Defect and electronic structures of calcium-doped lanthanum cuprate

Quenched calcium-doped lanthanum cuprate (La2CuO4) samples were examined at room temperature using extended X-ray absorption fine structure (EXAFS) an alysis and X-ray absorption near-edge spectroscopy (XANES) near the CaK edg e, and via atomic- and electronic-structure modeling, Calculations and expe riments show that the Ca atom, in a manner similar to that of Sr and Ba ato ms, replaces the La atom and maintains its nine-fold coordination above the center of a CuO4 square, which is the nominal position of the La atom. In contrast to the Sr and Ba atoms, the distances from Ca to its nearest neigh bors are similar to the size of the La3+ cation, The distances that are obt ained from the simulation are in reasonable agreement with the experimental results for calcium, as well as previous studies that involved strontium a nd barium. The binding of a calcium dopant to vacancies on the O(1) sites i s stable, but only slightly; therefore, there are probably many different t ypes of such clusters. This calculated binding also is supported by the EXA FS data. The overall electronic structure is similar to that observed in ca lculations for pure La2CuO4: the LaO planes remain ionic, even with the pre sence of the calcium dopant, and the Cu-O(1) bond is covalent. The calcium is ionic in character, with a net charge of 1.6, and the narrow Ca 3d band lies similar to 6 eV above the Fermi energy level.