CRYSTAL STRUCTURES AND THERMAL STABILITY OF LNHAL(C₂O₄)*NH₂O.

Sergey D.Kirik, Gennady T.Korolev , Leonid A. Solovyov, Mikhail Ya. Nikulin.

Institute of Chemistry and Chemical Technology, 660036,Krasnoyarsk, Russia, Academy of non-ferrous metals and gold, 660025,Krasnoyarsk, Russia

Keywords: structure determination, X-ray powder diffraction, halogen oxalates of rare earth metals.

A number of fluoride oxalates of some rare earth metals with common formula of LnF(C₂O₄)*4H2O, where Ln= Dy, Ho, Er, Tm, Yb, Lu, as well as chloride oxalates LnCl(C₂O₄)*3H₂O, Ln=La, Ce, Nd, Sm, and bromide oxalates LnBr(C₂O₄)*3H₂O, Ln=La,Ce. were prepared using original methods and investigated. They comprise two groups of isostructural compounds.

Crystal structures were ab initio determined from X-ray powder data. Structural investigation procedure included: lattice cell parameter determination, whole powder pattern profile fitting, direct and Patterson's methods and Rietveld refinement at the final stage of determination. Fluoride oxalates were investigated on the example of TmF(C₂O₄)*4H₂O (a=12.637 A, b=11.129 A, c=8.117 A, b=132.98^o, Z=4, S.G. C2/c, R_wp=0.126, R_p=0.088, R_Bragg=0.058, S=2.87), chloride oxalates - LaCl(C₂O₄)*3H₂O (a=5.892A, b=16.222A, c=9.098A, b=114.99^o, Z=4, S.G.. P2₁/c, R_wp=0.098, R_p=0.073, R_Bragg=0.031, S=1.32), and bromide oxalates - LaBr(C₂O₄)*3H₂O (a= 5.950 A, b=16.341 A, c=9.146 A, b=114.85^o, Z=4, S.G. P2₁/c, R_wp=0.070, R_p=0.054, R_Bragg=0.058, S=1.89).

The common feature of the discussed structures is the presence of infinite zigzag chains of ...Ln(C₂O₄)Ln(C₂O₄)…type. In the case of fluoride the chains are linked through bridged fluoride atoms. In the chlorides and bromides the chains are brought together so that oxygen atoms of oxalate groups co-ordinate metal atoms from neighbour chains. In the fluorides an irregular eight top polyhedron reminds octahedron with two adjacent split tops. It consists of four oxygen atoms of oxalates groups, two fluorides and two molecules of co-ordinating water. Two other molecules of water are situated in a hole formed by the zigzag chains. There is also irregular nine tops polyhedron around metal atoms in chlorides and bromides. It presents a combination of the pentagonal pyramid and trigonal prism. The metal atom is on the basis of the pyramid and the halogen is in the top. Three other co-ordinated atoms comprise a prism beneath the basis of the pyramid. The halogen atoms are directed out of the plain where the chain lays. All the molecules of water are co-ordinated to metal atoms.

Differences in the structures are responsible for different ways of decomposition. Fluorides have separate stages for detachment of every water molecule. At the final stage of the decomposition the substance looses the oxalate group giving LnOF. The chlorides and bromides output 2.5 H₂O in one stage and then the stages of water and oxalate detachment follow overlapping each other. The final product of decomposition is LnHalO.