Synthetic,Structural, and Spectroscopic Investigations of K3Nb8O21‐type Complex Oxides K3MTa7O21 (M = Ti,Zr)

Potassium‐containing zirconium(IV)/titanium(IV) tantalum(V) oxides, K₃TiTa₇O₂₁ ( 1 ) and K₃ZrTa₇O₂₁ ( 2 ), of K₃Nb₈O₂₁‐type of compounds are afforded from potassium‐molybdate flux. Both compounds crystallize in the hexagonal space group P6₃/mcm (no. 193) with a = 908.69(2), c = 1202.83(7) pm and c/a = 1.324 (Z = 2) for 1 and a = 913.30(3), c = 1219.21(6) pm and c/a = 1.335 (Z = 2) for 2 , respectively. The Structural motif of [MTa₇O₂₁]^3– (M = Ti⁴⁺ or Zr⁴⁺) consists of edge‐shared (M,Ta)₆O₂₄‐units that are similar to corner‐sharing Ta₆O₂₇ units of synthetic soro‐silicate K₃Ta₃Si₂O₁₃ and double borate K₃Ta₃B₂O₁₂. The solid state bandgap measurements revealed that calculated values (3.26 eV for K₃TiTa₇O₂₁ and 3.14 eV for K₃ZrTa₇O₂₁) are dependent on aperture of Ta–O–Ta bond angle as it was previously shown for perovskite‐type tantalate photocatalysts.     

A one-pot, non-catalytic approach to 1,2,4-benzothiadiazine-1,1-dioxides

Condensations of o-halo-substituted benzenesulfonyl chlorides with 2-aminopyridines and amidines may give the corresponding 1,2,4-benzothiadiazine-1,1-dioxides under mild, non-catalytic conditions in nearly quantitative yields. The successful one-pot cyclization depends on three factors: (i) the nature of the o-halogen, (ii) the electronic character of the benzene ring substituent, and (iii) the steric load around the amidine unit. O-Fluorobenzenesulfonyl chlorides bearing methylcarboxyl- or nitro-group and o-chloro- and o-bromobenzenesulfonyl chlorides bearing nitro-group are reactive enough to give the desired 1,2,4-benzothiadiazine-1,1-dioxides in a one-pot base-promoted reaction. In all other cases, open-chain sulfonylated amidine intermediates are isolated. The latter are converted to the title compounds either in the presence of potassium carbonate or upon the addition of a copper(I) catalyst.     

Synthesis, structure, and properties of supramolecular charge-transfer complexes between bis(18-crown-6)stilbene and ammonioalkyl derivatives of 4,4'-bipyridine and 2,7-diazapyrene

4,4'-Bipyridine and 2,7-diazapyrene derivatives (A) having two ammonioalkyl N-substituents were synthesized. The complex formation of these compounds with bis(18-crown-6)stilbene (D) was studied by spectrophotometry, cyclic voltammetry, (1)H NMR spectroscopy, and X-ray diffraction analysis. In MeCN, π-donor D and π-acceptors A form supramolecular 1:1 (D·A) and 2:1 (D·A·D) charge-transfer complexes. The D·A complexes have a pseudocyclic structure as a result of ditopic binding of the ammonium groups to the crown-ether fragments. The better the geometric matching between the components, the higher the stability of the D·A complexes (log K up to 9.39). A key driving force of the D·A·D complex formation is the excessive steric strain in the precursor D·A complexes. The pseudocyclic D·A complexes involving the ammoniopropyl derivative of 4,4'-bipyridine were obtained as single crystals. Crystallization of the related ammonioethyl derivative was accompanied by transition of the D·A complexes to a structure of the (D·A)(m) coordination polymer type.     

Pb2.85Ba2.15Fe4SnO13: A new member of the AnBnO3n−2 anion-deficient perovskite-based homologous series

Pb_2.85Ba_2.15Fe₄SnO₁₃, a new n=5 member of the anion-deficient perovskite based A_nB_nO_3n−2 (A=Pb, Ba, B=Fe, Sn) homologous series, was synthesized by the solid state method. The crystal structure of Pb_2.85Ba_2.15Fe₄SnO₁₃ was investigated using a combination of neutron powder diffraction, electron diffraction, high angle annular dark field scanning transmission electron microscopy and Mössbauer spectroscopy. It crystallizes in the Ammm space group with unit cell parameters a=5.7990(1) Å, b=4.04293(7) Å and c=26.9561(5) Å. The Pb_2.85Ba_2.15Fe₄SnO₁₃ structure consists of quasi two-dimensional perovskite blocks separated by 1/2[110](1?01)_p crystallographic shear (CS) planes. The corner-sharing FeO₆ octahedra at the CS planes are transformed into edge-sharing FeO₅ distorted tetragonal pyramids. The octahedral positions in the perovskite blocks between the CS planes are jointly taken up by Fe and Sn, with a preference of Sn towards the position at the center of the perovskite block. The chains of FeO₅ pyramids and (Fe,Sn)O₆ octahedra of the perovskite blocks delimit six-sided tunnels at the CS planes occupied by double chains of Pb atoms. The compound is antiferromagnetically ordered below T_N=368±15 K.     

