Complex aluminates RE_2SrAl_2O_7 (RE = La, Nd, Sm-Ho): Cation ordering and stability of the double perovskite slab-rocksalt layer P_2/RS intergrowth

摘要

The series of complex aluminates RE_2SrAl_2O_7 (RE = La, Nd, Sm, Eu, Gd, Tb, Dy, Ho) was prepared at 1450 deg C. Their structure is the double perovskite slab-rocksalt layer P_2/RS intergrowth of the n = 2 Ruddelsden-Popper phases. X-ray Rietveld refinement calculations point to a progressive RE-Sr ordering between the ninefold sites in the RS layers and the twelvefold on in the P_2 slabs, respectively. This ordering proves both to improve the ease to prepare these aluminates and stabilize their intergrowth structure. The limit observed for RE=Ho corresponds to a nearly fully ordered distribution of the Ho~(3+) and Sr~(2+) cations -91 percent. Due to an excess structure-bonding anisotropy in the ninefold sites, a further extent to smaller RE~(3+) cations is unsuccessful. These results well compare with those regarding the magnetoresistant manganites RE_(1+x)Sr_(2-x)Mn_2O_7: increasing RE-Sr ordering and limit found for Er. The big size A element long range ordering phenomena which are unlikely to depend on redox phenomena of the B element in the P_2 slabs, have to be considered as specific trends of the P_2/RS intergrowth type complex oxides.