Structural study of praseodymium oxalate halide trihydrates

The praseodymium oxalate halide trihydrates, PrC2O4X.3H2O, X = Cl and Br, crystallize in the monoclinic system with P21/n as the space group (Z = 4). The overall structure consists of layers parallel to the ac plane. The layers are formed by a network of two nearly perpendicular chains of Pr atoms linked by oxalate groups. The water molecules and halide ions reside between the layers and in the voids formed by the two Pr-oxalate chains. The main interaction between adjacent layers occurs by hydrogen bonding between water molecules and halide ions. The praseodymium atoms are coordinated to five oxygens from the two non-equivalent oxalate groups, to three oxygens from water molecules and to one halide. The coordination polyhedron is strongly distorted tricapped trigonal prism. One of the two oxalate groups acts as a hexadentate ligand while the other is only tetradentate. The thermal decomposition proceeds via anhydrous salt and corresponding oxyhalide PrOX to praseodymium oxide, Pr6O11. Several intermediate phases were observed but not identified with certainty. The relation between the thermal decomposition and the crystal structure was discussed