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.

The oxidation process of samariumoxysulfide was studied in the temperature range of 500–1000 °C. Our DTA investigation allowed for establishing the main thermodynamic (∆Hº_exp = −654.6 kJ/mol) and kinetic characteristics of the process (E_a = 244 kJ/mol, A = 2 × 10¹⁰). The enthalpy value of samarium oxysulfate (ΔHºf (Sm₂O₂SO_4(monocl)) = −2294.0 kJ/mol) formation was calculated. The calculated process enthalpy value coincides with the value determined in the experiment. It was established that samarium oxysulfate crystallizes in the monoclinic symmetry class and its crystal structure belongs to space group C2/c with unit cell parameters a = 13.7442 (2), b = 4.20178 (4) and c = 8.16711 (8)Å, β = 107.224 (1)°, V = 450.498 (9)ų, Z = 4. The main elements of the crystalline structure are obtained and the cation coordination environment is analyzed in detail. Vibrational spectroscopy methods confirmed the structural model adequacy. The Sm₂O₂SO₄luminescence spectra exhibit three main bands easily assignable to the transitions from ⁴G_5/2 state to ⁶H_5/2, ⁶H_7/2, and ⁶H_9/2 multiplets.

The kinetic dependences of the yield of Ln₂O₂S (Ln = La, Pr, Nd, and Sm) upon the treatment of Ln₂O₂SO₄ in a hydrogen flow at 950, 1120, and 1170 K are constructed. The plots are approximated by Avrami-Erofeev equations, equations of compressed volume and compressed surface, and Jander’s equation. The correctness of choosing these equations is evaluated using the numerical values of Fisher’s criterion. Images of the particles of Nd₂O₂SO₄ → Nd₂O₂S transformation in the flow are obtained on an Ntegra Aura atomic force probe microscope. A mechanism of the crystal growth of the Nd₂O₂S phase from the nucleation site of the Nd₂O₂SO₄ and Nd₂O₂S phase boundaries is proposed.

The exposure of anhydrous Ln₂(SO₄)₃ (Ln = Gd, Dy, Y, Er, Lu) sulfates to flowing hydrogen in the range 500–1050°C leads to the formation of Ln₂O₂S + Ln₂O₃ materials. With increasing reaction temperature, the mole fraction of Ln₂O₂S in the samples decreases. Single-phase Ln₂O₂S (Ln = Gd, Dy, Y, Er, Lu) compounds have been obtained by exposing the rare-earth sulfates first to flowing hydrogen in the range 500–600°C and then to hydrogen sulfide in the range 850–950°C.

Sequentially exposing coprecipitated yttrium, ytterbium, and erbium sulfates to flowing H₂ at 500–600°C for 6 h and then to flowing H₂S at 1000°C for ≤7 h, we obtained single-phase samples of the following solid solutions: (1) (Y_0.97Er_0.02Yb_0.01)₂O₂S (а = 3.779 Å, с = 6.579 Å), (2) (Y_0.94Er_0.05Yb_0.01)₂O₂S (а = 3.783 Å, с = 6.572 Å), (3) (Y_0.91Er_0.07Yb_0.02)₂O₂S (а = 3.775 Å, с = 6.568 Å), (4) (Y_0.87Er_0.10Yb_0.03)₂O₂S (а = 3.772 Å, с = 6.559 Å), (5) (Y_0.82Er_0.15Yb_0.03)₂O₂S (а = 3.770 Å, с = 6.545 Å), and (6) (Y_0.96Yb_0.03-Er_0.01)₂O₂S (a = 3.780 Å, c = 6.580 Å). The observed chemical transformations were accompanied by a transition from powder particles 2–25 μm in size to agglomerates 30–50 μm in size. The upconversion luminescence intensity in the samples was found to have a maximum: 400 (1), 3750 (2), 2100 (3), 1100 (4), and 150 arb. units (5). The presence of (Y_0.96Yb_0.03Er_0.01)₂O₃ and (Y_0.96Yb_0.03Er_0.01)₂S₃ as impurity phases in the samples was shown to considerably reduce the luminescence intensity.

Kinetic diagrams of Ln₂O₂SO₄ (Ln = La, Pr, Nd, Sm) systems reduction in a H₂ flow are plotted for the first time in temperature-duration of treatment coordinates in which there are five areas of phase states. The temperatures of formation are established for products of the Ln₂O₂SO₄ + 4H₂ = Ln₂O₂S + 4H₂O reaction in the temperature range of 880–900 K and products of the Ln₂O₂SO₄ + H₂ = Ln₂O₃ + SO₂+ H₂O reaction in the temperature range of 1090–1220 K. The ranges of the temperature of formation of the homo-geneous Ln₂O₂S phase were found to decrease: 880–1220, 900–1200, 900–1180, and 900–1090 K in the sequence La-Pr-Nd-Sm.