Abstract: Thr structural, electronic, and magnetic properties of the highly mismatched perovskite oxides, Th(0.35)A(0.65)MnO(3), where Ais for the alkaline earth divalent cations (Ca, Ba, Sr), which are all characterized by the same large tolerance factor (t=0.934), have been investigated by using electron microscopy, electrical resistivity, magnetic susceptibility, and magnetization. It is clearly established that a transition from ferromagnetic metallic towards spin-glass insulator samples is induced as the A-site cationic size mismatch is increased. Moreover, the magnetoresistance (MR) properties of these manganites are strongly reduced for the spin-glass insulators, demonstrating that the A-sire cationic disorder is detrimental for the colossal MR properties. Based on these results, a new electronic and magnetic diagram is established that shows that the A-site disorder, rather than the A-site average cationic size (or t) is the relevant factor for generating spin-glass insulating manganites. [S0163-1829(99)01746-4].

Abstract: For the composition (Sr0.61Pb0.18)(Fe0.75Mn0.25)O2.29, a new modulated crystallographic shear structure, related to perovskite, has been synthesized and structurally characterized by transmission electron microscopy. The structure can be described using a monoclinic supercell with cell parameters am = 27.595(2) Å, bm = 3.8786(2) Å, cm = 13.3453(9) Å, and βm = 100.126(5)°, refined from powder X-ray diffraction data. The incommensurate crystallographic shear phases require an alternative approach using the superspace formalism. This allows a unified description of the incommensurate phases from a monoclinically distorted perovskite unit cell and a modulation wave vector. The structure deduced from the high-resolution transmission electron microscopy and high-angle annular dark-field−scanning transmission electron microscopy images is that of a 1/2[110]p(203)p crystallographic shear structure. The structure follows the concept of a phasoid, with two coexisting variants with the same unit cell. The difference is situated at the translational interface, with the local formation of double (phase 2) or single (phase 1) tunnels, where the Pb cations are likely located.

Abstract: The exploration of the Mn4+-rich side of the Pr1-xSrxMnO3 system has allowed the extension of the domain of the cubic perovskite, by using a two-step process, combining synthesis under Ar flow at high temperature and O-2 pressure annealing at lower temperature. We show that these Pr-doped cubic perovskites exhibit a coupled structural (cubic-tetragonal) and magnetic (para-antiferro) transition connected with a resistivity jump at the same temperature. The strong interplay between lattice, charges, and spins for these oxides results from the appearance at low temperature of the distorted C-type antiferromagnetic structure. The Pr1-xSrxMnO3 magnetic phase diagram shows, for 0.9 less than or equal to x less than or equal to 1 (i.e., on the Mn4+-rich side), the existence at low temperature of C- and G-type antiferromagnetism. The absence of ferromagnetic-antiferromagnetic competition explains that magnetoresistante properties are not observed in this system, in contrast to Mn4+-rich Ln(1-x)Ca(x)MnO(3) systems.

Abstract: A new Tl-doped strontium ferrite Fe2(Sr2-Tl)Sr3Fe4O14.65, with an original structure, has been synthesized and structurally characterized by powder X-ray diffraction and transmission electron microscopy. The TGA and Mssbauer studies evidence a mixed valence of iron. The structure exhibits a commensurate modulation, with a F-type subcell a ≈ b ≈ 5.4 Å (≈ ap√2), c ≈ 42 Å with a modulation vector q = αa* with α = 0.4. The supercell parameters have been refined as a= 27.1101(8) Å, b= 5.5187(2) Å and c= 42.0513(9) Å, in the space group Fmmm. The electron diffraction and electron microscopy data of this novel ferrite show that it can be described as a FeTl-2234-type structure corresponding to the intergrowth of a quadruple perovskite slice [(SrFeO2.8)4], with a complex rock salt related slice [Fe2(Sr2-Tl)O3.4]∞, built up of one double iron layer [Fe2O2.4] sandwiched between two [SrO] layers. The HRTEM images show that the oxygen atoms and vacancies are randomly distributed in the perovskite layers while the HAADF STEM images evidence the absence of Tl segregation in the matrix. Fe2(Sr2-Tl)Sr3Fe4O14.65 exhibits a very large value of χ (11emu/mol) at 5 K, which remains large at 400 K; the M(H) loop presents a shape characteristic of ferrimagnetism, with a large coercive field of 0.3 T. The value of magnetization saturates at 400 K at 0.68 μB/Fe. At 10 K, the value of magnetization reaches a maximum of 2 μB/Fe. The resistivity presents a semiconducting-like behavior, with ρ 800 Ω·cm at 300 K.

Abstract: The origin of the incommensurability in the crystallographic shear (CS) structure of the ferri-Manganite (Sr0.61Pb0.18)(Fe0.75Mn0.25)O2.29, related to the cation deficient perovskite, has been determined by careful analysis of the boundaries between the two variants constituting the phasoid. High Resolution Electron Microscopy/HAADF-STEM images allow the structural mechanisms to be understood through the presence of structural units common to both phases, responsible of the incommensurate character observed in the electron diffraction patterns. The structural analysis allows for identifying different types of CS phases in the Pb−Sr−Fe(Mn)−O diagram and shows that the stabilization of the six-sided tunnels requires a higher A/B cationic ratio. A description of these phases is proposed through simple structural building units (SBU), based on chains of octahedra bordered by two pyramids. The (Sr0.61Pb0.18)(Fe0.75Mn0.25)O2.29 CS compound exhibits a strong antiferromagnetic and insulating behavior, similar to the Fe-2201 and terrace ferrites but differs by the presence of a hysteresis, with a small coercive field.

Abstract: A combination of CeO2 and PtO2 doping has been studied in melt-processed YBa2Cu3O7-x. This study was carried out using an optimized well established MTG process. The cerium-platinum doped samples exhibit a high fishtail effect with a J(c) of 4.3 x 10(4) A/cm(2) under an applied field of 1 T. Microstructural and nanostructural studies have been performed. The 211 and BaCeO3 inclusions are a few micrometers wide and the formation of a metastable nanocrystalline phase (Y4Ba4)Cu-8-x(Ce,Pt)(x)O-20+delta related to the perovskite is detected. The 123 grains show no specific extended defect generated by the doping process. The relationship between structure and properties are discussed.

Abstract: A new superconducting mercury oxycarbonate, Pb0.7Hg0.3Sr4Cu2CO3O7, has been synthesized. This tetragonal phase (a = 3.824 angstrom, c= 16.468 angstrom) consists of an intergrowth of two nonsuperconducting compounds, Sr2CuO2CO3 and Pb0.7Hg0.3Sr2CuO5. It exhibits after optimization a critical temperature of 70 K, with a sharp transition and a superconducting volume fraction of 50%. Its behavior can be compared to that of thallium oxycarbonates previously isolated. This study is completed by a reinvestigation of the 1212 cuprate of the system Hg-Pb-Sr-Ca-Nd-Cu. A superconducting phase with the 1212 structure, similar to that previously obtained but with a significantly different composition, Pb0.7Hg0.3Sr2+xCa0.7Nd0.3-xCu2O7, has been obtained, with a T(c onset) of 100 K. The behavior of the latter is compared with other lead-based 1212 cuprates.