Une nouvelle famille structurale: Les oxydes Ln4-2 xBa 2+2 xZn 2- xO 10-2 x ( Ln = La, Nd)

Two original compounds, Ln_{4-2 x}Ba _{2+2 x}Zn _{2- x}O _{10-2 x}, were isolated for Ln = La, Nd and 0 ≤ x ≤ 0.25. These oxides are tetragonal with a and c parameters close to 6.91 and 11.59 Å, respectively, for lanthanum, and 6.75 and 11.54 Å for neodymium. The structure of these phases was determined from X-ray powder patterns in the most symmetric space group, {I4 }/{mcm}, using Patterson and Fourier functions for x = 0. The structure should be compared to that of copper oxides La _{4-2 x}Ba _{2+2 x}Cu _{2- x}O _{10-2 x}: it is built up of identical Ln₂O ₅ layers formed from face- and edge-sharing LnO ₈ polyhedra, between which Ba ²⁺ and Zn ²⁺ ions are inserted. Contrary to the copper compounds, two successive Ln₂O ₅ layers are rotated by 90°, involving a doubling of c. The result for Zn ²⁺ is tetrahedral coordination, while the coordination of Ba ²⁺ becomes a bicapped antiprism.