Abstract: The new families of superconducting compounds ''1201'' (Hg0.4Pr0.6Sr2CuO4+delta), ''1212'' (Hg0.4Pr0.6Sr2(Ca1-x-ySrxPry) Cu2O6+delta), and ''1222'' (Hg0.4Pr0.6Sr2(Pr1.7Sr0.3)(2)Cu2O8+delta) all show ordering between praseodymium and mercury in the (Hg,Pr)O-delta plane, reducing the symmetry from tetragonal to orthorhombic or even to monoclinic. In the 1201 compound the ordered superstructure can be described as O-I (a(I) approximate to 2a(p), b(I), approximate to a(p), c(I) approximate to c(1201)). In the 1212 compound two distinct superstructures are identified, O-I (a(I) approximate to 2a(p), b(II) approximate to a(p), c(I) approximate to c(1212)) and O-II (a(II) approximate to 2a(p), b(II) approximate to a(p), c(II) approximate to 2c(1212)). In some compounds, such as Hg0.4Pr0.6Sr2(Ca0.4Sr0.4Pr0.2) Cu2O6+delta, the Hg-Pr ordering is only present in microdomains, but due to an ordering between Sr layers and Ca layers, the c-axis doubles and one obtains a tetragonal superstructure T-III (a(III) approximate to a(p), c(III) approximate to 2c(1212)). In the 1222 compound, ordering in the (Pr,Hg)O-delta plane is very defective; a large number of translation defects actually reduce the symmetry to triclinic. (C) 1995 Academic Press, Inc.

Abstract: The relationships between incommensurability, charge ordering and magnetotransport properties in Nd0.5Ca0.5Mn1-xCrxO3 manganites have been studied by electron diffraction and lattice imaging versus temperature with 0.02 less than or equal to x less than or equal to 0.07. All compositions show an incommensurate superstructure over the whole temperature domain, despite the fact that they are ferromagnetic and conductive below 140 K, The q-vector (1/2 – delta)a* decreases with increasing temperature for all compositions x. For a given temperature q also decreases with x. Lattice images obtained at low temperature give a clear view of the characteristics of the incommensurate structure. They also provide a better understanding of the charge ordering process. The low-temperature form of the Cr-doped manganites is not a perfectly doubled cell [[2a(p)root 2 x 2a(p) x a(p)root 2]], but defects inducing a tripled cell occur pseudo-periodically. (C) 2000 Elsevier Science B.V. All rights reserved.

Abstract: Intercalation of lithium in the ribbon structure Li4Mo5O17 has been achieved, using both electrochemistry and soft chemistry. The ab initio structure determination of the ¡°Mo−O¡± framework of Li12Mo5O17 shows that the [Mo5O17]¡Þ ribbons keep the same arrangement of edge sharing MoO6 octahedra and the same orientation as in the parent structure but that a topotactic antidistortion of the ribbons appears, as a result of the larger size of Mo4+ in ¡°Li12¡± compared to Mo6+ in ¡°Li4¡±. On the basis of bond valence calculations, it is observed that 12 octahedral sites are available for Li+ in the new structure so that an ordered hypothetical rock salt type structure can be proposed for Li12Mo5O17. After the first Li insertion, a stable reversible capacity of 100 mA¡¤h/g is maintained after 20 cycles. A complete structural reversibility leading back to the ribbon type Li4Mo5O17 structure is obtained using a very low rate of C/100. The exploration of the Li mobility in those oxides shows that Li4Mo5O17 is a cationic conductor with ¦Ò = 10−3.5 S/cm at 500 ¡ãC and Ea = 0.35 eV.

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 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.

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].