Nature of polyamorphic transformations in H2O under isothermal compression and decompression

Abstract

We report polyamorphic transformations of ${\mathrm{H}}_{2}\mathrm{O}$ under isothermal compression and decompression at 150 K, observed by in situ x-ray diffraction. Detailed structural information across the transformations among low-, high-, and very-high-density amorphs (LDA, HDA, and VHDA) reveals the mechanisms, the reversibility, and (dis)continuity of the transformations. During isothermal (de)compression, the polyamorphic transformations are characterized by a sharp and reversible LDA-VHDA transformation, with an HDA-like form (referred to as HDA\ensuremath{'}) appearing as an intermediate state. The LDA-VHDA transformation is found to occur in two steps: a discontinuous transition between LDA and the end member of HDA\ensuremath{'} on the low-pressure side, followed by a continuous change within HDA\ensuremath{'} involving structural reconfigurations and finally converging to VHDA. Both LDA and VHDA are found to be structurally stable showing mainly elastic behavior under (de)compression, while HDA\ensuremath{'} is structurally unstable. Possible relations of these observations to low- and high-density liquids are discussed.