Structure and thermal behaviour of lanthanum aluminium nitrates

Abstract

The structure of the new phase LaAl(NO3)6.12H2O has been determined from single-crystal diffraction data. The symmetry is trigonal [a=10.948(1) Å, c=16.802(2) Å, S.G. R3] and the structure consists of discrete lanthanum and aluminium polyhedra. La atoms are surrounded by six bidentate nitrate groups in the form of an icosahedron and Al atoms are octahedrally coordinated to six water molecules. The remaining H2O molecules are located in the holes of the structure, whose cohesion is ensured by hydrogen bonds. The thermal decomposition gives first the cubic phase LaAl(NO3)6.6H2O [a=12.301(1) Å, S.G. P213], whose structure has been solved from single-crystal diffraction data. The structure is also built from La(NO3)63− anions and Al(H2O)63+ cations. The final decomposition product is LaAlO3, which crystallises from an amorphous state at about 450°C. A study of the microstructure of LaAlO3 from diffraction line broadening shows the presence of a significant amount of microstrains at 900°C.