Non-ohmic electrical conductivity of β-rhombohedral boron in high electric fields

The low-field conductivity of β-rhombohedral boron follows Mott's law of variable-range hopping. Recent improvements in the energy band scheme attribute the hopping centers to specific, partly occupied states in the band gap, evoked by structural defects, in particular unoccupied B(13) sites. Band type conductivity is also possible, after valence electrons have been excited into unoccupied gap states. An experimental tool to gain an insight into the transport mechanism of semiconductors is the field-dependence of the electrical conductivity. For the interpretation of such experiments various theories are at disposal: the classical model of hot electrons, the Poole-Frenkel model, models of non-thermally activated hopping by Mott and Shklovskii, the Model of the field-dependence of small polarons mobility, the model of space-charge-limited currents. New measurements of the electrical conductivity in the temperature range 187-303 K at field strength up to 8 k V cm⁻¹ are presented and discussed according to the above-mentioned theories together with a critical review of the previous measurements of other authors. Three ranges are to be distinguished. (i) Up to about 200 V cm⁻¹: ohmic behavior; (ii) between about 200 V cm⁻¹ and about 20 kV cm⁻¹: non-ohmic behavior with a temperature-dependent field-dependence; (iii) above the “electrical breakdown” at about 20 kV cm⁻¹: non-ohmic behavior with I:E² independent of temperature.     

Infrared spectrum and absorption coefficient of the cofactor flavin in water

Flavins play a key role as redox cofactors of enzymes involved in important metabolic processes. Moreover, they undergo photochemical reactions as chromophores in sensors of blue light or magnetic field in many organisms. The reaction mechanisms of flavoproteins have been investigated by infrared spectroscopy and theoretical studies. However, basic information on flavins in the infrared spectral range has been missing, such as absorption spectra in water and absorption coefficients. Here, the cofactors flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) were investigated in aqueous medium by Fourier transform infrared spectroscopy. Transmission and attenuated total reflection (ATR) configuration were employed in direct comparison. Absorption spectra in the range of 920–1800 cm⁻¹ were determined after accurate subtraction of the contributions from the water vibrations. The important carbonyl vibrations were resolved at 1661 and 1712 cm⁻¹. The absorption spectra may serve as a reference for theoretical and experimental studies on the effect of the microenvironment on the flavin cofactor. Furthermore, the molar absorption coefficient of FAD at 1547 cm⁻¹ was determined to 2200 L mol⁻¹ cm⁻¹ with an integral absorption coefficient of ∼50,000 L mol⁻¹ cm⁻². These values are prerequisite for the determination of reaction yields in flavoproteins from reaction-induced difference spectra.     

Repulsive ∑ and low-lying (⩾ 1.9 eV) ∑ states of BeO

Ab initio calculations have been carried out on the lowest ³∑⁻ and ³∑⁺ states of beryllium oxide. A “double zeta plus polarization” set of Slater functions was used. The self-consistent-field wavefunction for the ³∑⁻ state dissociates properly to ground state Be and O atoms and is repulsive. Electron correlation was explicitly considered for the ³∑⁺ state using “first-order” wavefunctions, which have yielded reliable dissociation energies for other diatomic molecules. The ³∑⁺ state, which has not been observed experimentally, is predicted to lie 1.91 eV above the lowest ¹∑⁺ state. Predicted spectroscopic constants for the ³∑⁺ state are r_e = 1.384 Å, ω_e = 1234 cm⁻¹, and B_e = 1.527 cm⁻¹.     

Photophysical properties of gatifloxacin in aqueous solution by laser flash photolysis and pulse radiolysis

GFX in water, at pH 7.0, shows intense absorption bands with peaks at 284 and 333 nm, (ε=24,670 and 12,670 M⁻¹ cm⁻¹). Both the absorption and emission properties of GFX were pH-dependent; the pK_a values for the protonation equilibria of the ground state (5.7 and 8.9) and excited singlet state (3.6 and 7.5) of GFX were determined spectroscopically. GFX fluoresces weakly, with a maximum quantum yield for fluorescence emission (0.06) at pH 4.7. A series of experiments were performed to characterize the transient species of GFX in aqueous solution using laser flash photolysis and pulse radiolysis. GFX undergoes monophotonic photoionization with a quantum yield of 0.16 on a 355 nm laser excitation. This process leads to the formation of a long-lived cation radical with a maximum absorption at 380 nm. Triplet-triplet absorption had maximum absorption at 510 nm. The reaction of GFX with one-electron oxidant N₃• was investigated and the bimolecular rate constant was determined to be 3.1×10⁹ M⁻¹ s⁻¹.     

