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 C 2/ 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.