**4. Discussion**

The crystals of **1**–**4** could be distributed over two genetically distinct groups. The crystals of **1** and **2** were formed during the first stage of synthetic schoepite alteration. Their structures were based on the layered complexes with the edge-sharing linkage of uranyl pentagonal bipyramids, which reflected the heating of the reaction solution during the growth processes. It should be noted that original schoepite was obtained from nearly neutral solutions, whereas the aqueous medium in our experiment was significantly more acidic (pH ~ 2). Acidic conditions and the presence of additional Cs<sup>+</sup> cations destroyed the dense layer in the structure of schoepite, but high temperature allowed preserving an edge-sharing complexation of *Ur* coordination polyhedra. An initial solution in the experiment contained the Cs:U:S molar ratio ~ 1:1:1, which explained the predominance of the Cs[(UO2)(SO4)(OH)](H2O)0.25 (**1**) phase in the precipitate. The lower amount of crystals of **2** could be explained by the lower temperature of the experiment that is preferable for the formation of the zippeite-type structures.

The crystals of **3** and **4** could be attributed to the later genetic type because they were grown after cooling the system at room temperature conditions. It is of interest that both phases had in their structures uranyl pentagonal bipyramids that shared an edge with the sulfate tetrahedra. Similar arrangement of *Ur* and sulfate oxyanions were found in the structures of four natural uranyl sulfates: klaprothite, Na6(UO2)(SO4)4(H2O)4, its polymorph peligotite, Na6(UO2)(SO4)4(H2O)4, ottohahnite, Na6(UO2)2(SO4)5(H2O)8.5 [38], and lussierite Na10[(UO2)(SO4)4](SO4)2·3H2O [39]. The same clusters

that were observed in the structures of klaprothite and peligotite (Figure 4) were previously described in a few synthetic compounds [40–43], which were grown using low temperature (70 ◦C) hydrothermal experiments. The presence of such unusual arrangements of edge-sharing uranyl bipyramid and sulfate tetrahedra have never been observed during regular evaporation experiments at room temperature. Thus, we could assume that these clusters were formed on the first, hydrothermal stage of our experiment. Moreover, the presence of such S-enriched [(UO2)(SO4)4] <sup>6</sup><sup>−</sup> clusters in the heated solution could explain the local disturbance of the Cs:U:S ~ 1:1:1 concentration, which induced the formation of S-"depleted" zippeite-like compound **2**. There is one more known synthetic K-bearing uranyl sulfate, whose structure is based on the double klaprothite-type clusters [44]. This row could be continued by the further doubling of the cluster in K4(UO2(SO4)3) [44] to get the quadruple 0D unit in the structure of ottohahnite (Figure 4). Further increase of the cluster size led to the arrangement of the infinite chains in the structure of **3**, which in turn, during the dehydration process [34], would transform into the layer in the structure of **4**. The absence of the structures based on the 0D structural units in our experiment might come from the requirements for longer storage at elevated, but not high, temperatures, and higher concentrations of Cs<sup>+</sup> cations and [SO4] <sup>2</sup><sup>−</sup> oxyanions in the initial solution.

**Figure 4.** Scheme of structural evolution for the 1D and 2D uranyl sulfate complexes, which have an edge-sharing uranyl bipyramid and sulfate tetrahedron.
