Amorphous ices govern a range of cosmological processes and are potentially key materials for explaining the anomalies of liquid water. A substantial density gap between low-density amorphous (LDA) and the high-density amorphous ices (HDA) with liquid water in the middle is a cornerstone of our current understanding of water. However, we show that ball milling ‘ordinary’ ice Ih at low temperature gives a structurally distinct medium-density amorphous ice (MDA) within this density gap. These results raise the possibility that MDA is the true glassy state of liquid water or alternatively a heavily sheared crystalline state. Remarkably, the compression of MDA at low temperature leads to a sharp increase of its recrystallization enthalpy highlighting that H₂O can be a high-energy geophysical material.

1. Low-temperature compression experiments

MDA samples were transferred into indium gaskets located inside 8 mm-diameter harden-steel piston cylinders precooled with liquid nitrogen. Compression at 77 K for a range of pressures up to 1.6 GPa was performed at 5 kN min^-1 using the computerized Zwick Roell Proline Z100TN Universal Testing Machine. Following decompression, the samples were recovered under liquid nitrogen for further analysis. In one experiment, a compressed sample was heated to 250 K at ambient pressure to transform the sample to ice Ih and then compressed again to 1.6 GPa at 77 K.

2. Differential Scanning Calorimetry (DSC)

Stainless-steel pans were filled with ice samples under liquid nitrogen and transferred into a pre-cooled PerkinElmer DSC 8000 advanced double-furnace differential scanning calorimeter. The samples were heated from 95 to 260 K at 10 K min^-1 and transformed to ice Ih. A second scan from 95 to 260 K was then recorded and used as a background for the first scan. Finally, the amounts of H₂O in the DSC pan were determined by recording the endothermic melting at 0°C and using 6012 J mol^-1 as the enthalpy of melting.

3. X-ray diffraction measurements

Ice samples were transferred into a purpose-built sample holder with Kapton windows under liquid nitrogen. Powder X-ray diffraction patterns were then recorded by mounting the sample holders on a Stoe Stadi P diffractometer with Ge 111 monochromated Cu Kα1 radiation at 40 kV and 30 mA, and a Mythen 1K area detector. The temperature of the samples was controlled with an Oxford Instruments Cryojet HT.

4. Raman spectroscopy

Raman spectra were recorded at 80 K using a Renishaw Ramascope (632.8 nm) and an Oxford Instruments Microstat^N cryostat.

5. Small-angle X-ray scattering (SAXS)

Ball-milled ice samples were transferred into a self-built aluminum sample holder under liquid nitrogen. The sample holder consists of two 150 × 100 × 20 mm aluminum plates with a 4 mm-diameter bore in the center. One plate contains a 10 mm wide and 1.8 mm deep cylindrical recess with the bore in the middle which was covered with 0.025 mm thick Kapton foil and filled with the ice sample. The second plate with Kapton foil covering the bore and a conical exit at the back was placed on top of the ice sample creating a closed 1.8 mm deep sample environment. The two plates were firmly held together by four M6 screws at the corners. A hollow 3D-printed base made from PLA was attached to the bottom of the two aluminum plates and a Pt100 temperature sensor was firmly inserted through a hole from the side of the aluminum block, close to the sample compartment, and secured with heat-conducting paste. The entire assembly was transferred quickly from liquid nitrogen into the vacuum chamber of a Ganesha 300XL SAXS instrument. After connecting the temperature sensor to a Lakeshore 331 cryogenic temperature controller, the vacuum chamber was evacuated to a base pressure of ~1×10^-2 mbar within a few minutes. A microfocus X-ray beam was generated with a copper source with a motorized collimation system of 2 sets of 4-blade single crystal slits. SAXS data was collected with a solid-state photon-counting detector (PILATUS 300 K, Dectris AG, Switzerland) mounted on a 1.4 m transverse rail along the beamline. SAXS data in the 0.15 to 0.7 Å^-1 Q-range with a resolution of 0.00179 Å^-1 was continuously collected every 3 minutes from 80 to 220 K over the course of about 10 hours. The measurements were repeated with an empty sample compartment but including the Kapton foils for background subtractions.

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