Tb16Br23B4: Endohedrale Bor‐Atome in tetrameren Terbiumclustern

Tb₁₆Br₂₃B₄: Tetrameric Terbium Clusters with Endohedral Boron Atoms The new cluster compound Tb₁₆Br₂₃B₄ was prepared from a stoichiometric mixture of Tb‐metal, TbBr₃ and B‐powder under Ar‐atmosphere in sealed Ta ampoules at 920–950 °C. It crystallizes monoclinic in the space group C2/m with a = 17.523(4) Å, b = 12.008(2) Å, c = 11.901(2) Å und β = 103.95(3)°. In the crystal structure B‐centered Tb₆ octahedra are connected via common edges to form tetrameric units. The Br atoms connect the Tb₁₆B₄‐clusters 3‐dimensionally coordinating the unoccupied edges and corners of the octahedra. Tb₁₆Br₂₃B₄ is a semiconductor with an electrical band gap of E_g = 0.4 eV. The magnetic susceptibility follows a Curie‐Weiss law corresponding to an effective magnetic moment μ_eff = 9.55 μ_B at high temperatures with an antiferromagnetic ordering below 20 K.     

Microstructural and physicochemical study of the buried Fe/AlGaAs(100) interface by transmission electron microscopy and x‐ray emission spectroscopy

Among the magnetic metal/semiconductor contacts, the Fe/GaAs system has been widely studied owing to its potential applications in electronic devices. In contrast, there are not many studies concerning the Fe/Al_xGa_1?xAs contact, and in particular there are no reports concerning the changes induced in the interfacial zone by the presence of Al. In this work, thin polycrystalline iron films were deposited by ion beam sputtering at room temperature on a 300 nm thick Al_0.25Ga_0.75As layer grown by molecular beam epitaxy onto GaAs(001). X‐ray diffraction analysis showed that the iron films are polycrystalline, and indications of a (002) texture of the film were observed. The fine scale analysis of the interface was achieved by high‐resolution transmission electron microscopy (HRTEM) observations, the results of which are compared with the physicochemical information obtained from electron‐induced x‐ray emission spectroscopy, by analysing the Al 3p valence states at the Fe/Al_xGa_1?xAs interface. The HRTEM experiments on cross‐section samples indicate that the interfacial zone between iron and AlGaAs is limited to <1.5 nm in thickness. X‐ray emission spectroscopy showed the presence of Al atoms in an FeAl‐like environment at the interface, and the existence of wrong bonds and point defects. The estimated width of the perturbed interface (2.0 ± 0.5 nm) is in agreement with the HRTEM results. Copyright © 2003 John Wiley & Sons, Ltd.     

Thermodynamics of plasticized triblock copolymers. I. Theory

The thermodynamic theory of bulk ABA copolymers developed by Leary and Williams is extended to copolymer–solvent systems. Free energy expressions are derived for five hypothetical phase-separated morphologies and evaluated specifically for a polymer with approximately 25% of the A component. The separation temperature, T_s, at which a given morphology will be in equilibrium with a homogeneous mixture, is also evaluated. The major result is the prediction of the T_s(?S) depression, where ?s is the solvent fraction. Depression is maximized when δ_S is equidistant between δ_A and δ_B, but becomes rapidly less when δ_S is outside the δ_A–δ_B range. Morphological favoritism is independent of ?_S and δ_S (model does not apply to preferential precipitation), with a planar microstructure being favored along with microstructures containing domains of B in continuous A for the 25% A polymer.     

