| Summary: | Crystalline hydrate of double cesium europium sulfate [CsEu(H<sub>2</sub>O)<sub>3</sub>(SO<sub>4</sub>)<sub>2</sub>]·H<sub>2</sub>O was synthesized by the crystallization from an aqueous solution containing equimolar amounts of 1Cs<sup>+</sup>:1Eu<sup>3+</sup>:2SO<sub>4</sub><sup>2−</sup> ions. Anhydrous salt CsEu(SO<sub>4</sub>)<sub>2</sub> was formed as a result of the thermal dehydration of the crystallohydrate. The unusual effects observed during the thermal dehydration were attributed to the specific coordination of water molecules in the [CsEu(H<sub>2</sub>O)<sub>3</sub>(SO<sub>4</sub>)<sub>2</sub>]·H<sub>2</sub>O structure. The crystal structure of [CsEu(H<sub>2</sub>O)<sub>3</sub>(SO<sub>4</sub>)<sub>2</sub>]·H<sub>2</sub>O was determined by a single crystal X-ray diffraction analysis, and the crystal structure of CsEu(SO<sub>4</sub>)<sub>2</sub> was obtained by the Rietveld method. [CsEu(H<sub>2</sub>O)<sub>3</sub>(SO<sub>4</sub>)<sub>2</sub>]·H<sub>2</sub>O crystallizes in the monoclinic system, space group <i>P</i>2<sub>1</sub>/<i>c</i> (<i>a</i> = 6.5574(1) Å, <i>b</i> = 19.0733(3) Å, <i>c</i> = 8.8364(2) Å, β = 93.931(1)°, <i>V</i> = 1102.58(3) Å3). The anhydrous sulfate CsEu(SO<sub>4</sub>)<sub>2</sub> formed as a result of the thermal destruction crystallizes in the monoclinic system, space group <i>C</i>2/<i>c</i> (<i>a</i> = 14.327(1) Å, <i>b</i> = 5.3838(4) Å, <i>c</i> = 9.5104(6) Å, β = 101.979(3) °, <i>V</i> = 717.58(9) Å3). The vibration properties of the compounds are fully consistent with the structural models and are mainly determined by the deformation of non-rigid structural elements, such as H<sub>2</sub>O and SO<sub>4</sub><sup>2−</sup>. As shown by the diffused reflection spectra measurements and DFT calculations, the structural transformation from [CsEu(H<sub>2</sub>O)<sub>3</sub>(SO<sub>4</sub>)<sub>2</sub>]·H<sub>2</sub>O to CsEu(SO<sub>4</sub>)<sub>2</sub> induced a significant band gap reduction. A noticeable difference of the luminescence spectra between cesium europium sulfate and cesium europium sulfate hydrate is detected and explained by the variation of the extent of local symmetry violation at the crystallographic sites occupied by Eu<sup>3+</sup> ions, namely, by the increase in inversion asymmetry in [CsEu(H2O)<sub>3</sub>(SO<sub>4</sub>)<sub>2</sub>]·H<sub>2</sub>O and the increase in mirror asymmetry in CsEu(SO<sub>4</sub>)<sub>2</sub>. The chemical shift of the <sup>5</sup>D<sub>0</sub> energy level in cesium europium sulfate hydrate, with respect to cesium europium sulfate, is associated with the presence of H<sub>2</sub>O molecules in the vicinity of Eu<sup>3+</sup> ion.
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