Observation of lanthanum- and rare-earth-site ordering in T^*-phase La_2-x-yR_xSr_yCuO₄ (R=Sm, Eu, Gd, Tb)

There are two inequivalent lanthanum and/or rare-earth (R) sites in the T^*-phase La_2-x-yR_xSr_yCuO₄ structure. One site M is locally similar to that in the T structure (K₂NiF₄-type) and the other site (M’) is locally similar to that in the T’ structure (Nd₂CuO₄-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 T^*-phase materials, we find unambiguously that the La and R ions enter the two inequivalent sites in an ordered way in the T^*-phase La_0.9R_0.9Sr_0.2CuO₄ (R is one of Sm, Eu, Gd, or Tb). The La and the rare-earth ions preferentially occupy the M and M’ sites, respectively.