The stepwise Diels-Alder reaction of 4-nitrobenzodifuroxan with Danishefsky's diene

The Diels–Alder reaction of 4‐nitrobenzodifuroxan (NBDF) with 1‐methoxy‐3‐trimethylsilyloxy‐1,3‐butadiene has been investigated experimentally and theoretically. Treatment of NBDF with excess diene in chloroform at room temperature was found to afford a single product that contained a carbonyl functionality. Based on an X‐ray structure and NMR spectroscopic data, the product appeared to be a result of the hydrolysis of the OSiMe₃ moiety of the thermodynamically more stable endo [2+4] cycloadduct, characterized by a cis arrangement of the MeO and NO₂ functionalities. In situ NMR investigations of the interaction were carried out at room temperature in CDCl₃ and at ?40 °C in deuterated acetonitrile. Calculations at the B3LYP/6‐31G* level in the gas phase and in acetonitrile were carried out under the assumption that the most stable cis conformation of the diene is also the most reactive in the interaction. The analysis revealed the NBDF/cis diene interaction involves the formation of a zwitterionic intermediate. Importantly, this intermediate is formed in two preferred conformations, which correspond to the endo and exo modes of approach of the reagents. Cyclization of these two identified conformations afforded the experimentally characterized endo and exo [2+4] cycloadducts. According to the calculations, the interconversion of the two conformers can either take place through a return to the pre‐reaction complexes or it can occur by rotation through an intermediate conformation of lesser stability. In view of the stepwise character of the interaction, the possibility that the intermediate zwitterion is the result of the interaction between NBDF and the trans diene could not be excluded. Calculations carried out with the most stable and more populated s‐trans conformer confirmed this idea and supported the role of the zwitterion in the overall interaction.     

Single-mode 212 W tapered fiber laser pumped by a low-brightness source

With a tapered double-clad all-glass ytterbium fiber as a gain medium, a maximum output power of over 200 W at 1079 nm and a slope efficiency of over 70% were demonstrated. The tapered double-clad fiber concept allows for using low-brightness diode bars and results in cost-effective and efficient high-power fiber lasers. The adiabatic conical fiber gain waveguide combines improved pump absorption owing to enhanced mode mixing in the pump cladding, low-noise single fundamental mode operation with M(2)< or =1.02, and strong potential for significant power scaling.     

Structure of heterogeneous states in nanocrystalline CoPd films

Thin films of equiatomic alloy CoPd have been produced by condensation at high substrate temperatures. The films obtained are characterized by microdiffraction patterns in the form of diffuse halo in the initial state. This is polymorphic martensitic transient state, formed during martensitic transformation of the low-temperature hcp phase to the high-temperature fcc phase. The metastable crystal structures arising in these alloy films are identified.     

Perpendicular magnetic anisotropy in the Co50Pt50, Co50Pd50, and Co50Pt50 ? xPdx thin films

Films of the Co₅₀Pt₅₀, Co₅₀Pd₅₀, and Co₅₀Pt_{50 − x} Pd _x alloys (where x = 1–10 at %) have been prepared. The processes of atomic ordering and its influence on the perpendicular magnetic anisotropy and the coercive force in these films have been investigated. The dependence of the coercive force on the film thickness has been analyzed. It has been shown that thin films of ordered alloys become magnetically uniaxial with the easy axis normal to their plane and can be used for magnetic and thermomagnetic recording and storage of information.     

Higher borides and oxygen-enriched Mg–B–O inclusions as possible pinning centers in nanostructural magnesium diboride and the influence of additives on their formation

The study of high pressure (2 GPa) synthesized MgB₂-based materials allows us to conclude that higher borides (with near MgB₁₂ stoichiometry) and oxygen-enriched Mg–B–O inclusions can be pinning centers in nanostructural magnesium diboride matrix (with average grain sizes of 15–37 nm). It has been established that additions of Ti or SiC as well as manufacturing temperature can affect the size, amount and distribution of these inclusions in the material structure and thus, influence critical current density. The superconducting behavior of materials with near MgB₁₂ stoichiometry of matrix is discussed.     

Piezoelectric Current Generation in Wurtzite GaAs Nanowires

Generation of electric current is observed when GaAs nanowires with wurtzite crystal structure are bent by the probe of an atomic force microscope. The current originates from a piezo active phase in the nanowires due to the piezoelectric effect. Increasing of the piezo‐potential in bent nanowires enhances tunneling through the probe–nanowire Schottky barrier due to the thermionic field emission. Laser illumination amplifies short‐circuit current pulses by two orders of magnitude from 9 pA to 1 nA due to the piezo‐phototronic effect. Utilization of such piezo‐phototronic effect in GaAs nanowires is a solution to accelerate the efficiency of hybrid energy sources “piezoelectric nanogenerator ? solar cell” comprised of III–V nanowires.