TEM study of the electron-doped layered La2-2xCa1+2xMn2O7:: Orthorhombic phase in the 0.8<x<1.0 composition range

A series of Ruddlesden-Popper compounds with n = 2 and stoichiometry La2-2xCa1+2xMn2O7 were synthesized by a citrate gel technique and studied by TEM for the composition range 0.8 < x < 1.0. In situ experiments and analysis of substructure/twins identified structural changes of the 14/mmm phase at low temperatures (200-350 degreesC), Reciprocal space analysis identified superlattice reflections (not previously detected by X ray) and possible space groups. The low-temperature phase belongs to the noncentrosymmetric Cmc2(1) or Ama2 space group, and the transient Cmcm space group is also possible. These structural transitions do not coincide with the magnetic transitions, which occur below room temperature. A structural model based on tilting of almost-rigid octahedra is suggested for Cmcm and Cmc2(1). The proposed model for the low-temperature Cmc2(1) phase has a Phi Phi Psi (z)/Phi Phi Psi (z) combination of tilts, while the transient Cmcm has a Phi Phi0/Phi Phi0 combination of tilts. (C) 2001 Academic Press