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) analysis and X-ray absorption near-edge spectroscopy (XANES) near the CaK edge, and via atomic- and electronic-structure modeling. Calculations and experiments show that the Ca atom, in a manner similar to that of Sr and Ba atoms, 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 neighbors are similar to the size of the La³⁺ cation. The distances that are obtained from the simulation are in reasonable agreement with the experimental results for calcium, as well as previous studies that involved strontium and barium. The binding of a calcium dopant to vacancies on the O(1) sites is stable, but only slightly; therefore, there are probably many different types of such clusters. This calculated binding also is supported by the EXAFS data. The overall electronic structure is similar to that observed in calculations for pure La₂CuO₄: the LaO planes remain ionic, even with the presence 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 approx. 6 eV above the Fermi energy level.