DEVELOPMENT OF THERMOLUMINESCENCE BANDS DURING GREENING OF WHEAT LEAVES UNDER CONTINUOUS AND INTERMITTENT ILLUMINATION

Abstract— Mature wheat leaves excited by 1-min illumination at a low temperature of -60° C showed five thermoluminescence bands at -45, -10, +25, +40 and +55° C (denoted as Z_u, A, B₁ B₂ and C bands, respectively). The development of these bands during greening of etiolated wheat leaves under continuous and intermittent illumination was investigated, and the following results were obtained. (1) Etiolated leaves showed only the C band. When these leaves were greened under continuous light, the B₁ and B₂ bands appeared at 3 h and the Z_u band appeared at 10 h. The B₁ and B₂ bands were intensified during prolonged greening under continuous illumination to be the strong bands observed for mature leaves. The A band and the group of B₁ and B₂ bands were alternative: Similar experiments by excitation of thermoluminescence at -20° C showed the development of the A band instead of these B₁ and B₁ bands. (2) When the etiolated leaves were greened under intermittent illumination (1-ms light + 5-min dark), the Z_u band first appeared after 5 h of illumination (60 flashes) and was gradually intensified during further illumination with 340 flashes but, interestingly, neither the B₁ nor the B₂ band appeared even after 24–28 h of illumination with 280–340 flashes. (3) On exposure of such leaves greened under intermittent illumination to continuous light, the B₁ and B₂ bands were rapidly developed. The appearance of these bands was accompanied by the generation of the Hill activity (DCIP photoreduction with water as electron donor). (4) These results were discussed in relation to the previous study on photoactivation of the latent water-splitting system accumulated in the leaves greened under intermittent illumination. It was deduced that the structure responsible for the A band or the group of B₁ and B₂ bands is involved in the evolution of oxygen in chloroplasts, and probably involves cations of the Mn²⁺ catalyst generated by the action of light.     

Interpenetration of a 3D Icosahedral M@Ni12 (M=Al,Ga) Framework with Porphyrin‐Reminiscent Boron Layers in MNi9B8

Two ternary borides MNi₉B₈ (M=Al, Ga) were synthesized by thermal treatment of mixtures of the elements. Single‐crystal X‐ray diffraction data reveal AlNi₉B₈ and GaNi₉B₈ crystallizing in a new type of structure within the space group Cmcm and the lattice parameters a=7.0896(3) Å, b=8.1181(3) Å, c=10.6497(4) Å and a=7.0897(5) Å, b=8.1579(4) Å, c=10.6648(7) Å, respectively. The boron atoms build up two‐dimensional layers, which consist of puckered [B₁₆] rings with two tailing B atoms, whereas the M atoms reside in distorted vertices‐condensed [Ni₁₂] icosahedra, which form a three‐dimensional framework interpenetrated by boron porphyrin‐reminiscent layers. An unusual local arrangement resembling a giant metallo‐porphyrin entity is formed by the [B₁₆] rings, which, due to their large annular size of approximately 8 Å, chelate four of the twelve icosahedral Ni atoms. An analysis of the chemical bonding by means of the electron localizability approach reveals strong covalent B?B interactions and weak Ni?Ni interactions. Multi‐center dative B?Ni interaction occurs between the Al–Ni framework and the boron layers. In agreement with the chemical bonding analysis and band structure calculations, AlNi₉B₈ is a Pauli‐paramagnetic metal.     

Magnetic focusing of redox molecules at ferromagnetic microelectrodes

The electrochemical behavior of Fe and Ni disk-shaped microelectrodes (25 to 250 μm radius) in a uniform magnetic field (1 T) is reported. Magnetization of the ferromagnetic microelectrode generates a magnetic field gradient across the depletion layer that can be used to focus paramagnetic molecules toward the electrode surface. The magnetic force acting on the depletion layer is given by F_∇B=(χ/μ)B·∇B, where χ and μ are the volume magnetic susceptibility and magnetic permeability of the depletion layer, respectively, and B is the magnetic field. Magnetic field focusing of the nitrobenzene radical anion at Fe and Ni microelectrodes is demonstrated.     

Crystal Structure of SrAl2B2O7 and Eu Luminescence

The crystal structure of strontium dialuminodiborate SrAl₂B₂O₇ has been established by single-crystal X-ray diffraction methods. The compound crystallizes in the trigonal system (space group R c, Z=6) with cell parameters a=4.893(1) Å and c=47.78(1) Å. Aluminium and boron atoms are, respectively, in tetrahedral and triangular oxygen coordination. The assembly of Al₂O₇ units and BO₃ triangles forms double layers between which Sr²⁺ ions are located. The Eu²⁺-doped crystalline powder exhibits a luminescence band with maximum at 415 nm. Luminescence characteristics are compared to those of other strontium borates.     

