Structural refinement of RE(2)ACu(2)O(6) from powder X-ray diffraction data (RE = La, Nd, A = Sr, Ca)

Crystal structures of La2SrCu2O6, Nd1.8Sr1.2Cu2O6, LaNdSrCu2O6 and La2CaCu2 O6 are refined from powder X-ray diffraction data. The size effect plays a dominant role in controlling the distribution of the RE3+ and A(2+) ions in the lattices. Small ions Nd3+ and Ca2+ show strong preference for occupyin g the 2a site located between two CuO2 layers, while large ions Sr2+ and La 3+ prefer the 4e site in rock salt layers. When RE3+ ions occupy the 2a sit e, extra oxygen may transfer to the 2b site at the apical position to keep the local charge equilibrium, and superconductivity of the compounds is det eriorated. Resistance data for samples annealed in nitrogen and oxygen atmo sphere agree well with this structural model. These results indicate that L a2CaCu2O6 is the best superconductor candidate among the 0212 compounds stu died. (C) 1999 Elsevier Science B.V. All rights reserved.