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.

Reaction of Er₂S₃ powder with aqueous HF solution (49%) followed by drying of the resultant precipitate at 25–95°C has afforded X-ray amorphous phase preferably as ErF₃ · 0.87H₂O hydrate. The resultant phase have been shown to undergo thermal dissociation at 50–165°C (ΔH = 97.8 J/g, Δm = 5.5 wt %), Δm is 1 wt % in temperature range 165–270°C. Exothermal effect at 236–270°C (ΔH =–64.3 J/g) has correlated with formation of polycrystalline compound ErF₃: space group Pnma, a = 0.634(8) nm, b = 0.684(2) nm, c = 0.438(2) nm. The size of particles of ErF₃ phase formed at 170°C calculated by the Selyakov–Scherrer formula has been found to be 45 × 60 nm. The reaction of Tm₂S₃ with HF solution at 25°C has led to a phase of composition [(H₃O)Tm₃F₁₀] · 1.7H₂O: space group Fd3-m, a = 1.5389(5) nm whose particles are regular octahedra in shape and 4–8 μm in size. Upon heating of the prepared compound to 150°C (ΔH = 51.1 J/g, Δm = 4.2 wt %), water molecules go to gas phase, while crystal structure of the phase retains: a = 1.5353(3) nm. Three-quarter of water molecules nave been lost in temperature range 150–225°C (ΔH = 23.4 J/g, Δm = 3.8 wt %). Exothermal effect at 225–270°C (ΔH =–18.8 J/g) has been accompanied by formation of polycrystalline structure of compound TmF₃: space group Pnma, a = 0.627(8) nm, b = 0.681(3) nm, c = 0.440(9) nm. Particles of compound TmF₃ have been found to be 110–250 nm in size.

Complex sulfides with the SrS: Ln₂S₃ ratio equal to 3: 1, 1: 3, or 1: 4 for Ln = Y or Lu have been predicted on the basis of thermodynamic analysis of previously designed SrS-Ln₂S₃ (Ln = Tb, Dy, or Er) phase diagrams. Phase diagrams for the SrS-Ln₂S₃ (Ln = Tm, Lu, or Sc) systems have been designed for the first time. These systems form congruently melting compounds SrLn₂S₄ (CaFe₂O₄ type structure, orthorhombic crystal system). Unit cell parameters, heats of melting, and microhardnesses have been determined. For SrTm₂S₄, the respective values are as follows: a = 1.181 nm, b = 1.421 nm, c = 0.396 nm, 2040 K, ΔH _m = 188 kJ/mol, 3500 MPa; for SrLu₂S₄: a = 1.187 nm, b = 1.416 nm, c = 0.392 nm, 2070 K, ΔH _m = 190 kJ/mol, 3540 MPa; and for SrSc₂S₄: a = 1.180 nm, b = 1.410 nm, c = 0.390 nm, 2100 K, ΔH _m = 206 kJ/mol, 3650 MPa. The increase in the melting temperatures and the heats of melting calculated for the SrLn₂S₄ compounds correlate with their classification as thio salts.