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Abstract
Since the first report of very-high density amorphous ice (VHDA) in 2001 [T. Loerting et al., Phys. Chem. Chem. Phys. 3, 5355-5357 (2001)], the status of VHDA as a distinct amorphous ice has been debated. We here study VHDA and its relation to expanded high density amorphous ice (eHDA) on the basis of isobaric heating experiments. VHDA was heated at 0.1 ≤ p ≤ 0.7 GPa, and eHDA was heated at 1.1 ≤ p ≤ 1.6 GPa to achieve interconversion. The behavior upon heating is monitored using in situ volumetry as well as ex situ X-ray diffraction and differential scanning calorimetry. We do not observe a sharp transition for any of the isobaric experiments. Instead, a continuous expansion (VHDA) or densification (eHDA) marks the interconversion. This suggests that a continuum of states exists between VHDA and HDA, at least in the temperature range studied here. This further suggests that VHDA is the most relaxed amorphous ice at high pressures and eHDA is the most relaxed amorphous ice at intermediate pressures. It remains unclear whether or not HDA and VHDA experience a sharp transition upon isothermal compression/decompression at low temperature.
Highlights
Water, the molecule of life, is ubiquitous in nature and yet bears astonishing properties
While the very high-density amorphous ice (VHDA) samples expand at all pressures studied here, unannealed HDA (uHDA) is known to expand during isobaric heating below 0.35 GPa,29,65 but it compacts at higher pressures
We studied the behavior of VHDA and expanded high density amorphous ice (eHDA) by in situ volumetry upon isobaric heating at 0.1 ≤ p ≤ 0.7 GPa and 1.1 ≤ p ≤ 1.6 GPa, respectively, in search of sharp limits of stability
Summary
The molecule of life, is ubiquitous in nature and yet bears astonishing properties. Those properties, often called anomalies, are nicely listed on the web page of Martin Chaplin.. Since the third amorphous ice form called very high-density amorphous ice (VHDA) was reported in 2001,10 naturally the question arose whether water could even form three distinct liquids. In attempting to answer this question, one has to clarify whether VHDA is a truly distinct polyamorph or not. We state that if VHDA is a distinct polyamorph it has to share a first-order-like transition a)Present address: Department of Physics, Sapienza University of Rome, Piazzale Aldo Moro 5, I-00185 Roma, Italy. Given that studies of VHDA both during isobaric heating at low pressures and isothermal decompression at T ≥ 125 K17,18 showed that VHDA transforms to HDA before it transforms to LDA, the key question on the status of VHDA boils down to the clarification of the HDA-VHDA relation
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