Heavy doping of H ion in 12CaO·7Al2O3

12CaO·7Al₂O₃ (C12A7, mayenite), which has a nanoscale porous structure that can accommodate extraframework species such as hydride (H⁻), oxide (O²⁻), hydroxide (OH⁻) ions, and electrons, has been doped with H⁻ ions to investigate its effects as dominant extraframework species. Chemical doping with CaH₂ enables the concentration of H⁻ ions to reach almost the theoretical maximum. The concentration of H⁻ ions is characterized by optical absorption intensity ascribed to photoionization of H⁻ ions, and ¹H magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. Persistent electron generation, which is accompanied by the formation of an F⁺ absorption band and electrical conductivity, by irradiation with ultraviolet light at room temperature increases as the H⁻ ion doping increases until it reaches half the theoretical maximum and then decreases as the H⁻ ion concentration increases further. This dependence indicates that both H⁻ and O²⁻ ions are necessary for the generation of persistent electrons.     

Light induced magnetic properties of spiropyrane tris(oxalato)chromate (III) single crystals

The effect of UV light on Weiss temperature and ESR spectra in 1-isopropyl-3, 3, 5′, 6′-tetramethylspiro[indolin-2,2′-[2H]pyrano[3,2-b]pyridinium] tris(oxalato)chromate (III) (Sp₃Cr(C₂O₄)₃) has been found. Additional line has been observed in the ESR spectra of irradiated samples in “strong” magnetic fields of ~15 kOe. The analysis of angular dependences of the ESR spectra allowed a contribution of Cr³⁺ ions to magnetic properties of Sp₃Cr(C₂O₄)₃ to be determined. The zero-field splitting parameters D=0.619 cm⁻¹, E=0.024 cm⁻¹ were derived from the experimental data. The parameters were typical for Cr³⁺ in the chromium oxalate. Weiss temperature changed sign from 25 to −25 K under UV irradiation. The value of Weiss temperature and its changing cannot be explained by exchange interaction, dipole-dipole interaction or the effect of crystal field. The existence of Weiss temperature is explained by the changes in amount and spin of paramagnetic particles. The change is due to thermoactivated redistribution of electrons between chromium ions and spiropyrane molecules. Light-induced transfer of electrons is also explaining the change in sign of Weiss temperature under UV irradiation.     

Intramolecular energy transfer in actinide complexes of 6-methyl-2-(2-pyridyl)-benzimidazole (biz): comparison between Cm and Tb systems

Coordination of the 6-methyl-2-(2-pyridyl)-benzimidazole ligand with actinide and lanthanide species can produce enhanced emission due to increased efficiency of intramolecular energy transfer to metal centers. A comparison between the curium and terbium systems indicates that the position of the ligand's triplet state is critical for the enhanced emission. The energy gap between the ligand's triplet state and the acceptor level in curium is about 1000 cm⁻¹, as compared to a ~600 cm⁻¹ gap in the terbium system. Due to the larger gap, the back transfer with curium is reduced and the radiative yield is significantly higher. The quantum yield for this “sensitized” emission increases to 6.2%, compared to the 0.26% value attained for the metal centered excitation prior to ligand addition. In the terbium case, the smaller donor/acceptor gap enhances back transfer and the energy transfer is less efficient than with the curium system.     

Synthesis of a novel bistriazene reagent 4,4′-bis[3-(4-phenylthiazol-2-yl)triazenyl]biphenyl and its highly sensitive color reaction with mercury(II)

