**3.4 NH3 and its hydrates**

### *3.4.1 NH3*

NH3 I is a thermodynamically stable phase of solid NH3 at temperatures <200 K under low pressure [43]. Olovsson and Templeton [24] and Reed and Harris [25] showed by X-ray diffraction and neutron diffraction, respectively, that NH3 I at 77 K is chiral, including the positions of the hydrogen atoms (space group: *P*213). Hewat and Riekel [44] also confirmed this by high-accuracy neutron diffraction at temperatures between 2 and 180 K. It has been observed by IR spectroscopy that the crystallization temperature of a-NH3 is 80 K [44, 45].

#### **Figure 5.**

*Transmission electron microscopy image and corresponding electron diffraction pattern of (A) amorphous CH3OH (a-CH3OH) deposited on amorphous H2O (a-H2O) at 82 K and (B) those of crystalline CH3OH (α-CH3OH) at 110 K formed during the heating of a-CH3OH.*

*3.4.2 NH3H2O*

NH3H2O I is a thermodynamically stable phase of solid NH3H2O at temperatures <194 K under low pressure [46]. Olovsson and Templeton [24] showed by X-ray diffraction that NH3H2O I at 113 K is chiral, including the positions of the hydrogen atoms (space group: *P*212121). Loveday and Nelmes [26] confirmed the space group by neutron diffraction at 110 K.

*3.4.3 NH32H2O*

NH32H2O I is a thermodynamically stable phase of solid NH32H2O at temperatures <176 K under low pressure [47]. Bertie and Shehata [17] showed by X-ray diffraction and IR spectroscopy that NH32H2O I at 100 K is chiral, including the positions of the hydrogen atoms (space group: *P*212121). However, neutron diffraction studies showed that the space group is *P*213 and the hydrogen is partially ordered at temperatures between 4 and 174 K [26, 27]. Among the four hydrogen sites, the hydrogen occupancy of two of the sites was unity (order), while those of the other two sites were one-third and two-thirds. They considered that the transition to the ordered phase is frustrated by kinetics, as in the transition of pure ice Ih to ice XI.

Fortes et al. [27] suggested the occurrence of a hydrogen-ordered phase (space group: *P*212121) at temperatures <140 K because the ordered phase must be thermodynamically more stable than the disordered phase at low temperatures. We therefore suggested the occurrence of a hydrogen-ordered phase (space group: *P*212121) at lower temperatures in space, although the equilibrium structure at low temperatures still remains unclear. Further studies on the formation of the ordered phase using a dopant should be undertaken.

#### **3.5 CH4**

CH4 II is a thermodynamically stable phase of solid CH4 at temperatures <20.4 K under low pressure [48]. Savoie and Fourier [28] suggested that the CH4 II space group is *P*213 based on the measurement of far-IR spectra at 12 and 25 K. Hashimoto et al. [29] also suggested the same space group based on the calculation of pairinteraction potentials. Press [30] showed by neutron diffraction at 24.5 K that six of the eight molecules were ordered while the remaining two were orientationally disordered, with a space group of *F*m3c. Kobashi et al. [49], based on IR and Raman spectra, suggested that, theoretically, the space group of CH4 II is *F*m3c. Greiger et al. [50] analyzed the total neutron cross section assuming the structure proposed by Press [30]. Although recent studies have only referred to *F*m3c, these results do not rule out the existence of the space group *P*213. Therefore, we cannot eliminate the possibility that both space groups exist.
