Thermal stability against crystallization upon isobaric heating at pressure 0.1 ≤ P ≤ 1.9 GPa is compared for five variants of high- (HDA) and very high-density amorphous ice (VHDA) with different preparation history. At 0.1–0.3 GPa expanded HDA (eHDA) and VHDA reach the same state before crystallization, which we infer to be the contested high-density liquid (HDL). Thus, 0.3 GPa sets the high-pressure limit for the possibility to observe HDL for timescales of minutes, hours, and longer. At P > 0.3 GPa the annealed amorphous ices no longer reach the same state before crystallization. Further examination of the results demonstrates that crystallization times are significantly affected both by the density of the amorphous matrix at the crystallization temperature Tx as well as by nanocrystalline domains remaining in unannealed HDA (uHDA) as a consequence of incomplete pressure-induced amorphization.

The pressure dependence of the crystallization temperature of different forms of expanded high-density amorphous ice (eHDA) was scrutinized. Crystallization at pressures 0.05-0.30 GPa was followed using volumetry and powder x-ray diffraction. eHDA samples were prepared via isothermal decompression of very high-density amorphous ice at 140 K to different end pressures between 0.07-0.30 GPa (eHDA). At 0.05-0.17 GPa the crystallization line T (p) of all eHDA variants is the same. At pressures>0.17 GPa, all eHDA samples decompressed to pressures <0.20 GPa exhibit significantly lower T values than eHDA and eHDA. We rationalize our findings with the presence of nanoscaled low-density amorphous ice (LDA) seeds that nucleate in eHDA when it is decompressed to pressures <0.20 GPa at 140 K. Below ~0.17 GPa, these nanosized LDA domains are latent within the HDA matrix, exhibiting no effect on T of eHDA. Upon heating at pressures  ⩾0.17 GPa, these nanosized LDA nuclei transform to ice IX nuclei. They are favored sites for crystallization and, hence, lower T . By comparing crystallization experiments of bulk LDA with the ones involving nanosized LDA we are able to estimate the Laplace pressure and radius of ~0.3-0.8 nm for the nanodomains of LDA. The nucleation of LDA in eHDA revealed here is evidence for the first-order-like nature of the HDA  →  LDA transition, supporting water's liquid-liquid transition scenarios.

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