Search for room temperature high-TCR manganite/silver composites

We have synthesized La_0.7Ba_0.3MnO₃ (LBMO):wt%Ag_x and La_0.7Sr_0.3MnO₃ (LSMO):wt%Ag_x composites with x=0.0, 0.1, 0.2, 0.3 and 0.4 at different sintering temperatures of 1300 and 1400 °C by solid-state reaction route. In our previous work [Awana et al. Solid State Commun. 140 (2006) 410] on La_2/3Ca_1/3MnO₃ (LCMO):Ag_x (x=0.0, 0.1, 0.2, 0.3, 0.4) composites, we reported about 12% temperature coefficient of resistance (TCR) for Ag_0.4 samples near metal–insulator (MI) transition, which was explained on the basis of better grains connectivity. A sharp MI transition results in high TCR. Such high-TCR material can be used as a bolometer or infrared detector at room temperature. In the present work, it is shown that the TCR is improved with Ag addition for both LBMO:wt%Ag_x and LSMO:wt%Ag_x composites. In addition to improvement in TCR, high MR is also achieved in the vicinity of room temperature.     

Pyrazine Based Type-I Sensitizing Assemblies for Photoreduction of Cu(II) in ‘One-Pot Three-Component’ CuAAC Reaction Under Aerial Conditions

Photosensitizing assemblies of pyrazine derivative PDA have been developed which exhibit a high photostability, ‘lighted’ excited state, balanced redox potential, high transportation potential and activate oxygen via type-I pathway only. These PDA assemblies in combination with Cu(II) ions catalyze the CuAAC reaction via in situ reduction of Cu(II) ions without any reducing or stabilizing agent. The present protocol has wide substrate scope with recyclability of the catalytic system up to six catalytic cycles and is applicable to gram-scale synthesis.     

Low field magnetoresistance and conduction noise in layered manganite La1.4Ca1.6Mn2O7

Temperature dependence of conduction noise and low field magnetoresistance of layered manganite La_1.4Ca_1.6Mn₂O₇ (DLCMO) are reported and compared with the infinite layered manganite La_0.7Ca_0.3MnO₃ (LCMO). The double layered manganite was prepared using standard solid state reaction method and had a metal-insulator transition temperature (T_M-I) of 155 K. The temperature dependence of susceptibility showed evolution of ferromagnetic ordering at 168 K. The observed voltage noise spectral density (S_V) shows 1/f^α type of behaviour at all temperatures from 77 K to 300 K. In the ferromagnetic region (T<168 K), S_V/V² shows two peaks at 164 K and 114 K. The observed two peaks in normalised conduction noise of DLCMO is attributed to the excess noise generated due to setting up of short range 2D-ferromagnetic ordering and long range 3D-ferromagnetic ordering at two different temperatures T_C2 and T_C1. In temperature range between T_C1 and T_C2, the magnetoresistance (MR) showed a gradual increase with the magnetic field. The observed MR has been explained in the framework of the two phase model [ferromagnetic (FM) domains and paramagnetic (PM) regions].     

Relativistic calculation of atomic X-ray transition rates

The fully relativistic transition probabilities in atomic X-ray emission are calculated for a wide range of selected elements of the periodic table, fromZ=21 toZ=93. The treatment includes the effect of the retardation of the radiation field. The atomic model used is the self-consistent relativistic Hartree-Fock, with the Slater approximation for the exchange term. The formulas for EL and ML multipole transition rates are derived, and transition rates are calculated for all transitions to theK-shell,L-subshells, andM-subshells. The contributions of all multipoles, as determined by the angular momentum and parity selection rules, are included in the present work. The calculated values of (Kα ₂/Kα ₁), (Kβ ₁/Kα ₁), (Kβ ₃/Kβ ₁) and (Kβ ₂ Kβ ₁) transition rates are in good agreement with the available experimental data. Rates for the forbidden (M1) 2s _1/2→1s _1/2 transitions are also presented.     

Mass spectra of refractory mixed-oxide compounds which contain an alkaline earth

The mass spectra of the high-temperature vapors over a series of barium tungstates have been measured. It is shown that the dominant vapor species over these mixed oxides is BaO(g); Ba(g) also is present as a minor vapor species over some of these oxides. The spectrum for Ba₂CaWO₆ shows Ca(g) in addition to BaO(g) and Ba(g). It appears that the BaO is formed by the solid state dissociation, e.g.

     

Pseudo-Halide Complexes of Biscyclopentadienyl Niobium(IV) and Bisindenyl Niobium(IV) Dichloride

Reactions of biscyclopentadienyl niobium(IV) dichloride and bisindenyl niobium(IV) dichloride with sodium or potassium salts of various pseudohalides have been studied and the pseudohalide complexes so formed of the formulae (C₅H₅)₂Nb(Ps)₂ and (C₂H₇)₂Nb(Ps)₂ where Ps may be NC, NCO, NCS, or N₃, have been isolated. The complexes have been characterised on the basis of physical measurements, analytical data and infrared spectral studies.     

