In situ x-ray diffraction study of polyamorphism in H₂O under isothermal compression and decompression

Amorphous-amorphous transformations in H₂O have been studied under rapid isothermal compression and decompression in a diamond anvil cell together with in situ x-ray diffraction measurements using brilliant synchrotron radiation. The experimental pathways provide a density-driven approach for studying polyamorphic relations among low-, high-, and very high-density amorphs (LDA, HDA, VHDA) in a pressure range of 0-3.5 GPa at temperatures of 145-160 K. Our approach using rapid (de)compression allows for studying the polyamorphic transformations at higher temperatures than the conditions previously studied under slow (de)compression or isobaric annealing. Multiple compression-decompression cycles can be integrated with in situ x-ray measurements, thus facilitating the study of repeatability and reversibility of the polyamorphic transformations. Fast in situ x-ray diffraction measurements allow for obtaining detailed insight into the structural changes across polyamorphic transformations regarding the (dis)continuity, reversibility, and possible intermediate forms. As demonstrated at isothermal conditions of 145 K and 155 K, the polyamorphic transformations are characterized by a sharp and reversible LDA-VHDA transformation, with an HDA-like form (referred to as HDA') appearing as an intermediate state. The LDA-VHDA transformation is found to occur in two steps: a discontinuous transition between LDA and HDA' and a continuous change within HDA' involving structural reconfigurations and finally converging to VHDA.