Cs3H(SeO4)2 exhibits three different crystal structure with different phases at 3 different temperatures. It is composed of SeO4 tetrahedrons throughout the material. There is a 3-fold rotation in the tetrahedron. The Cs atom is surrounded by 6 of such SeO4 tetrahedrons. As each tetrahedrons tilt in a different manner in every phase, protons will adjust itself to fit into different spaces at each phase. At room temperature, the compound is at phase III whereby its crystal belongs to a monoclinic system with the space group C2/m. The lattice parameters are a III= 10.903(3) Å, b III = 6.3904(8) Å, c III = 8.452(2) Å, βIII = 112.46(1)° and Z = 2. When we look at the compound in 2 dimensions, we can see that there is a high symmetry as we are able to form horizontal and vertical mirror lines. Glide lines that are parallel to the a III axis are also be observed. Between each tetrahedron, a 2-fold rotation is present. At 369K, the compound exhibits phase II which is a monoclinic phase with the space group A2/a. Interestingly, the increase in temperature led to a decrease in symmetry. The lattice parameters are now a II= 11.037(1) Å, b II = 6.415(1) Å, c II = 16.042(4) Å, βII = 102.69(1)° and Z = 4. Hydrogen bonds are formed between SeO4 tetrahedrons along the [310] direction. A two-fold screw axis, inversion center and a two-fold rotation axis is found here. At around 470K, its crystal structure changes again. It is in phase I with a trigonal system and the space group R3m. The lattice constants become aI= 6.4260(6) Å, cI = 23.447(2) Å, Z = 3. Since hydrogen bonds are composed between 2 oxygen atoms, three equivalent positions for a proton appear in phase I, forming a hexagonal shape throughout the structure. Therefore, we can see that at each of these temperatures, the symmetry changes and this is due to the different bonding between the atoms.
Along the (001) plane, it is evident that there is a change in the domain structure at the phase transition of TII–III and TI–II.