Impedance measurements using ionic contacts on purified and doped silver bromide crystals

Impedance measurements are made on purified, vacancy-doped and interstitial-doped silver bromide crystals of thicknesses from 0.061 cm to 0.260 cm using constant ionic strength solution contacts at temperatures from 20°C to 35°C. Results characterize the macroscopic transport properties from 316 kHz to 0.01 Hz and at d.c. In all cases the high frequency limit resistive component of the impedance, R_∞, is equal to the d.c. resistance R₀. This result indicates rapid ion exchange at the surfaces and neither Warburg diffusion nor surface kinetic-limitation of transport. As expected, R_∞ is a function of charge carrier concentration, which depends on extrinsic dopant concentration over the temperature range measured. For pure crystals the resistance, R_∞, is linearly dependent on crystal thickness, δ, while thickness correlation could not be tested for the doped crystals as dopant level was not uniform. The geometric capacitance for all crystals, C_g, is linear with δ^?1. The dimensionless dielectric constant, κ, calculated from C_g and crystal dimentsions, is 13.9±0.7. The electric relaxation time, τ_el, defined as the product R_∞C_g, at 25°C is found to be 50±3 μs for pure crystals, 1 to 4 μs for the Cd²⁺-doped crystals and 12 to 66 μs for the S^2?-doped crystals. Temperature dependance of R_∞ allows determination of crystal transport activation energies. For pure and Cd²⁺-doped crystals a value of 0.33±0.02 eV is found. For the S²⁺-doped crystals values from 0.48 to 0.28 eV are found. The parameter α, in the non-ideal Cole-Cole representation of impedance plane arcs, is 0.96±0.01 for purified crystals, 0.93±0.03 for Cd²⁺-doped, and 0.92±0.02 for S^2?-doped crystals.     

THERMOLUMINESCENCE and OXYGEN EVOLUTION FROM A THERMOPHILIC BLUE-GREEN ALGA OBTAINED AFTER SINGLE-TURNOVER LIGHT FLASHES*

Abstract— Oxygen evolution and thermoluminescence (TL) studies on a thermophilic blue-green alga Synechococcus vulcanus Copeland revealed the following: (a) The deactivations of the S₃ and S₂ states of the Oxygen Evolving Complex, at room temperature, have half-times of ～200 and 75 s, respectively, instead of 30 and 20 s found in mesophilic plants, (b) The TL band(s) “B”, due to the recombination of the state S₂ or S₃ and Q_B, the reduced secondary quinone acceptor, is(are) at50–55°C instead of25–30°C; the intensity of this band oscillates with a period of 4 with maxima on the 2nd and the 6th flashes, (c) The TL band “D” in the presence of diuron, due to the recombination of S₂ and the reduced primary quinone acceptor Q_A,^? occurs at ～35°C instead of0–10°C. (d) Furthermore, the ratio of Q_B^? to Q_B in dark-adapted S. vulcanus cells is close to 1 as in intact spinach leaves, but not 0.43 as in isolated thylakoids from spinach.     

Magnetic field effects detected by exciplex fluorescence and preferential solvation in binary solvents. The effect of temperature

At 20–70 °C, the temperature has almost no effect on the semisaturation field,B _1/2, or on the magnetic effects detected by exciplex fluorescence (pyrene/N,N-dimethylaniline) in binary benzene-DMSO mixtures. In individual solvents (ethanol, methanol) heating leads to a noticeable increase in the magnetic effect, whileB _1/2 decreases. The results obtained corroborate a previously proposed hypothesis that polar microclusters are formed in binary solvents with components of different polarity. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1821–1823, October, 1993.     

Synthesis and Crystal Structure of Cesium Hexamminesodium Decahydro‐closo‐decaborate‐Ammonia(1/1), Cs[Na(NH3)6][B10H10]·NH3

Cs[Na(NH₃)₆][B₁₀H₁₀]·NH₃ was synthesised from cesium and disodium‐decahydro‐closo‐decaborate Na₂B₁₀H₁₀ in liquid ammonia, from which it crystallized in form of temperature sensitive colorless plates (triclinic, P1¯, a = 8.4787(7) Å, b = 13.272(1) Å, c = 17.139(2) Å, α = 88.564(1)°, β = 89.773(1)°, γ = 81.630(1)°, V = 1907.5(3) ų, Z = 4). The compound is the first example of an alkali metal boranate with two different types of cations. The decahydro‐closo‐decaborate dianions [B₁₀H₁₀]^2— and the cesium cations form a equation/tex2gif-stack-1.gif[Cs₂(B₁₀H₁₀)₂]^2— layer parallel to the ac plane. These layers are separated by N—H···N‐hydrogen bonded hexamminesodium cations.     

