Sol-gel synthesis and structural characterisation of the perovskite type pseudo solid solution LaNi0.5Cu0.5O3

The composition LaNi0.5Cu0.5O3-delta has been prepared by heat treating a polyacrylamide xerogel in flowing O-2. A perovskite type structure starts to crystallise at 700 degreesC and is achieved at 1000 degreesC. TGA data point to the formation of a rather strongly oxidised compound (delta less than or equal to 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 degreesC 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 less than or equal to Ni% less than or equal to 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)root2 x 2a(p)root2 x a(p)] supercell. Thus, the composition LaNi0.5Cu0.5O3-delta has to be understood in terms of a nanoscale segregated pseudo solid solution formed of two lanthanum cupronickelates, enriched in copper and nickel, respectively.