The structural characteristics of Sr1−x Lax TiO3+δ (0≤x≤0.4) at 1400 °C have been investigated as a function of ambient oxygen partial pressure. A modified Rietveld pattern-fitting structure-refinement program [H. M. Rietveld, J. Appl. Crystallogr. 2, 65 (1969)] was used to determine the nature of the distortions of the fundamental perovskite unit cell, the degree of lattice perfection, and the cation vacancy concentrations. Specimens equilibrated in forming gas displayed a linear relation between x, the lattice parameters, and the degree of lattice perfection while those samples annealed in air and oxygen deviated significantly from linearity. A monoclinic distortion of the perovskite structure was seen in the samples at low oxygen partial pressures while a second phase or layer type of distortion appeared in samples with x>0.2 under oxidizing conditions.
The use of the profile shape function based on the direct convolution product described in paper I [Howard & Snyder (1989). J. Appl. Cryst. 22,[238][239][240][241][242][243] in profile and Rietveld pattern fitting algorithms has been investigated further. These algorithms aided in the fabrication of a better instrument-profile standard and in determining the crystallite size and strain in a series of specimens. Analysis of an A1 specimen using a profile fitting algorithm yielded its strain and the strain recovery rate as a function of annealing time and temperature. The results from a modified Rietveld program showed the strain in La-substituted SrTiO3 specimens to be a function of the La content and the oxygen partial pressure in which the specimens were annealed. In all cases, the specimen's contributions to line broadening as a result of both small crystallite size and strain in this study were similar to the results obtained in paper I, i.e. they tended to be Lorentzian in nature.
The La(Crl-,Mn,)03 (x = 0 to 0.25) system exhibited a displacive phase transformation from an orthorhombic to a rhombohedral structure at temperatures ranging from about 256°C at x = 0 to 305°C at x = 0.25. The Rietveld method of X-ray analysis was used to determine structural characteristics and to assay phase concentrations when multiple phases were present. Indications were that the Mn ions on the B site affected the transition temperature through an ionic size effect; i.e., Mn3+, with a larger ionic size than Cr3+, increased the transition temperature while the presence of the smaller Mn4+ ions reduced it. A hysteresis was also observed with respect to the phase transformation as a function of temperature. The phase transformation was associated with a change in the rotation of the oxygen octahedra from an antiphase to an in-phase tilt sequence. [Key words: phase transformation, lanthanum oxide, X-ray diffraction, chromium, manganese.] Stufe Chem., in press. "K. J. Rao and C. N. R. Rao, "Crystal Structure Transformations of Alkali Sulphates, Nitrates, and Related Substance: Thermal Hysteresis in Re-0 versible Transformation," J. Muter. Sci., 1, 238 (1966).
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