Observation of lanthanum- and rare-earth-site ordering in ${\mathit{T}}^{\mathrm{*}}$-phase ${\mathrm{La}}_{2\mathrm{-}\mathit{x}\mathrm{-}\mathit{y}}$${\mathit{R}}_{\mathit{x}}$${\mathrm{Sr}}_{\mathit{y}}$${\mathrm{CuO}}_4$ (R=Sm, Eu, Gd, Tb)

There are two inequivalent lanthanum and/or rare-earth (R) sites in the ${\mathit{T}}^{\mathrm{*}}$-phase ${\mathrm{La}}_{2\mathrm{-}\mathit{x}\mathrm{-}\mathit{y}}$ ${\mathit{R}}_{\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{y}}$ ${\mathrm{CuO}}_4$ structure. One site M is locally similar to that in the T structure (${\mathrm{K}}_2$ ${\mathrm{NiF}}_4$-type) and the other site (M’) is locally similar to that in the T’ structure (${\mathrm{Nd}}_2$ ${\mathrm{CuO}}_4$-type). By studying the x-ray-absorption near-edge structure and extended fine structure of both La and rare-earth elements in T-, T’-, and ${\mathit{T}}^{\mathrm{*}}$-phase materials, we find unambiguously that the La and R ions enter the two inequivalent sites in an ordered way in the ${\mathit{T}}^{\mathrm{*}}$-phase ${\mathrm{La}}_{0.9}$ ${\mathit{R}}_{0.9}$ ${\mathrm{Sr}}_{0.2}$ ${\mathrm{CuO}}_4$ (R is one of Sm, Eu, Gd, or Tb). The La and the rare-earth ions preferentially occupy the M and M’ sites, respectively.