Correlations of structural, magnetic, and dielectric properties of undoped and doped CaCu3Ti4O12

The present work reports synthesis, as well as a detailed and careful characterization of structural, magnetic, and dielectric properties of differently tempered undoped and doped CaCu3Ti4O12 (CCTO) ceramics. For this purpose, neutron and x-ray powder diffraction, SQUID measurements, and dielectric spectroscopy have been performed. Mn-, Fe-, and Ni-doped CCTO ceramics were investigated in great detail to document the influence of low-level doping with 3d metals on the antiferromagnetic structure and dielectric properties. In the light of possible magnetoelectric coupling in these doped ceramics, the dielectric measurements were also carried out in external magnetic fields up to 7 T, showing a minor but significant dependence of the dielectric constant on the applied magnetic field. Undoped CCTO is well-known for its colossal dielectric constant in a broad frequency and temperature range. With the present extended characterization of doped as well as undoped CCTO, we want to address the question why doping with only 1% Mn or 0.5% Fe decreases the room-temperature dielectric constant of CCTO by a factor of ~100 with a concomitant reduction of the conductivity, whereas 0.5% Ni doping changes the dielectric properties only slightly. In addition, diffraction experiments and magnetic investigations were undertaken to check for possible correlations of the magnitude of the colossal dielectric constants with structural details or with magnetic properties like the magnetic ordering, the Curie-Weiss temperatures, or the paramagnetic moment. It is revealed, that while the magnetic ordering temperature and the effective moment of all investigated CCTO ceramics are rather similar, there is a dramatic influence of doping and tempering time on the Curie-Weiss constant.Comment: 10 pages, 11 figure

Local structure study about Co in YBa2_2(Cu1−x_{1-x}Cox_x)3_3O7−δ_{7-\delta} thin films using polarized XAFS

We have studied the local structure around Co in YBa$_2$(Cu$_{1-x}$Co$_x$)$_3$O$_{7-\delta}$ thin films with three different concentrations: x=0.07, 0.10, 0.17, and in a PrBa$_2$(Cu$_{1-x}$Co$_x$)$_3$O$_{7-\delta}$ thin film of concentration x=0.05 using the X-ray Absorption Fine Structure (XAFS) technique. Data were collected at the Co $K$-edge with polarizations both parallel and perpendicular to the film surface. We find that the oxygen neighbors are well ordered and shortened in comparison with YBCO Cu-O values to 1.80 \AA{} and 1.87 \AA{} in the $c$-axis and $ab$-plane, respectively. A comparison of further neighbors in the thin film and powder data show that these peaks in the film are suppressed in amplitude relative to the powder samples, which suggests there is more disorder and/or distortions of the Co environment present in the thin films.Comment: 14 pages; To be submitted to Phys. Rev.

The Crystal Structure and RNA-Binding of an Orthomyxovirus Nucleoprotein

Genome packaging for viruses with segmented genomes is often a complex problem. This is particularly true for influenza viruses and other orthomyxoviruses, whose genome consists of multiple negative-sense RNAs encapsidated as ribonucleoprotein (RNP) complexes. To better understand the structural features of orthomyxovirus RNPs that allow them to be packaged, we determined the crystal structure of the nucleoprotein (NP) of a fish orthomyxovirus, the infectious salmon anemia virus (ISAV) (genus Isavirus). As the major protein component of the RNPs, ISAV-NP possesses a bi-lobular structure similar to the influenza virus NP. Because both RNA-free and RNA-bound ISAV NP forms stable dimers in solution, we were able to measure the NP RNA binding affinity as well as the stoichiometry using recombinant proteins and synthetic oligos. Our RNA binding analysis revealed that each ISAV-NP binds ,12 nts of RNA, shorter than the 24ﾖ28 nts originally estimated for the influenza A virus NP based on population average. The 12-nt stoichiometry was further confirmed by results from electron microscopy and dynamic light scattering. Considering that RNPs of ISAV and the influenza viruses have similar morphologies and dimensions, our findings suggest that NP-free RNA may exist on orthomyxovirus RNPs, and selective RNP packaging may be accomplished through direct RNA-RNA interactions

High pressure synthesis and structure of the superconducting mercury cuprates (Hg1-xMx)Ba2Can-1CunO2+2n+delta with M = C, S.

In mercury superconducting cuprates synthesized at high pressure, partial carbon substitution on the mercury site occurs when the precursors are not absolutely carbon free. Hg-12(n-1)n samples made from carbonate-containing precursors show lower Tcs and smaller c parameter. A model of the structural arrangement of the CO3 -2 groups is proposed from neutron diffraction data and crystal-chemistry considerations. The third oxygen of the oxyanion is situated in the (Hg,C)O∂ layer and hinders the incorporation of extra-oxygen atoms. The (Hg1-xCx)Ba2CuO4+∂ phase diagram shows three different phases, namely HgBa2CuO4+∂, Hg0.5C0.5Ba2CuO4+∂ and CBa2CuO4+∂. The latter two phases are not superconducting due to the insufficient hole concentration on the superconducting layer. Sulphur can also partially replace mercury in Hg1-xSxBa2CuO4+∂ and forms a solid solution up to x=0.15. Neutron diffraction shows that the sulphur atoms arrange as S04 -2 oxyanions. The larger S-O distances induce steric limitations to the presence of other sulphate anions in the same mesh

Effects of silicon addition on the electrical and magnetic properties of copper-doped (La,Ca)MnO3 compounds

In this paper we report about the electrical properties of La0.7Ca0.3MnO3 compounds substituted by copper on the manganese site and/or deliberately contaminated by SiO2 in the reactant mixture. Several phenomena have been observed and discussed. SiO2 addition leads to the formation of an apatite-like secondary phase that affects the electrical conduction through the percolation of the charge carriers. On the other hand, depending on the relative amounts of copper and silicon, the temperature dependence of the electrical resistivity can be noticeably modified: our results enable us to compare the effects of crystallographic vacancies on the A and B sites of the perovskite with the influence of the copper ions substituted on the manganese site. The most original result occurs for the compounds with a small ratio Si/Cu, which display double-peaked resistivity vs. temperature curves. (C) 2003 Elsevier B.V. All rights reserved

HIGH-PRESSURE SYNTHESIS AND PROPERTIES OF THE HGBA2CAN-1CUNO2N+2+DELTA (N=1-6) SUPERCONDUCTORS

High-pressure high temperature synthesis has allowed us to prepare members of the new superconducting mercury cuprates series with n = 1-6. The variation of the structural and magnetic properties, oxygen content and Tc across the series, which appears to be the richest among the different superconducting cuprates, will be presented and discussed. Tc and the oxygen content δ increases with n from Hg-1201 (Tc=95K, δ=0.06) to a maximum of 135 K with δ=0.40 for Hg-1223. Structural refinement from neutron diffraction data on Hg-1245 showed a further decrease of δ = 0.30 in the high pressure as prepared compound (Tc= 101K). Tentatives to increase δ by a treatment under 200 bar of oxygen in Hg-1234, Hg-1245 and Hg-1256 showed instead a loss of oxygen from the overdoped as prepared samples. © 1994