A second distinct structural state of high-density amorphous ice at 77 K and 1 bar

High-density amorphous ice (HDA), further densified on isobaric heating from 77 K to 165 (177) K at 1.1 (1.9) GPa, relaxes at 77 K and 1 bar to the same structural state with a density of 1.25 +/- 0.41 g cm(-3). Its density is higher by approximate to9% than that of HDA, and thus it is called very-high-density amorphous ice (VHDA). X-ray diffractogram and Raman spectrum of VHDA clearly differs from that of HDA, and the hydrogen-bonded O-O distance increases from 2.82 Angstrom in HDA to 2.85 Angstrom in VHDA. Implications for the polyamorphism of the amorphous forms of water are discussed.