Metallic and Mott Insulating Spin‐Frustrated Antiferromagnetic States in Ionic Fullerene Complexes with a Two‐Dimensional Hexagonal C60.− Packing Motif

(MDABCO⁺)(C₆₀^.?)(TPC) ( 1 ), in which MDABCO⁺ is N‐methyldiazabicyclooctanium, TPC is triptycene, and both have threefold symmetry, is a rare example of a fullerene‐based quasi‐2D metal and contains closely packed hexagonal fullerene layers with interfullerene center‐to‐center distances of 10.07 Å at 300 K. Evidence for the metallic nature of 1 was obtained by optical and microwave conductivity measurements on single crystals. The metal is characterized by a nontypical Drude response and relatively large optical mass (m*/m₀=6.7). The latter indicates a narrow‐band nature, which is consistent with the calculated bandwidth of 0.10–0.15 eV. The coexistence of metallic and antiferromagnetic nonmetallic 2D layers was observed in 1 above 200–230 K. It was assumed that the nonmetallic layers undergo a transition to the metallic state below 200 K due to ordering of the fullerene and cationic sublattices. New layered complex (MQ⁺)(C₆₀^.?)(TPC) ( 2 ) with a hexagonal arrangement of C₆₀^.? was obtained by increasing the interfullerene distance with the bulkier N‐methylquinuclidinium cations (MQ⁺) having threefold symmetry. The structure of 2 is characterized by increased interfullerene center‐to‐center distances in the layers (10.124, 10.155, and 10.177 Å at 250 K). Unit‐cell doubling parallel to the 2D layer (along the b axis) was observed at low temperatures. In contrast to metallic 1 , 2 exhibits a nonmetallic spin‐frustrated state with an antiferromagnetic interaction of spins (the Weiss temperature is ?27 K) and no magnetic ordering down to 1.9 K. It was supposed that the expanded interfullerene distances in the triangular arrangement decrease the bandwidth and suppress metallic conductivity in 2 , and thus a Mott–Hubbard insulating state with antiferromagnetically frustrated spins results.     

Combined effect of the screening of a donor ion and the conduction band nonparabolicity on the binding energy of a donor at the center of a spherical quantum dot

We carried out variational model calculations for the assessment of the combined effect of the nonparabolicity of the electron effective mass and the screening of the donor ion by the valence electrons of GaAs for a donor placed at the center of a spherical quantum dot. We considered finite confining potentials between the GaAs QD and the surrounding Ga_1-xAl_xAs matrix. We found that the combined effect becomes more pronounced as the radius of the quantum dot decreases. © 1996 John Wiley & Sons, Inc. Int J Quant Chem 60: 507–510, 1996     

The temperature dependence and the mechanism of the SO2 (3B1) quenching reactions

The Arrhenius parameters have been determined for the SO₂(³B₁) quenching reaction (9), SO₂(³B₁) + M → (SO₂ ? M), for 21 different molecules as quenching partner M. The rate constants were calculated from phosphorescence lifetime measurements made over a range of reactant pressures and temperatures. Excitation of the SO₂ (³B₁) molecules was accomplished by two very different methods: (1) a 3829 Å laser pulse generated the triplet directly through absorption within the “forbidden” SO₂ (³B₁) → SO₂ (¹A₁) band; (2) a broadband Xe-flash system generated SO₂(³B₁) molecules and triplets were formed subsequently by intersystem crossing, SO₂(¹B₁) + M → SO₂(³B₁) + M. The measured rate constants were independent of the method of triplet formation employed. For the atmospheric gases, the activation energies (kcal/mole) were identical within the experimental error: N₂, 2.9 ± 0.4; 0₂, 3.2 ± 0.5; Ar, 2.8 ± 0.6; CO₂, 2.8 ± 0.4; CO, 2.7 ± 0.4; CH₄, 2.5 ± 0.6. This energy corresponds to the first region of the SO₂(³B₁) → SO₂(¹A₁) absorption spectra in which Brand and coworkers observe strong perturbations. It is suggested that the quenching in these cases results largely from the physical process involving potential energy surface crossing to another electronic state. Activation energies for SO₂(³B₁) quenching by the paraffinic hydrocarbons show a regular decrease in the series ethane, neopentane, propane, n-butane, cyclohexane, and isobutane, which parallels closely the decrease in C? H bond energies in these compounds. These and other data are most consistent with the dominance of chemical quenching in these cases. The rate constants for the olefinic and aromatic hydrocarbons and nitric oxide show only very small variations with temperature change, and they are near the kinetic collision number. These data support the hypothesis that quenching in these cases is associated with the formation of a charge-transfer complex and subsequent chemical interactions between the SO₂(³B₁) molecule and the π-system of these compounds.     

