The Pr2Se3-PrSe2 system: Studies of the phase relationships and the modulated crystal structure of PrSe1.85

Thermochemical investigations have been carried out to elucidate the phase relationships in the system Pr₂Se₃-PrSe₂ and to construct the p_Se-T-x diagram. The result of the tensimetric and thermogravimetric studies showed the thermal decomposition of PrSe₂ to be a step-wise process due to the formation of a sequence of discrete intermediate phases with the compositions PrSe_1.9, PrSe_1.85, PrSe_1.8, and Pr₂Se₃. PrSe_1.85 and PrSe_1.8 have been previously considered as selenium-deficient non-stoichiometric phases. Conclusions derived from the thermodynamic study were verified by X-ray diffraction and electron microscopy studies on single crystals of PrSe_1.85 which were obtained from reactions of praseodymium and selenium in stoichiometric amounts in a KCl flux at 1070 K. A two-dimensional modulation could be observed for PrSe_1.85 in X-ray and high-resolution transmission electron microscopy. The structure of PrSe_1.85 was solved and refined in superspace group P4/n()(β-α)00 with lattice parameters of a=4.137(1) Å, c=8.398(2) Å of the basic unit cell and α=β=0.293(1). The origin of the modulation can be attributed to a site occupancy wave and a charge density wave in its planar selenium layer. The experimentally determined magnetic moments indicate Pr³⁺ for the polyselenides PrSe₂, PrSe_1.9 and PrSe_1.85.