STRUCTURAL PECULIARITIES OF 2 LAYERED CUPRATES, NDSRCUO3.5 AND ND1.8SR1.2CU2O6-DELTA

The structures of the layered copper oxides NdSrCuO3.5 and Nd1.8Sr1.2Cu2O6-δ are solved from single crystal X-ray diffraction. The analysis of the results compared to those of analogous compounds (ACuO3-x)m(A′O)n (n = 1, m = 1,2) involving La shows several important structural features. In the rock-salt layers [Nd1-xSrxO]∞, the [Nd1-xSrx]∞ layers are systematically shifted out of the plane of the oxygen layer along c. In NdSrCuO3.5 partial site vacancies are detected on the two oxygen sites, whereas in Nd1.8Sr1.2Cu2O6-δ no other oxygen vacancy is observed except the empty site located between two neighboring Cu layers. A short apical distance AO2 between an oxygen and a metallic site (Nd, Sr) is a pecularity of the two structures and also of the similar La oxides. This distance appears as an invariant over all these intergrowths whatever the size of A, the inserted cation. Another remarkable feature deals with the influence of the size of the A cations upon the CuO2 apical distance of the CuO6 octahedra in La2CuO4-type oxides: the replacement of cations like lanthanum or strontium by smaller ions like neodymium in the rock salt layer, leads to a significant compression of the CuO6 octahedra along c. © 1991.