OXYGEN NONSTOICHIOMETRY AND CRYSTAL AND DEFECT STRUCTURE OF PRMNO3+Y AND NDMNO3+Y

Praseodymium and neodymium manganates LnMnO(3+y) have been prepared and characterized by powder X-ray and neutron diffraction together with thermogravimetric and chemical titration analysis. Both ternary oxides have an orthorhombically distorted perovskite-like structure (Pbnm space group) with unit cell parameters a = 5.4562(7), b = 5.5914(7), c = 7.672(1)Angstrom and a = 5.4116(9), b = 5.6928(9), c = 7.599(1) Angstrom for praseodymium and neodymium manganates, respectively. A small deficiency in manganese has been found together with an oxygen excess, which can likely be described by the cation disorder: Pr0.95Mn0.94O3 and Nd0.97Mn0.95O3. The thermodynamics of the oxygen exchange process have been described in terms of partial molar enthalpy and partial molar entropy of the oxygen dissolution in the lattice. (C) 1995 Academic Press, Inc.