(Nitronyl Nitroxide)‐Substituted Trioxytriphenylamine Radical Cation Tetrachlorogallate Salt: A 2p‐Electron‐Based Weak Ferromagnet Composed of a Triplet Diradical Cation

The synthesis and the solid state magnetic properties of (nitronyl nitroxide)‐substituted trioxytriphenylamine radical cation tetrachlorogallate, NNTOT+·GaCl4? , are reported. In the temperature region between 300 and 3 K, the magnetic behavior is characterized by the strong intramolecular ferromagnetic interaction (J/k_B=+400 K) between the radical ( NN ) and the radical cation ( TOT ⁺) and the weak intermolecular antiferromagnetic interaction (J/k_B=?1.9 K) between NNTOT⁺ ions. Below 3 K, a 3D‐type long‐range magnetic ordering into a weak ferromagnet was observed (T_N=2.65 K). The magnetic entropy (S_mag=8.97 J K^?1 mol^?1) obtained by the heat capacity measurement is in good agreement with the theoretical value of R ln3=9.13 J K^?1 mol^?1 based on the S=1 state.     

Spectroscopic studies of conducting polymers

Electronic and vibrational spectra of poly(p-phenylenevinylene) doped with donors or acceptors have been studied, together with the spectra obtained for the radical ions and divalent ions of its oligomers. The electronic absorption spectra of doped poly(p-phenylenevinylene) in the region from visible to near-infrared show two bands for the H₂SO₄-doped species and one very broad band for the Na-doped species. On the basis of the analyses of resonance Raman spectra, the two electronic absorption bands of the H₂SO₄-doped species are attributed to a polaron-lattice structure, whereas the broad band of the Na-doped species is attributed to overlapping absorptions associated with localized electronic levels of polarons and bipolarons. A Pauli spin susceptibility of the H₂SO₄-doped species is explained by the polaron-lattice structure.     

Strong Antiferromagnetic Coupling of Spins in the (MDABCO+)(C60.−) Salt with 3D Close Packing of the C60.− Radical Anions (MDABCO+: N‐Methyldiazabicyclooctanium Cation)

A new salt, (MDABCO⁺)(C₆₀^.?) ( 1 ; MDABCO⁺=N‐methyldiazabicyclooctanium cation), was obtained as single crystals. The crystal structure of 1 determined at 250 and 100 K showed 3D close packing of fullerenes with eight fullerene neighbors for each C₆₀^.?. These neighbors are located at 10.01–10.11 Å center‐to‐center distances (250 K) and van der Waals interfullerene C???C contacts are formed with four fullerene neighbors arranged in the bc plane. Fullerene ordering observed below 160 K is accompanied by the appearance of one and a half independent C₆₀^.? and trebling of the unit cell along the b axis. Fullerenes are packed closer to each other at 100 K. As a result, fullerenes are located in the three‐dimensional packing at 9.91–10.12 Å center‐to‐center distances and 18 short interfullerene C???C contacts are formed for each C₆₀^.?. Although they are closed packed, fullerenes are not dimerized down to 1.9 K. Magnetic data indicate strong antiferromagnetic coupling of spins in the 70–300 K range with a Weiss temperature of Θ=?118 K. Magnetic susceptibility shows a round maximum at 46 K. Such behavior can be described well by the Heisenberg model for square two‐dimensional antiferromagnetic coupling of spins with an exchange interaction of J/k_B=?25.3 K. This magnetic coupling is one of the strongest observed for C₆₀^.? salts.     

