Order-disorder in grossly non-stoichiometric SrFeO2.5

Configurational lattice energy techniques are used to investigate oxygen vacancy ordering and the order–disorder transition in SrFeO2.50. Vacancy disorder is shown to present many new challenges, largely due to the extensive relaxation in such grossly non-stoichiometric systems. With large supercells it is not feasible to optimise each individual configuration. Efficient methods for choosing a small number of representative configurations are discussed. Oxygen vacancy–vacancy interactions are considerable in SrFeO2.50 and lead to the formation of preferred local structural entities. While the low-temperature structure consists of an ordered arrangement of octahedra and tetrahedra, the disordered high-temperature structure may be described as a mixture of tetrahedra, square pyramids and octahedra. Fe atoms with coordination numbers lower than four are negligible. The assumption of an ideal solution of oxygen vacancies in such systems, commonly made in standard thermodynamic treatments, is questionable.

[1]  I. Todorov,et al.  Ab initio calculation of phase diagrams of ceramics and minerals , 2001 .

[2]  J. Alonso,et al.  Preparation and crystal structure of the deficient perovskite LaNiO2.5, solved from neutron powder diffraction data , 1995 .

[3]  S. Stølen,et al.  On the entropic contribution to the redox energetics of SrFeO3−δ , 2001 .

[4]  S. Hull,et al.  The crystal structures, microstructure and ionic conductivity of Ba2In2O5 and Ba(InxZr1-x)O3-x/2 , 2002 .

[5]  A. W. Overhauser,et al.  Theory of the Dielectric Constants of Alkali Halide Crystals , 1958 .

[6]  Neil L. Allan,et al.  FREE-ENERGY DERIVATIVES AND STRUCTURE OPTIMIZATION WITHIN QUASIHARMONIC LATTICE DYNAMICS , 1997 .

[7]  N. Allan,et al.  Ab initio calculation of phase diagrams of oxides , 2001 .

[8]  C. R. A. Catlow,et al.  Computer simulation of solids , 1982 .

[9]  S. Kachi,et al.  Refinement of the Crystal Structure of Dicalcium Ferrite, Ca2Fe2O5. , 1971 .

[10]  Hybrid Monte Carlo and lattice dynamics simulations: the enthalpy of mixing of binary oxides , 1998 .

[11]  Julian D. Gale,et al.  GULP: A computer program for the symmetry-adapted simulation of solids , 1997 .

[12]  N. Allan,et al.  Ionic solids at elevated temperatures and/or high pressures: lattice dynamics, molecular dynamics, Monte Carlo and ab initio studies , 2000 .

[13]  I. Todorov,et al.  Free energy of solid solutions and phase diagrams via quasiharmonic lattice dynamics , 2001 .

[14]  P. Hagenmuller,et al.  Structural transitions at high temperature in Sr2Fe2O5 , 1985 .

[15]  M. Schmidt,et al.  Crystal and Magnetic Structures of Sr2Fe2O5 at Elevated Temperature , 2001 .

[16]  B. Dabrowski,et al.  Evolution of Oxygen-Vacancy Ordered Crystal Structures in the Perovskite Series SrnFenO3n−1 (n=2, 4, 8, and ∞), and the Relationship to Electronic and Magnetic Properties , 2000 .

[17]  B. Raveau,et al.  La8−xSrxCu8O20: An oxygen-deficient perovskite built of CuO6, CuO5, and CuO4 polyhedra , 1988 .

[18]  S. Stølen,et al.  Redox energetics of perovskite-related oxides , 2002 .

[19]  Y. Takeda,et al.  Phase relation in the oxygen nonstoichiometric system, SrFeOx (2.5 ≤ x ≤ 3.0) , 1986 .

[20]  N. F. Mott,et al.  Conduction in polar crystals. I. Electrolytic conduction in solid salts , 1938 .

[21]  D. M. Smyth,et al.  Defects and transport of the brownmillerite oxides with high oxygen ion conductivity − Ba2In2O5 , 1995 .

[22]  V. Caignaert,et al.  Sr2Mn2O5, an oxygen-defect perovskite with Mn(III) in square pyramidal coordination , 1985 .