Darrin S. Richeson

EFFECT OF CU-O LAYER SPACING ON THE MAGNETIC FIELD INDUCED RESISTIVE BROADENING OF HIGH-TEMPERATURE SUPERCONDUCTORS

Abstract For H‖c-axis, the magnetic field induced broadening of the resistive transitions of high-Tc superconductors (HTS) is shown to depend strongly on the Cu-O layer spacing. For the highly anisotropic HTS, we show experimental evidence that flux motion results from a thermally activated crossover from three dimensional (3D) vortex lines to 2D independent pancake-like vortices in the Cu-O layers, which is intrinsic to the material and occurs when kBT exceeds the Josephson coupling energy of these layers. At low temperatures, however, thermally activated conventional depinning (which can be sample dependent) or melting in the uncoupled 2D Cu-O layers is also required for flux motion. For YBa2Cu3O7, this dimensional crossover does not occur belowHc2, presumably because the conducting Cu-O chains short-circuit the Josephson interlayer coupling, leading to better superconducting properties in a magnetic field. These results show that strong interlayer coupling is a key to finding good alternatives.

Single-Molecule Magnet Behavior with a Single Metal Center Enhanced through Peripheral Ligand Modifications

Bis(imino)pyridine pincer ligands in conjunction with two isothiocyanate ligands have been used to prepare two mononuclear Co(II) complexes. Both complexes have a distorted square-pyramidal geometry with the Co(II) centers lying above the basal plane. This leads to significant spin-orbit coupling for the d(7) Co(II) ions and consequently to slow relaxation of the magnetization that is characteristic of Single-Molecule Magnet (SMM) behavior.

Epitaxial growth of BaTiO3 thin films by organometallic chemical vapor deposition

Epitaxial BaTiO3 thin films were grown in situ on (100) LaAlO3 by low‐pressure organometallic chemical vapor deposition using the precursors Ba (hexafluoroacetylacetonate)2 (tetraglyme) and titanium tetraisopropoxide. The phase composition and epitaxial quality were sensitive to the reactant partial pressures and growth temperature. Deposition at 800 °C yielded [100]‐oriented BaTiO3 films. In‐plane epitaxy was confirmed for the BaTiO3 films by x‐ray diffraction.

Phase‐selective route to high Tc superconducting Tl2Ba2Can−1CunO2n+4 films: Combined metalorganic chemical vapor deposition using an improved barium precursor and stoichiometry‐controlled thallium vapor diffusion

Films of the Tl2Ba2Can−1CunO2n+4 high Tc superconductors (n=2 or 3) can be prepared with a high degree of phase selectivity using a combination of metalorganic chemical vapor deposition (MOCVD) and vapor diffusion. Ba‐Ca‐Cu‐O films are first prepared by MOCVD using the volatile metalorganic precursors Ba(hexafluoroacetyl‐ acetonate)2(tetraglyme), Ca(dipivaloylmethanate)2, and Cu(acetylacetonate)2. The ‘‘second‐generation’’ barium precursor exhibits significantly improved thermal stability and volatility over previously used compounds. Thallium is then incorporated into these films by vapor diffusion using a Tl‐Ba‐Ca‐Cu oxide mixture of controlled composition as the source of volatile thallium oxides. Phase control is achieved by a combination of improved stoichiometry of the deposited film, a result of the new Ba source, and annealing with the appropriate oxide mixture. The resultant films consist predominantly of the Tl2Ba2Ca2Cu3Ox or Tl2Ba2CaCu2Ox phase, each having preferential orientation of the cryst...

Organometallic chemical vapor deposition routes to high Tc superconducting Tl-Ba-Ca-Cu-O films

Films of the Tl‐Ba‐Ca‐Cu‐O high Tc superconductor can be prepared by either of two organometallic chemical vapor deposition routes. Ba‐Ca‐Cu‐O films are first prepared on yttria‐stabilized zirconia using the volatile precursors Ba(heptafluorodimethyloctanedionate)2, Ca(dipivaloylmethanate)2, and Cu(acetylacetonate)2. Deposition is carried out at 5 Torr pressure with argon as the carrier gas and water vapor as the reactant gas. Thallium is next incorporated in these films either by vapor diffusion using bulk Tl‐Ba‐Ca‐Cu‐O as the source, or by organometallic chemical vapor deposition using Tl(cyclopentadienide) as the source. The latter deposition is carried out at atmospheric pressure with an argon carrier and water‐saturated oxygen reactant, followed by rapid thermal annealing. Both types of films consist primarily of the TlBa2Ca2Cu3Ox phase, have preferential orientation of the CuO planes parallel to the substrate surface, and exhibit onset of superconductivity at ∼120 K with zero resistance by 100 K.

