Undoped LaFeAsO, parent compound of the newly found high-T c superconductor, exhibits a sharp decrease in the temperature-dependent resistivity at ~160 K. The anomaly can be suppressed by F doping and the superconductivity appears correspondingly, suggesting a close associate of the anomaly with the superconductivity. We examined the crystal structures, magnetic properties and superconductivity of undoped (normal conductor) and 14 at.% F-doped LaFeAsO (T c = 20 K) by synchrotron X-ray diffraction, DC magnetic measurements, and ab initio calculations to demonstrate that the anomaly is associated with a phase transition from tetragonal (P4/nmm) to orthorhombic (Cmma) phases at ~160 K as well as an antiferromagnetic transition at ~140 K. These transitions can be explained by spin configuration-dependent potential energy surfaces derived from the ab initio calculations. The suppression of the transitions is ascribed to interrelated effects of geometric and electronic structural changes due to doping by F − ions.Parent compounds to the high-T c superconductors show a rapid decrease in their electrical resistivity (ρ), which is clearly seen on the resistivity-temperature (T) curves with a kink at ~160 K (T anom ). This anomaly has been attributed to the combined effect of a crystallographic phase transition at ~160 K, and an antiferromagnetic ordering of the Fe spins at a slightly lower temperature of ~140 K [27][28][29][30][31]. Both transitions can be simultaneously suppressed by the electron or hole doping, suggesting a close association of these phase transitions with the superconductivity observed in the doped compounds.The Fe-based and the Cu-based superconductors have a common feature in that superconductivity is attained by providing itinerant electron or hole carriers to the two-dimensional transport layers containing 3d transition metal elements. However, they differ distinctly from each other in that nine 3d electrons (one hole) are involved for Cu 2+ , which forms ionic bond with oxide ions, whereas six 3d electrons participate in a more complex interplay of Fe−Fe and Fe−As bonding.In this study, we examine the crystal structures, magnetic properties and superconductivity of undoped and 14 at.% F-doped LaFeAsO (T c = 20 K) by Rietveld refinement of synchrotron X-ray diffraction, DC magnetic measurements, and ab initio calculations. We demonstrate that the undoped LaFeAsO undergoes a phase transition from tetragonal (P4/nmm) to orthorhombic (Cmma) phases at ~160 K as well as an antiferromagnetic transition at ~140 K. These transitions can be explained by spin configuration-dependent potential energy surfaces derived from the ab initio calculations. Doping by F − ions in the LaO layers suppresses both transitions, which is ascribed to interrelated effects of geometric and electronic structural changes. Our results

We report a metallic state in a nanostructured porous crystal 12CaO x 7Al2O3 by incorporating electrons in the inherent subnanometer-sized cages, in which a three-dimensionally closely packed cage structure acts as an electronic conduction path. High-density electron doping ( approximately 2 x 10(21) cm(-3)), which was achieved by a thermal treatment in Ti metal vapor at approximately 1100 degrees C, induces homogenization of the cage geometry to a symmetric state, resulting in an insulator-metal transition with a sharp enhancement of the electron drift mobility from approximately 0.1 to 4 cm(2) V(-1) s(-1). The results provide an approach for the realization of electroactive functions in materials composed only of environmentally benign elements by utilizing the appropriate nanostructures.

A new quaternary fluoroarsenide CaFeAsF with the tetragonal ZrCuSiAs-type structure composed of alternate stacking of (FeAs)delta- and (CaF)delta+ layers was synthesized. CaFeAsF is a poor metal and shows the anomaly at approximately 120 K in temperature dependence of electrical conductivity. The electron doping by the partial replacement of the iron with cobalt suppresses the anomaly and induces the bulk superconductivity (optimal Tc = 22 K for CaFe0.9Co0.1AsF), analogous to recently discovered FeAs-based superconductors. The present results suggest that CaFeAsF is a promising candidate as a parent compound for high Tc superconductors.

Isotopic ratios of radioactive releases into the environment are useful signatures for contamination source assessment. Uranium is known to behave conservatively in sea water so that a ratio of uranium trace isotopes may serve as a superior oceanographic tracer. Here we present data on the atomic 233 U/ 236 U ratio analyzed in representative environmental samples finding ratios of (0.1-3.7)Á10 À2 . The ratios detected in compartments of the environment affected by releases of nuclear power production or by weapons fallout differ by one order of magnitude. Significant amounts of 233 U were only released in nuclear weapons fallout, either produced by fast neutron reactions or directly by 233 U-fueled devices. This makes the 233 U/ 236 U ratio a promising new fingerprint for radioactive emissions. Our findings indicate a higher release of 233 U by nuclear weapons tests before the maximum of global fallout in 1963, setting constraints on the design of the nuclear weapons employed.

