**3.2 H2O**

## *3.2.1 Ice Ih, Ic, and XI*

Hydrogen-disordered hexagonal ice (ice Ih) is a thermodynamically stable phase of solid H2O under low pressures at temperatures >72 K [33]. At temperatures <72 K, hydrogen-ordered ice XI becomes the thermodynamically stable phase [34, 35]. It is widely accepted that doping (e.g., KOH) is essential for the formation of ice XI at low temperatures [36]. A thermodynamically metastable phase of hydrogen-disordered cubic ice (ice Ic) also exists at temperatures between 100 and 200 K [33]. The crystal structures of ice Ih and ice Ic are very similar except for the stacking sequences of their layers: ice Ih is ABABAB and ice Ic is ABCABC. The space groups of ice Ih, ice XI, and ice Ic are *P*63/mmc, *C*mc21, and *F*d3m, respectively, and all these crystals are not chiral.

#### **Figure 3.**

*Transmission electron microscopy image of (A) crystalline CO (α-CO) deposited on amorphous H2O at 19 K, and (B) the electron diffraction pattern of α-CO on amorphous Si at 19 K. White scale bar = 500 nm.*
