The magnetic, transport and thermal properties of alkaline-earth-filled skutterudites AFe 4 Sb 12 (A = Ca, Sr, Ba) are reported. All three compounds show maximum magnetic susceptibility and thermopower at 50 K and a large electronic specific heat coefficient of 100 mJ/mol K 2 . The electrical resistivity exhibits a shoulder at 70 K and follows quadratic temperature dependence below 9 K. These properties are the characteristics of a nearly ferromagnetic metal. The suppression of ferromagnetic spin fluctuations of the Fe 3d band by applying magnetic fields was manifested as a negative magnetoresistance between 50 and 80 K.The concept of itinerant magnetism has been developed over the past three decades. 1) In weakly or nearly ferromagnetic systems, the temperature dependence of the amplitude of local spin density results in the Curie-Weiss behavior of magnetic susceptibility 1 at high temperatures. In an itinerant ferromagnet, the ordered moment can be much smaller than the effective moment. Low-energy spin fluctuations (SFs) significantly influence the transport and thermal properties at low temperatures. For example, SF scattering gives rise to quadratic T dependence of the electrical resistivity, and the temperature variation of SFs leads to the significant enhancement of the electronic specific heat. Furthermore, SFs in ZrZn 2 and Sr 2 RuO 4 are believed to act as the glue for electron pairing in unconventional superconducting states. 2,3) NaFe 4 Sb 12 is a newly discovered weak itinerant ferromagnet with a rather high T c of 85 K and a small ordered moment of 0.25 " B /Fe atom. 4) This compound belongs to the family of filled skutterudites with the chemical formula MT 4 X 12 , where M is an alkali, alkaline-earth or a rare-earth metal, T is Fe, Ru or Os, and X is P, As, or Sb. The T atom is located at the center of an X 6 octahedron, and the cornersharing TX 6 octahedra form a tilted array. Within this family, rare-earth filled compounds have been extensively studied because of their novel physical properties, such as the unconventional superconductivity in PrOs 4 Sb 12 . 5) However, the role of d electrons of T atoms in the physical properties has not been well understood so far. The discovery of itinerant ferromagnetism in NaFe 4 Sb 12 with Na þ ions has revealed the strong dependence of the d electron magnetism on the valence of the filler ion. In fact, LaFe 4 Sb 12 with La 3þ ions remains in an enhanced paramagnetic state. 6) These contrasting magnetic behaviors in turn suggest that the ground state of AFe 4 Sb 12 with divalent A ions should be close to a magnetic critical point at which an itinerant ferromagnetic state emerges. In this paper, we report the magnetic, transport, and thermal properties of AFe 4 Sb 12 (A = Ca, Sr, and Ba). The results show that all three are in a nearly ferromagnetic state with strong SFs of the Fe 3d electrons.The first investigation of the magnetic properties of AFe 4 Sb 12 (A = Ca, Sr, Ba) was performed by Danebrock et al. 7) The Curie-Weiss behavior with an effective magn...
1,3-Enynes are core structures of various natural products or pharmaceuticals and are broadly used synthons in organic synthesis. Metal-catalyzed alkyne dimerization is a desirable preparation method, due to its perfect atom economy and the readily available alkyne substrates. Controlling the regio- and stereoselectivity remains a challenge, due to the competing formation of the head-to-tail (gem) and head-to-head (E/Z) isomers. Although catalytic systems based on noble metals have been extensively studied, there has been a growing interest to replace these noble metals with environmentally benign and inexpensive alternatives. In this Perspective, we highlight recent advances in catalytic alkyne dimerization, without the use of noble metals.
We report a novel mesoionic N-heterocyclic olefin (mNHO) derived from a 1,2,3-triazolium salt. With the carbenic position unprotected, the mNHO can tautomerize into the mesoionic carbene (MIC) form at room temperature. The reactivity of this new mNHO with various group 13 Lewis acids and CO2 have been studied experimentally and computationally.
A series of piano-stool Fe−NHC complexes have been prepared and characterized. The NHC ligands used herein possess a benzyl and a mesityl wingtip groups and have different electronic structures within the NHC rings. The catalytic activities of these Fe complexes have been examined for the homodimerization of terminal alkynes.
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