Reversible first-order transition between two H2O amorphs at ∼0.2 GPa and ∼135 K

Transition between the low- and high-density amorphous H2O was observed with a piston-cylinder apparatus from 77 K to 140 K, almost the glass-transition temperature of the amorphs. The transition was found to occur reversibly and abruptly at ∼135 K and ∼0.2 GPa with a volume change of 0.20±∼0.01 cm3/g and some hysteresis. This confirmed the transition to be first order. Absence of intermediate amorphous states between the low-density and high-density amorphs suggested existence of two substantially different liquid phases near the glass-transition temperature.