciences 114(31), 8193–8198 (2017)