Preparation of Cu, Ag, Fe and Al nanoparticles by the exploding wire technique

We describe a novel process for the production of nanoparticles of Cu, Ag, Fe and Al which involves exploding their respective wires, triggered by large current densities in the wires. The particles are characterised by X-ray diffraction (XRD) and atomic force microscopy (AFM). Particle sizes in the range 20–100nm were obtained employing this technique. The XRD results reveal that the nanoparticles continue to retain lattice periodicity at reduced particle sizes, displaying in some cases evidence of lattice strain and preferential orientation. In the case of Fe, Mossbauer spectroscopy reveals loss of ferromagnetism as a result of the reduced size of the particles. Dedicated to Professor C N R Rao on his 70th birthday     

Enthalpies of formation of gas-phase N3, N3-, N5+, and N5- from Ab initio molecular orbital theory, stability predictions for N5(+)N3(-) and N5(+)N5(-), and experimental evidence for the instability of N5(+)N3(-)

Ab initio molecular orbital theory has been used to calculate accurate enthalpies of formation and adiabatic electron affinities or ionization potentials for N3, N3-, N5+, and N5- from total atomization energies. The calculated heats of formation of the gas-phase molecules/ions at 0 K are DeltaHf(N3(2Pi)) = 109.2, DeltaHf(N3-(1sigma+)) = 47.4, DeltaHf(N5-(1A1')) = 62.3, and DeltaHf(N5+(1A1)) = 353.3 kcal/mol with an estimated error bar of +/-1 kcal/mol. For comparison purposes, the error in the calculated bond energy for N2 is 0.72 kcal/mol. Born-Haber cycle calculations, using estimated lattice energies and the adiabatic ionization potentials of the anions and electron affinities of the cations, enable reliable stability predictions for the hypothetical N5(+)N3(-) and N5(+)N5(-) salts. The calculations show that neither salt can be stabilized and that both should decompose spontaneously into N3 radicals and N2. This conclusion was experimentally confirmed for the N5(+)N3(-) salt by low-temperature metathetical reactions between N5SbF6 and alkali metal azides in different solvents, resulting in violent reactions with spontaneous nitrogen evolution. It is emphasized that one needs to use adiabatic ionization potentials and electron affinities instead of vertical potentials and affinities for salt stability predictions when the formed radicals are not vibrationally stable. This is the case for the N5 radicals where the energy difference between vertical and adiabatic potentials amounts to about 100 kcal/mol per N5.     

Polynitrogen chemistry: preparation and characterization of (N5)2SnF6, N5SnF5, and N5B(CF3)4

Metathetical processes were used to convert N5SbF6 into N5[B(CF3)4] and (N5)2SnF6. The latter salt is especially noteworthy because it contains two N5+ ions per anion, thus demonstrating that salts with touching polynitrogen cations can be prepared. This constitutes an important milestone towards our ultimate goal of synthesizing a stable, ionic nitrogen allotrope. The stepwise decomposition of (N5)2SnF6 yielded N5SnF5. Multinuclear NMR spectra show that in HF the SnF5- ion exists as a mixture of Sn2F(10)(2-) and Sn4F(20)(4-) ions. Attempts to isolate FN5 from the thermolysis of (N5)2SnF6 were unsuccessful, yielding only the expected decomposition products, FN3, N2, trans-N2F2, NF3, and N2.     

Methyl Tin(IV) derivatives of HOTeF(5) and HN(SO(2)CF(3))(2): a solution multinuclear NMR study and the X-ray crystal structures of (CH(3))(2)SnCl(OTeF(5)) and [(CH(3))(3)Sn(H(2)O)(2)][N(SO(2)CF(3))(2)

The new tin(IV) species (CH(3))(2)SnCl(OTeF(5)) was prepared via either the solvolysis of (CH(3))(3)SnCl in HOTeF(5) or the reaction of (CH(3))(3)SnCl with ClOTeF(5). It was characterized by NMR and vibrational spectroscopy, mass spectrometry, and single crystal X-ray diffraction. (CH(3))(2)SnCl(OTeF(5)) crystallizes in the monoclinic space group P2(1)/n (a = 5.8204(8) A, b =10.782(1) A, c =15.493(2) A, beta = 91.958(2) degrees, V = 971.7(2) A(3), Z = 4). NMR spectroscopy of (CH(3))(3)SnX, prepared from excess Sn(CH(3))(4) and HX (X = OTeF(5) or N(SO(2)CF(3))(2)), revealed a tetracoordinate tin environment using (CH(3))(3)SnX as a neat liquid or in dichloromethane-d(2) (CD(2)Cl(2)) solutions. In acetone-d(6) and acetonitrile-d(3) (CD(3)CN) solutions, the tin atom in (CH(3))(3)SnOTeF(5) was found to extend its coordination number to five by adding one solvent molecule. In the strong donor solvent DMSO, the Sn-OTeF(5) bond is broken and the (CH(3))(3)Sn(O=S(CH(3))(2))(2)(+) cation and the OTeF(5)(-) anion are formed. (CH(3))(3)SnOTeF(5) and (CH(3))(3)SnN(SO(2)CF(3))(2) react differently with water. While the Te-F bonds in the OTeF(5) group of (CH(3))(3)SnOTeF(5) undergo complete hydrolysis that results in the formation of [(CH(3))(3)Sn(H(2)O)(2)](2)SiF(6), (CH(3))(3)SnN(SO(2)CF(3))(2) forms the stable hydrate salt [(CH(3))(3)Sn(H(2)O)(2)][N(SO(2)CF(3))(2)]. This salt crystallizes in the monoclinic space group P2(1)/c (a = 7.3072(1) A, b =13.4649(2) A, c =16.821(2) A, beta = 98.705(1) degrees, V = 1636.00(3) A(3), Z = 4) and was also characterized by NMR and vibrational spectroscopy.