LANTHANUM L3-, EUROPIUM L3- AND COPPER K-EDGE EXAFS INVESTIGATION OF LANTHANUM COPPER OXIDES

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 co-ordination 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 La1.85Sr0.15CuO4 and La2CuO4.13 is similar to that in La2CuO4. The results obtained from La2CuO4.4 formed by treatment of La2CuO3.6 in oxygen in a thermal analyser are consistent with the formation of an Eu2CuO4-type structure.