Abstract
Two structures, all consisting of alternative stacking of hexagonal perovskite layer and graphite-like Ca 2O layer, were identified in Ln 2Ca 2MnO 7 systems ( Ln=La, Nd and Sm). La 2Ca 2MnO 7 (1), crystallizing in the space group R 3 ̄ m with the lattice constants a=5.62231(7) Å and c=17.3192(4) Å, contains almost ideal close packed [ LnO 3] arrays. While for the smaller rare earth cations, e.g., Nd 2Ca 2MnO 7 (2) and Sm 2Ca 2MnO 7 (3), the structure distorts to large unit cell ( R 3 ̄ , a′=2 a and c′= c). Study of the substituted systems, LnLn′Ca 2MnO 7 ( Ln or Ln′=La, Ce, Pr, Nd, Sm, Eu, Gd) and La 2− x Sm x Ca 2MnO 7, shows a phase transformation from (1) to (2) at certain value of cation size. The MnO 6 octahedra in these compounds are isolated, thus the magnetic property is mainly paramagnetic.
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