Effect of surface treatments on interfacial characteristics and band alignments of atomic‐layer‐deposited Al2O3 films on GaAs substrates

The GaAs surfaces were passivated by two kinds of chemical pretreatments (using NH₄OH and (NH₄)₂S as passivation agents) for atomic layer deposited (ALD) Al₂O₃ dielectric film growth. The chemical information at Al₂O₃/GaAs interfaces was carefully characterized. The impact of surface treatments on the band alignments of ALD Al₂O₃ films on n? GaAs (100) substrates was also investigated. After postdeposition annealing, the NH₄OH passivated samples not only have a slight increase of the As? As peaks with an appearance of As suboxide (AsO_x) feature at Al₂O₃/GaAs interfaces but also exhibit a serious interfacial interdiffusion between Al₂O₃ and GaAs. However, the (NH₄)₂S passivated samples produce the Ga? S and As? S overlayers on GaAs, effectively preventing from the intermixed diffusion between Al₂O₃ films and GaAs substrates with a sharper interface. Both NH₄OH and (NH₄)₂S passivated Al₂O₃ samples show the same band gap of 6.67 eV. The conduction band offset at Al₂O₃/GaAs interface for the (NH₄)₂S passivated samples have a slight enhancement of 0.14 eV in comparison to NH₄OH passivated ones. Copyright © 2010 John Wiley & Sons, Ltd.     

SINGLET-TRIPLET SPIN TRANSITION IN AN IRON(II) COMPLEX OF 1,10-PHENANTHROLINE-2-CARBOTHIOAMIDE

Abstract

The ⁵⁷Fe Mössbauer effect for the solid complex FeLCl₂·H₂O (L=10-phenanthroline-2-carbothioamide) has been studied between 4.2°K and 298°K. Two overlapping doublets (I and II) are observed, their relative intensity being strongly temperature dependent. The doublets are characterized by δE_Q(I)=0.53 mm sec^?1, δ^IS(I)=+ 0.22 mm sec^?1 and δE_Q(II)=1.33 mm sec^?1, δ^IS(II)=+0.23 mm sec^?1 at 4.2°K. In conjunction with magnetic data, the Mössbauer spectra are interpreted in terms of a singlet-triplet spin transition of the central iron(II) ion. In an applied magnetic field at 4.2°K, H_int=0, V_ZZ(I)>0, and V_ZZ(II)>0. The results are consistent with optical electronic and infrared vibrational spectra.     

2-Chloro-3-fluoropyridine copper(II) complexes and the effect of structural changes on magnetic behavior

Abstract

A family of copper(II) compounds has been prepared with the general formula (2-chloro-3-fluoro-pyridine)₂CuX₂·n(Y), where X?=?Cl, Br, n?=?0,1, and Y?=?methanol or water. For the copper chloride complexes, only solvated structures were obtained (1, X?=?Cl, Y?=?H₂O; 2, X?=?Cl, Y?=?CH₃OH) while for the copper bromide compounds, both a desolvated structure and the methanol solvate were prepared (3, X?=?Br, Y?=?none; 4, X?=?Br, Y?=?CH₃OH). Each compound has been characterized by IR, powder X-ray diffraction, single-crystal X-ray diffraction, and temperature-dependent magnetic susceptibility measurement. Compounds 1 and 4 are isostructural, in the space group Cm, and neither exhibits significant magnetic exchange although superexchange pathways are present. Compound 2 also crystallizes in the Cm space group and exhibits weak ferromagnetic interactions (J/k_B?=?1.72(2) K from the 1D-ferromagnetic chain model) which are likely propagated by hydrogen bonding. Compound 3 crystallizes in the space group P2₁/n with the Cu(II) ion sitting on a crystallographic inversion center. Compound 3 exhibits weak antiferromagnetic exchange via two-halide superexchange typical for similar complexes. The magnetic data for 3 were fit to the 1D-antiferromagnetic chain model resulting in J/k_B?=??1.21(1).