Yoshida et al., Flexibility of Hydrogen Bond and Lowering of Symmetry in Proton Conductor, Symmetry 2012, 4, 507-516.
In this paper, we take a look at the different phases of the Cs3H(SeO4)2 polymorphs. There are 3 different phases that is observed and each of them is influenced by the temperatures. In addition, in each phase, the polymorph exhibits different crystal structure. At the room temperature of 298K, it will be in phase 3 whereby it takes the structure of monoclinic-C2/m phase. When the temperature rises to 369K, it undergoes a transition into another phase (phase 2) also known as the high-temperature monoclinic-A2/a phase. The last transition will be into phase 1 when the temperature crosses above 456K. This final phase 1 transition takes the crystal structure of trigonal with space group of R3-m. The rise in temperature plays a role in the thermal motion of atoms, where higher temperature causes the atoms to have a higher energy and vibrate at higher frequency resulting in higher disorder, hence lower symmetry. Next, the hydrogen bonding also evolves as the compound transits into the different phases due to the change in temperature. At room temperature when the compound is in phase 3, as shown in figure 2(a), the hydrogen bonds between of the tetrahedrons are found to be parallel to the a-axis, resulting in high degree of symmetry. In addition, from a 2-d perspective, the movement of the SeO4 tetrahedrons along the a-axis projects a translation movement. Therefore, this is also known as glide line parallel along the a-axis. On the other hand, from a 3-d perspective, the compound in phase 3 contains glide planes and also a 2-fold rotation axis. In addition, it also shows a plane symmetry. When the temperature rises to above 369K, it will transition into phase 2, where the approximate alignment is along the [3 -1 0] and [3 1 0] direction. In this phase, a vertical mirror line exists between the SeO4 tetrahedrons. Hence, it maintains the...
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