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 [310].
