Structural transitions in ice samples at low temperatures and pressures

The structure of ice samples formed in the decay of a water impurity gel at temperatures above $4$ K and atmospheric pressure has been examined. The X-ray diffraction analysis indicates that three phases coexist in the initial sample at temperatures of $85$–$110$ K. These phases are amorphous ice occupying up to $30\%$ of the sample volume, cubic-phase ice $I_c$ metastable at low pressures ($\sim60\%$), and normal hexagonal ice $I_h$ ($\leq 6\%$). The characteristic sizes of crystals of the cubic and hexagonal phases are about $6$ and $30$ nm, respectively. The amorphous phase at annealing above $110$ K is gradually transformed to the crystalline phase both cubic and hexagonal. This transition is accompanied by two processes, including a fast increase in the sizes of cubicphase nanocrystals and the partial transition of the cubic phase $I_c$ to the hexagonal one $I_h$. Hexagonal ice $I_h$ prevails in the bulk of the sample above $200$ K.