Crystalline hydrate of double cesium europium sulfate [CsEu(H₂O)₃(SO₄)₂]·H₂O was synthesized by the crystallization from an aqueous solution containing equimolar amounts of 1Cs⁺:1Eu³⁺:2SO₄²⁻ ions. Anhydrous salt CsEu(SO₄)₂ 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(H₂O)₃(SO₄)₂]·H₂O structure. The crystal structure of [CsEu(H₂O)₃(SO₄)₂]·H₂O was determined by a single crystal X-ray diffraction analysis, and the crystal structure of CsEu(SO₄)₂ was obtained by the Rietveld method. [CsEu(H₂O)₃(SO₄)₂]·H₂O crystallizes in the monoclinic system, space group P2₁/c (a = 6.5574(1) Å, b = 19.0733(3) Å, c = 8.8364(2) Å, β = 93.931(1)°, V = 1102.58(3) Å3). The anhydrous sulfate CsEu(SO₄)₂ formed as a result of the thermal destruction crystallizes in the monoclinic system, space group C2/c (a = 14.327(1) Å, b = 5.3838(4) Å, c = 9.5104(6) Å, β = 101.979(3) °, V = 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 H₂O and SO₄²⁻. As shown by the diffused reflection spectra measurements and DFT calculations, the structural transformation from [CsEu(H₂O)₃(SO₄)₂]·H₂O to CsEu(SO₄)₂ 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 Eu³⁺ ions, namely, by the increase in inversion asymmetry in [CsEu(H2O)₃(SO₄)₂]·H₂O and the increase in mirror asymmetry in CsEu(SO₄)₂. The chemical shift of the ⁵D₀ energy level in cesium europium sulfate hydrate, with respect to cesium europium sulfate, is associated with the presence of H₂O molecules in the vicinity of Eu³⁺ ion.

Praseodymium sulfate was obtained by the precipitation method and the crystal structure was determined by Rietveld analysis. Pr₂(SO₄)₃ is crystallized in the monoclinic structure, space group C2/c, with cell parameters a = 21.6052 (4), b = 6.7237 (1) and c = 6.9777 (1) Å, β = 107.9148 (7)°, Z = 4, V = 964.48 (3) ų (T = 150 °C). The thermal expansion of Pr₂(SO₄)₃ is strongly anisotropic. As was obtained by XRD measurements, all cell parameters are increased on heating. However, due to a strong increase of the monoclinic angle β, there is a direction of negative thermal expansion. In the argon atmosphere, Pr₂(SO₄)₃ is stable in the temperature range of T = 30–870 °C. The kinetics of the thermal decomposition process of praseodymium sulfate octahydrate Pr₂(SO₄)₃·8H₂O was studied as well. The vibrational properties of Pr₂(SO₄)₃ were examined by Raman and Fourier-transform infrared absorption spectroscopy methods. The band gap structure of Pr₂(SO₄)₃ was evaluated by ab initio calculations, and it was found that the valence band top is dominated by the p electrons of oxygen ions, while the conduction band bottom is formed by the d electrons of Pr³⁺ ions. The exact position of ZPL is determined via PL and PLE spectra at 77 K to be at 481 nm, and that enabled a correct assignment of luminescent bands. The maximum luminescent band in Pr₂(SO₄)₃ belongs to the ³P₀ → ³F₂ transition at 640 nm.