Lanthanum L3-, europium L3- and copper K-edge EXAFS investigation of lanthanum copper oxides

Abstract

Some cation-substituted and oxygen-rich compounds derived from the lanthanum copper oxide of composition La₂CuO₄ have been examined by measurement of the copper K-, lanthanum L₃- and europium L₃-edge extended X-ray absorption fine structure. The results obtained from La₂CuO₄ are in good agreement with previously reported data and are consistent with the six-fold oxygen co-ordination of copper and nine-fold coordination by oxygen of lanthanum. The results obtained from the compound Eu₂CuO₄ show co-ordination numbers of four and eight for copper and europium as would be expected for a structure containing copper in a square-planar oxygen environment and europium in square antiprismatic oxygen co-ordination.

The compound La₂CaCu₂O₆ has a structure in which copper adopts a square-pyramidal oxygen environment whilst the lanthanum atoms are found to be in nine-fold oxygen co-ordination. The oxygen co-ordination of lanthanum and copper in the high-temperature superconductors La_1.85Sr_0.15CuO₄ and La₂CuO_4.13 is similar to that in La₂CuO₄. The results obtained from La₂CuO_4.4 formed by treatment of La₂CuO_3.6 in oxygen in a thermal analyser are consistent with the formation of an Eu₂CuO₄-type structure.