Exploration of the Crystal Structure and Thermal and Spectroscopic Properties of Monoclinic Praseodymium Sulfate Pr(2)(SO(4))(3)

Praseodymium sulfate was obtained by the precipitation method and the crystal structure was determined by Rietveld analysis. Pr(2)(SO(4))(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) Å(3) (T = 150 °C). The thermal expansion of Pr(2)(SO(4))(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, Pr(2)(SO(4))(3) is stable in the temperature range of T = 30–870 °C. The kinetics of the thermal decomposition process of praseodymium sulfate octahydrate Pr(2)(SO(4))(3)·8H(2)O was studied as well. The vibrational properties of Pr(2)(SO(4))(3) were examined by Raman and Fourier-transform infrared absorption spectroscopy methods. The band gap structure of Pr(2)(SO(4))(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 Pr(3+) 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 Pr(2)(SO(4))(3) belongs to the (3)P(0) → (3)F(2) transition at 640 nm.