The reaction of 1,3-dimesitylimidazolium chloride with Ti(NMe(2))(4) results in the 1,3-dimesitylimadazol-2-ylidene complex of Ti(NMe(2))(2)Cl(2)(3); the X-ray crystal structure of 3 evidences short intramolecular Cl...C(carbene) contacts.
Actinides such as uranium (U) and plutonium (Pu) are essential materials for nuclear power plants and nuclear weapons, but their long-term storage safety has always been a serious concern. The U.S. alone currently has an inventory of over 10 9 pounds of depleted uranium waste materials from the manufacturing of nuclear weapons and from the nuclear power industry. Even simple issues such as oxidation become major problems in the storage of radioactive materials. For example, the oxidation of UO 2 generates a powder-like U 3 O 8 that can cause the splitting of the storage sheath, [1][2][3] which significantly complicates the handling and storage of UO 2 fuel and impacts the safety of long term storage of actinides. A better control of the surface chemistry and the phases of oxides (UO 2 , U 3 O 7 , U 3 O 8 , and UO 3 ) is, therefore, crucial for interpreting the surface chemistry of the oxides and for developing new applications of depleted uranium. So far, almost all research on the oxidation states and surface chemistry of uranium oxides has been carried out using sintered polycrystalline materials. As data obtained from single crystal substrates are, without exception, easier to interpret than data obtained from polycrystalline materials, it is essential that high quality single-crystal-like uranium oxides be prepared for our attempts to better understand the intrinsic physical properties of the materials. Herein we report the first such attempt to date to control the oxidation states in uranium oxides through epitaxial stablization to grow single-crystal-like uranium oxide films. These epitaxial uranium oxides are stable in air because their oxidation states are crystallographically pinned. The difficulty in preparing single crystal uranium oxides with controllable phases lies in the coexistence of many oxides with different oxidation states, where conversions between the oxides are relatively facile. An additional challenge in the growth of single crystal and single phase uranium oxides stems from the abundance of polymorphic structures in these oxides. For example, there exist both orthorhombic (Amm2) and hexagonal (P6 2m) phases of U 3 O 8 . Using polycrystalline materials in experiments to extract intrinsic properties of the materials can create tremendous ambiguity. For instance, experimental results indicate that the kinetic rate of U 3 O 7 formation on UO 2 depends heavily on the initial form of materials studied: powders, polycrystalline pellets, or crystals.[2] It has also been shown that intra-granular oxidation proceeds more slowly than oxidation along grain boundaries. [4] This is, however, not unexpected considering that grain boundaries and/or crystallographic imperfections often have a profound effect on chemical reactions. Neglecting the impact of the preferential diffusion of oxygen along grain boundaries can eventually lead to inappropriate interpretations of the surface chemistry of the materials. Epitaxial films, whose crystallographic alignment is mainly controlled by that of the substrate,...
Doped manganites, RE 1-x A x MnO 3 (RE = rare-earth and A= alkaline-earth elements), have attracted much attention because of their potential applications in magnetic sensors and other devices. [1][2][3][4][5] A prominent feature of these materials is a metallic-insulating (MI) transition associated with the ferromagnetic-paramagnetic (FM-PM) transition.[4] The figure of merit of these colossal magnetoresistance (CMR) materials for many applications is the magnetoresistance (MR = (q Hq 0 )/q 0 , where q H and q 0 are the resistivities with and without a magnetic field, respectively). For practical applications, it is important to obtain a high MR at room temperature and at low magnetic fields. It has been observed that the MR near or at the transition temperature, T c , for a given field strength, is generally larger for samples with lower T c .[2] Thus, simply changing the composition of these manganites is not effective in obtaining large MR values near room temperature. Many recent efforts in searching for enhanced MR have been focused on transport properties across artificial grain boundaries and interfaces by making FM manganite/spacer superlattices and FM-insulator-FM tunneling junctions. [6][7][8] However, the enhancements of the MR were mostly achieved at low temperatures (< 150 K). [5]In this work, we report our efforts to obtain a large MR near room temperature by preparing multilayer-coated La 0.67 Sr 0.33 MnO 3 /La 0.67 Ca 0.33 MnO 3 (LSMO/LCMO) films using polymer assisted deposition (PAD). PAD is a solution technique that has been successfully used to prepare both simple and complex metal oxide films. [11,12] The LSMO and LCMO compounds have similar lattice parameters and have a Curie temperature above and below room temperature, respectively, which makes them very attractive for preparing multilayers. The PAD technique was used to prepare the solutions of La 0.67 Sr 0.33 Mn and La 0.67 Ca 0.33 Mn. These solutions were spincoated onto single-crystalline (001) LaAlO 3 (LAO) substrates. After each coating, the films were heated at 600°C for 5 min. For all the films, a total of 10 coatings were applied on the substrate and final annealing was performed at 950°C for 2 h in oxygen. The thicknesses of the films were ca. 130 nm. Our initial study focused on optimizing the LSMO/LCMO ratio to get a maximum MR close to room temperature. The films with LSMO/LCMO volume ratios of 70:30, 60:40, and 50:50 had MR values of -61 %, -63 %, and -57 %, respectively, at 300 K at an applied field of 5 T. Hence, in this work we prepared multilayer-coated films by holding the LSMO/ LCMO volume ratio constant (60:40 with maximum MR) and changing the number and the thickness of the individual layers. Details of the layered configuration with their sample identifications (ML1 and ML2) are illustrated in Figure 1. To make a direct comparison of the multilayer-coated films, a single-phase film of La 0.67 Sr 0.198 Ca 0.132 MnO 3 (LSCMO), which is a uniformly mixed phase of the LSMO/LCMO at a volume ratio of 60:40, was also prepa...
Amines Amines Q 0120 Dialkyl Aluminum Amides: New Reagents for the Conversion of C=O into C=NR Functionalities. -α-Diimines (III) and β-aminoenones (V) are prepared using dialkyl aluminium amides (II). -(GORDON*, J. C.; SHUKLA, P.; COWLEY, A. H.; JONES, J. N.; KEOGH, D. W.; SCOTT, B. L.; Chem.
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