Pulsed organometallic beam epitaxy of complex oxide films

We report the results of a pulsed organometallic beam epitaxy (POMBE) process for growing complex oxide films at low background gas pressure (10−4–10−2 Torr) and low substrate temperature (600–680 °C) using organometallic precursors in an oxygen plasma environment. Our results show that POMBE can extend the capability of organometallic chemical vapor deposition to growing complex oxide films with high precision both in composition and structure without the need for post‐deposition oxidation and heat treatments. The growth of phase‐pure, highly oriented Y‐Ba‐Cu‐O superconducting oxide films {[Tc (R=0)=90.5 K] and Jc (77 K, 50 K gauss)=1.1×105 A/cm2} is given as an example. Similar to the pulsed laser deposition process, the POMBE method has the potential for in situ processing of multilayer structures (e.g., junctions).

Capturing In+ Monomers in a Neutral Weakly Coordinating Environment

The application of a new bis(imino)pyridine ligand allowed the isolation and characterization of [{2,4-(t)Bu(2)C(6)H(3)N=CPh}(2)(NC(5)H(3))]In(+)(OTf)(-) as the first low-valent, main-group metal complex of this ligand scaffold. Structural analysis revealed a unique monomeric In(I) species with a surprisingly long metal-ligand bond. In conjunction with a density functional theory investigation, this complex is shown to display only nominal donor-acceptor interactions between the metal and the neutral ligand. The mixing of the occupied 5s metal orbital with the occupied ligand orbitals reduces the reactivity of the central atom and thus stabilizes this species. An In(III) species, [{2,4-(t)Bu(2)C(6)H(3)N=CPh}(2)(NC(5)H(3))]InCl(2)(+)InCl(4)(-) was also isolated and structurally characterized utilizing this ligand frame.

Catalytic C=N bond metathesis of carbodiimides by group 4 and 5 imido complexes supported by guanidinate ligands.

A family of guanidinate-supported imido metal complexes are novel, effective catalysts for C=N metathesis of alkyl and aryl carbodiimides and evidence suggests that this reaction proceeds via a sequential addition/elimination pathway.

Synthesis and structural characterization of the first trialkylguanidinate and hexahydropyramidopyramidinate complexes of tin

Abstract The first guanidinate complexes of tin have been prepared using N,N′,N′′-trialkylguanidinates ([(RN)2C(NRH)]−; R=cyclohexyl; isopropyl) and 1,3,4,6,7,8-hexahydro-2H-pyramido[1,2-a]pyramidinate (hpp−) as ligands. The direct reaction between triisopropylguanidine and SnCl4 provided the complex {[iPrN]2C[NHiPr]}SnCl3 (1) along with concomitant formation of the guanidinium salt {C[NHiPr]3}+[SnCl5(THF)]− (2). The Sn(II) guanidinate complexes {(C6H11N)2C[NH(C6H11)]}2Sn (3) and {CN[N(CH2)3]2}2Sn (4) were prepared through metathesis reactions between 0.5 equiv. SnCl2 and (C6H11N)2C[NH(C6H11)]Li or hppLi, respectively. Complex 4 is the first reported mononuclear complex of this ligand. In contrast, the reaction of hppLi with 1 equiv. SnCl2 afforded a bridging dinuclear species, {CN[N(CH2)3]2SnCl}2 (5). A second mononuclear complex of the hpp− ligand, {CN[N(CH2)3]2}2SnCl2 (6), was the product obtained from the reaction of 2 equiv. of hppLi with SnCl4. The full structural details of compounds 1 and 3–6 are reported. In the case of compounds 1 and 3 these results revealed a distinctly unsymmetrical bonding mode for the bidentate guanidinate ligand and suggest variable degrees of π delocalization with the ligand. The geometries of the Sn centers in 3, 4 are derived from distorted trigonal bipyramidal coordination with a stereochemically active lone pair occupying one coordination site. In contrast, complex 5 displayed a geometry derived from a tetrahedral ligand array with one vertex occupied by a lone pair of electrons. Complex 6 is six coordinate and possesses 2 equiv. chelating bidentate hpp− ligands and two cis-chloro groups.

Amidolithium and Amidoaluminum Catalyzed Synthesis of Substituted Guanidines : An Interplay of DFT Modeling and Experiment

The synthesis of substituted guanidines is of significant interest for their use as versatile ligands and for the synthesis of bioactive molecules. Lithium amides supported by tetramethylethylenediamine have recently been shown to catalyze the guanylation of amines with carbodiimide. In this report, density functional theory (DFT) calculations are used to provide insight into the mechanism of this transformation. The mechanism identified through our calculations is a carbodiimide insertion into the lithium-amide bond to form a lithium guanidinate, followed by a proton transfer from the amine. The proton transfer transition state requires the dissociation of one of the chelating nitrogen centers of the lithium guanidinate, proton abstraction from the amine, and bond formation between the lithium center and the amine nitrogen. On the basis of this mechanism, further calculations predicted that aluminum amides would also function as active catalysts for the guanylation of amines. We confirm this experimentally and report the development of aluminum amides as a new main group catalyst for the guanylation of a range of electron-poor amines with carbodiimide.