Kinetically Controlled Two-Step Amorphization and Amorphous-Amorphous Transition in Ice

We report the results of in situ structural characterization of the amorphization of crystalline ice Ih under compression and the relaxation of high-density amorphous (HDA) ice under decompression at temperatures between 96 and 160 K by synchrotron x-ray diffraction. The results show that ice Ih transforms to an intermediate crystalline phase at 100 K prior to complete amorphization, which is supported by molecular dynamics calculations. The phase transition pathways show clear temperature dependence: direct amorphization without an intermediate phase is observed at 133 K, while at 145 K a direct Ih-to-IX transformation is observed; decompression of HDA shows a transition to low-density amorphous ice at 96 K and $\sim 1\mathrm{Pa}$, to ice Ic at 135 K and to ice IX at 145 K. These observations show that the amorphization of compressed ice Ih and the recrystallization of decompressed HDA are strongly dependent on temperature and controlled by kinetic barriers. Pressure-induced amorphous ice is an intermediate state in the phase transition from the connected H-bond water network in low pressure ices to the independent and interpenetrating H-bond network of high-pressure ices.

Figure 1

(a) Structural evolution of compressed ice Ih at 100 K in the quasihydrostatic condition using the back-and-forth approach [34]. The background was subtracted for the typical integrated x-ray diffraction patterns (Fig. S2). At 0.49 GPa, the diffraction pattern is indexed with hexagonal structure of ice Ih (Table S1). (b) and (c)The detailed structural evolutions of (010) and (110) reflections for crystalline ice Ih under compression. (d) Density of the crystalline phases as a function of pressure. The density is calculated using more than six indexed diffraction peaks. BF: back-and-forth approach. The inset in Fig. 1 shows pressure-dependent $d$ spacings of three typical diffraction reflections of ice Ih. The slight difference in the initial values of $d$ spacings could be due to the cooling history.

Figure 2

Theoretical crystal structures of (a) ice Ih and (b) intermediate crystalline phase. The intermediate crystalline phase is a shear-distorted form of ice Ih in the $ab$ basal plane due to the softening of $C_{66}$ elastic modulus. The oxygen atom in the intermediate phase have shifted slightly from the original position of ice Ih. Thus, when view down the $c$ axis, the oxygen atoms are no longer aligned along the $c$ axis as in ice Ih.

Figure 3

Structural evolution of amorphous ice under decompression (a) at 96, (b) at 135, and (c) at 145 K. Amorphous ice at 145 K was obtained by heating VHDA from 133 to 145 K. (d) The pressure-dependent $Q$ values of the first sharp peak position of the amorphous ice. The dashed arrows indicate compression and decompression route. The solid arrows indicate the phase transition of HDA under decompression. The peaks marked by stars are from gasket.

Figure 4

Summary of phase transitions observed in experiments (a) under slow compression of ice Ih and (b) under slow decompression of amorphous ice. IP stands for the intermediate phase (see texts for more details). HDA or VHDA above 140 K was obtained by heating amorphous samples from 100 or 133 K to corresponding temperatures. The right and left triangle symbols indicate experimental data under compression and decompression, respectively. Crystalline ice Ih and Ic: black, right and left; HDA: blue, right and left; VIII’: green, right; IX: olive, right and left; IP: red, right; VI: orange, left; VI: purple, left.