Group A: Model Answer
Yoshida et al., Flexibility of Hydrogen Bond and Lowering of Symmetry in Proton Conductor, Symmetry 2012, 4, 507-516.
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There are three phases whereby each has a different crystal structure at three different temperatures. At room temperature (298K), Phase III is present whereby Cs3H(SeO4)2 has a crystal structure of a monoclinic with a space group of C2/m. At 400K, Phase II is present whereby Cs3H(SeO4)2 has a crystal structure of a monoclinic-A2/a symmetry. At 470K, Phase I is present whereby Cs3H(SeO4)2 has a crystal structure of a trigonal with a space group of R3-m. In Phase III, as we can see in Figure 2(a), the positioning of the tetrahedrons is parallel to the a-axis, and in between these SeO4 tetrahedrons are the hydrogen bonds. Looking at a 2dimensional perspective, we can also see that there is a translation movement of the SeO4 tetrahedrons along the a-axis; hence the symmetry operator would be a glide line parallel to a-axis. In a 3-dimensional perspective, we can see that Phase III has a 2-fold rotation axis and contains glide planes. In Phase II, from Figure 2(b), we can see that the positioning of the SeO4 tetrahedrons are along the approximate direction . Observing the schematic of the crystal structure in Phase II, we can see that there is a vertical mirror line in between the SeO4 tetrahedrons. There is also an a-glide reflection vertically. In Phase I, from Figure 2(c), the positioning of SeO4 tetrahedron is similar to that of Phase II, however the difference is the crystal structure and the hydrogen bonding. Comparing both Phase II and Phase III crystal structures of the compound, Phase II contains two-fold screw axis, inversion center and a two-fold rotation axis, which is the sole reason for Phase II to be twice of that of Phase III in terms of geometrical arrangement of...
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