Structural studies of rare earth/transition metal complex ion systems as a basis for understanding their thermal decomposition to mixed oxides

Thermal decomposition in air of the series of complexes [Co(NH3)(6)][Ln(dipic)(3)]center dot xH(2)O, [Cr(en)(3)][Ln(dipic)(3)]center dot yH(2)O and [Cr(urea)(6)][Ln(dipic)(3)]-zH(2)O (Ln = La - Lu, excluding Pm but plus Y; dipic = 2,6-pyridinedicarboxylate) to give oxide phases has been examined. The products were characterised by powder X-ray diffraction, magnetic and surface area measurements. From the Co-containing species, either Co3O4 + Ln(2)O(3) or perovskite (CoLnO(3)) products result, while the Cr-containing species provide CrLnO(4) phases in addition, the monazite form of CrNdO4 being detected for the first time in this way. Underpinning consideration of the solid state decomposition mechanisms, single-crystal X-ray structure determinations have been performed on [Co(NH3)(6)][Ln(dipic)(3)] nH(2)O across the Ln series, defining differing phases (all triclinic P (1) over bar) for n = 5 (encompassing Ln = La-Pr), n = 8(.5) (Ln = Tm - Eu) and n = 10 (Ln = Pr - Er, including Y), and also on one of the low-temperature decomposition products, [Co(NH3)(6)]Cl-dipic center dot 2H(2)O (1). The structure of 1 provides evidence that conventional pi-stacking of the dipicolinate anions may be of some importance in determining the solid state array, whereas consideration of all structures involving [Ln(dipic)(3)](3-) anions indicates that "edge-to-face" interactions between dipicolinate ligands of adjacent complex anions may also be determinants of the formation of sheetlike arrays of the trianions commonly observed in these compounds. ((c) Wiley-VCH Verlag GmbH T Co. KGaA, 69451 Weinheim, Germany, 2005).