*2.1. Synthesis*

An analog of the hydroxy-hydrated uranyl oxide mineral schoepite ([(UO2)8O2(OH)12](H2O)12 [6,7]) was synthesized according to the procedure discussed in [9], and its purity was checked using powder XRD; H2SO4 (Sigma-Aldrich, 98%) and Cs2SO4 (Vekton, Russia, 99%) were used as received. The 0.2 g of synthetic schoepite (0.03 mmol), 0.12 g of cesium sulfate (0.33 mmol), and 0.01 (0.19 mmol) ml of sulfuric acid were dissolved in 10 mL of deionized water. The solution was stirred and loaded in a 23 mL Teflon-lined steel autoclave, which was placed in a box furnace and heated to 110 ◦C. After 24 h, the furnace was cooled at ~20 ◦C/h to room temperature. A fine crystalline precipitate covered by a translucent light-yellow solution was found at the bottom of the Teflon capsule. Afterward, the product was poured into a watch glass. The detailed examination under the optical microscope revealed the presence of two types of crystals (Figure 1): The bulk of the precipitant was tiny yellowish isometric crystals with nearly diamond luster (compound **1**), interspersed with larger in size, but of much worse quality orange rhombus lamina crystals (compound **2**). After picking several crystals for further diagnostics, the product was left in a watch glass to evaporate at room temperature. In a few days, two more kinds of crystals were detected in a small amount at the edge of the solution (Figure 1): green rosettes of thin plates (compound **3**) and light green flattened rhombic crystals (compound **4**).

**Figure 1.** Crystals of **1** (**a**,**b**), **2** (**a**,**b**), **3** (**c**) and **4** (**d**) formed in the described synthetic experiment.
