Effect of non-stoichiometry on properties of La1-tMnO3+delta .2. Crystal structure

Crystal structure data at 298 K were deduced from high-resolution powder neutron diffraction measurements of seven samples within the homogeneity range of the grossly non-stoichiometric La1-tMnO3+delta phase. During simultaneous refinements of powder X-ray (synchrotron) and neutron diffraction data for La0.96MnO3.05 several simple and complex defect models were tested out. There is no significant evidence for a more complex model than random vacancies at the La and Mn sites. The refined composition La0.979MnO3.057 proves the existence of unequal amounts of vacancies at the La and Mn sites. The fully reduced La1-xMnO3-3t/2 samples contain solely Mn-III. The degree of Jahn-Teller distortion for the reduced samples having an orthorhombic structure (ORT1-type; space group Pnma) is independent of the concentration of La and O vacancies. The more oxidized samples with 10-30% Mn-IV take either the GdFeO3 (Pnma) or the LaAlO3-type (R-3c) structure, both with regular MnO6 octahedra. The valency of Mn is discussed on the basis of bond strength considerations.