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 pSe–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 PrSe1.9, PrSe1.85, PrSe1.8, and Pr2Se3. PrSe1.85 and PrSe1.8 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 PrSe1.85 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 PrSe1.85 in X-ray and high-resolution transmission electron microscopy. The structure of PrSe1.85 was solved and refined in superspace group P4/n(image)(β–αimage)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, PrSe1.9 and PrSe1.85.