X-ray-absorption studies of cation ordering and valence in the T^*-phase La_2-x-yR_xSr_yCuO₄ (R=Sm, Eu, Gd, Tb, and Dy)

X-ray-absorption measurements have been made at the lanthanide L edges, Sr and Cu K edges in the T^*-phase La_2-x-yR_xSr_yCuO₄ compounds with R=Sm, Eu, Gd, Tb, and Dy. The La and rare-earth x-ray-absorption near-edge (XANES) data and the extended x-ray-absorption fine-structure data both clearly indicate that the R and La ions selectively enter the M’ site and M site, respectively. The degree of this site ordering is nearly perfect and is similar for the Sm, Eu, Gd, Tb, and Dy members of this family. The L₃ XANES data show that the La, Sm, Eu, Gd, and Dy are trivalent in these compounds, but about 20% of the Tb ions are tetravalent in the Tb material. The Cu K-edge XANES spectra are consistent with a divalent state for copper, and are essentially identical for the as-prepared nonsuperconducting samples and the high-pressure oxygenated superconducting counterparts. This indirectly suggests that the hole carrier in the T^*-phase superconductors is largely associated with oxygen atoms as in La_1.85Sr_0.15CuO₄. Based on the results of cation-ordering, cation-valence, and oxygen-content measurements, the superconducting behavior of Sm, Em, and Gd T^* phases and the lack of superconductivity in the Tb and Dy compounds are discussed.