Abstract

The composition LaNi_0.5Cu_0.5O_3−δ has been prepared by heat treating a polyacrylamide xerogel in flowing O₂. A perovskite type structure starts to crystallise at 700 °C and is achieved at 1000 °C. TGA data point to the formation of a rather strongly oxidised compound (δ ≤ 0.09). As the solid state reaction of a mixture of La₂O₃/NiO/CuO powder oxides never yields a perovskite, the part of the sol–gel route in this synthesis proves to be crucial. On the basis of X-ray and neutron diffraction results, the existence of a cupronickelate having a rhombohedral perovskite type structure is proved. Rather unexpectedly, an electron diffraction study of the as-prepared composition at 1000 °C reveals the presence of two sets of perovskite type cupronickelate microcrystals: a copper rich one with a fixed composition 67% Cu and a nickel rich one with a small range of compositions 59 ≤ Ni% ≤ 67. The latter microcrystals feature the rhombohedral LaNiO₃ perovskite whereas the former one, due to the partial presence of Cu(II), frequently shows an orthorhombic [2a_p√2 × 2a_p√2 × a_p] supercell. Thus, the composition LaNi_0.5Cu_0.5O_3−δ. has to be understood in terms of a nanoscale segregated pseudo solid solution formed of two lanthanum cupronickelates, enriched in copper and nickel, respectively.