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We have synthesized a quaternary fluoroarsenide SrFeAsF with the ZrCuSiAs-type structure (P4/nmm, a = 0.3999 and c = 0.8973 nm), which is composed of an alternately stacked (FeAs) δ− and (SrF) δ+ layers, analogous to the FeAs-based superconductor LaFeAsO. SrFeAsF shows metallic type conduction with the anomaly at ~180 K. The partial replacement of the Fe with Co suppresses the anomaly and induces the superconductivity, while the maximal T c (4 K for SrFe 0.875 Co 0.125 AsF) is much lower than that of the Co-substituted LaFeAsO. Replacement of (LaO) δ+ layers with (SrF) δ+ layers results in a enlargement of the c-axis length (+2.6%). These results suggest the importance of interlayer interaction as a critical T c -controlling factor in FeAs-based superconductors.

Electrides 1 are quasi-ionic crystals in which electrons serve as anions. The anionic electrons are spatially separated from molecular cations by cavity and/or channel structures formed by organic complexants or crystallographic cages or channel walls in zeolitic crystals. In 2003, we succeeded in the synthesis of an inorganic electride using a Ca 12 Al 14 O 33 crystal (C12A7), which is an air-and room-temperature-stable material in the category of the electride. 2 C12A7 has a mayenite-type crystal structure, whose chemical formula of the unit cell is expressed by [Ca 24 Al 28 O 64 ] 4+ (O 2-) 2 . The "free oxygen ions" (O 2-) are trapped as counteranions in the cages embedded in the positively charged framework ([Ca 24 Al 28 O 64 ] 4+ ). The free oxygen ions can be selectively removed via appropriate reduction treatments or knock-on processes by energetic ions. 3 The removal results in the electron injection to the cage, which in turn imparts persistent electronic conductivity to C12A7. The electron-encaging C12A7 exhibits a metal-insulator transition at the critical electron concentration of ∼1 Â 10 21 cm -3 , and the C12A7 electride with the theoretical maximum electrons (2.3 Â 10 21 cm -3 ), i.e., [Ca 24 Al 28 O 64 ] 4+ (e -) 4 , undergoes superconducting transition at ∼0.4 K. 4 Moreover, the skeleton structure of the electride provides an unique playground for various anionic chemical species stabilized by strong Madelung potential of the cages, which hardly exist under usual conditions. Typical examples are O 2 -, O -, H -, and Au -. 5-7 Furthermore, several applications have been found for the C12A7 electride. Among them, one expects the electride usable for vacuum electronics as electron source materials for cold and thermo-field emissions by utilizing an small work function (2.4 eV). 8 Another promising application field is organic syntheses. It has been reported recently that the C12A7 electride acts as a selective reducing agent in organic reactions in water media, for instance, pinacol coupling of aromatic aldehydes. 9 These reactions are conventionally performed in nonaqueous solutions with an aid of reducing agents such as alkali and alkali-earth metal compounds.The C12A7 electrides have been synthesized by reducing bulk C12A7 single crystals grown by Czochralski method and the electride powders have been prepared by grinding the bulk electride crystals. Thus, alternative mass productive methods for the powder, such as a direct synthesis from powder mixtures, are highly required to realize the chemical applications of the electride. Here we report a direct synthesis method for the preparation of the C12A7 electride powder with the electron density up to the theoretical maximum of 2.33 Â 10 21 cm -3 . Furthermore, we demonstrate a comprehensive technique for the precise determination of the oxygen stoichiometry of the C12A7 electride powders. The technique involves X-ray fluorescence (XRF), powder X-ray diffraction (XRD), neutron powder diffraction (NPD), thermo-gravimetric/ differential thermal ana...

We examined doping effect of 3d transition metal elements (TM: Cr, Mn, Co, Ni, and Cu) at the Fe site of a quaternary fluoroarsenide CaFeAsF, an analogue of 1111-type parent compound LaFeAsO. The anomaly at ~120 K observed in resistivity (ρ) vs. temperature (T) plot for the parent compound is suppressed by the doping of each TM element. Furthermore, Co-and Ni-doping (CaFe 1-x TM x AsF, TM = Co, Ni) induces superconductivity with a transition temperature maximized at the nominal x = 0.10 for Co (22 K) and at x = 0.05 for Ni (12 K). These optimal doping levels may be understood by considering that Ni 2+ (3d 8 ) adds double electrons to the FeAs layers compared with Co 2+ (3d 7 ). Increased x for Co or Ni breaks the superconductivity while metallic nature dρ/dT > 0 is still kept. These observations indicate that Co and Ni work as electron donors. In contrast, neither of Cr, Mn nor Cu-doping induce superconductivity, yielding dρ/dT < 0 in the below the ρ-T anomaly temperature, indicating that these transition metal ions act as scattering centers. The two different behavior of TM replacing the Fe site is discussed in relation to the changes in the lattice constants with the doping. Effect of 3d Transition Metal Doping on the Superconductivity in CaFeAsF PACS: 74.70.-b, 74.70.Dd, 74.25.Fy 74.62.-c, 74.62.Dh Effect of 3d Transition Metal Doping on the Superconductivity in CaFeAsF