We report the synthesis of a novel bistriazene, 4,4′-bis(3-(4-phenylthiazol-2-yl)triazenyl)biphenyl (BPTTBP), and its highly sensitive color reaction with Hg²⁺. The new reagent was synthesized in good yield by coupling 2-amino-4-phenylthiazole with 4,4′-biphenyldiamine bisdiazonium salt. Using a blend of surfactants N-cetylpyridinium chloride (CPC) and polyethylene glycol n-octanoic phenyl ether (OP) as a micelle sensitizer, the red colored reagent assembles with Hg²⁺ in pH 9.8 borate buffer according to a 1:1 stoichiometry, forming a blue oligomeric/polymeric chelating complex with a high apparent stability constant (1.1 × 10⁸ M⁻¹). Whereas the maximum absorption of reagent occurs at 510 nm with an extinct coefficient of 1.35 × 10⁴ M⁻¹ cm⁻¹, the complex absorbs at 611 nm, with an apparent extinct coefficient of 1.04 × 10⁵ M⁻¹ cm⁻¹. Beer's law is obeyed in the range of 0-15 μg/25 mL Hg²⁺, and Sandell's sensitivity is 1.92 × 10⁻³ μg/cm². In the presence of thiourea and Na₄P₂O₇ as masking agents, the method was found free from interferences of foreign ions commonly occurring with mercury. The optimized protocol has been successfully applied to spectrophotometric determination of mercury in waste water samples. The features of the new reagent associated with its special structure were discussed, and an unprecedented “domino effect” was proposed to account for its unique chelating stoichiometry with Hg²⁺.     

The determination of pesticidal and non-pesticidal organotin compounds in water matrices by in situ ethylation and gas chromatography with pulsed flame photometric detection

The concurrent determination of pesticidal and non-pesticidal organotin compounds in several water matrices, using a simultaneous in situ ethylation and liquid–liquid extraction followed by splitless injection mode capillary gas chromatography with pulsed flame photometric detection, is described. The speciation analysis of nine organotin compounds includes low molecular weight–low boiling (non-pesticidal) and high molecular weight–high boiling analytes (pesticidal) of significant environmental interest. The minimum time for sodium tetraethylborate alkylation, using mechanical agitation, is determined to be 15 min in order to ensure the complete derivatization of the entire list of analytes. The utilization of a “hot needle” and a rapid injection rate is shown to be an efficacious means to eliminate “mass” or “needle” discrimination when determining the mixture of organotin compounds. Method detection limits are calculated to be in the low ng L^{− 1} range. The final method is applied to various water samples; storm water from the Cincinnati area demonstrated low native levels of three of the organotin compounds.     

Halide doping of ferromagnetic europium chalcogenides

The europium chalcogenides EuO, EuS, and EuSe are insulating ferromagnets at low temperatures. Rather large increases in their Curie temperature can be effected by doping to increase their electrical resistivity. Previously the doping species have been cation substitutions of “trivalent” rare earth ions or Eu-metal. It is shown here that the monovalent halide ions, particularly Cl⁻, are very effective in increasing the conductivity and Curie temperature of EuS and EuSe.     

New concept for the preparation of potassium sodides: the use of hexane as a non-polar solvent

NaK alloy in contact with 15-crown-5 hexane solution became potassium sodide K⁺(15-crown-5)₂Na⁻. After the evaporation of hexane the crystalline solid product was analyzed by X-ray diffraction and the lattice parameters were calculated. The potassium sodide thus obtained could be easily dissolved in tetrahydrofuran. A deep blue solution containing sodium anions and complexed potassium cations was formed with a very low concentration of solvated electrons, i.e. of the order of 10⁻⁷ M. Potassium anions were not detected in this case. A new crystalline potassium sodide K⁺(DCH-24-crown-8)₂Na⁻ was obtained using NaK alloy and dicyclohexano-24-crown-8 hexane solution.     

A Raman microspectroscopic study of organic inclusions in “watermelon” tourmaline from the Paprok mine (Nuristan, Afghanistan)

An exceptional mineral specimen, particularly interesting for collectors and researchers, is “watermelon” tourmaline, mined in the Paprok region in Nuristan, Afghanistan. A pale pink core (zone III) is surrounded by a thin, colorless zone (zone II), and a green outer zone (zone I). Differences between zones are clearly seen in the SEM picture, i.e., diversity in the size of constituent forms, and also the presence of inclusions of alien mineral phases which range from a few dozen to a few μm. In the line mapping experiment the Raman spectra were collected from the inner core towards the outer rim. There are observed typical for tourmaline (elbaite) groups of bands: at ca. 3600, 1100, 700, 400 and 250 cm⁻¹. Raman measurements determine the differences in the local chemical composition of particular zones. Interestingly, there is a new band emerging in the outer zone at ca. 840 cm⁻¹, and it could be due to the SiO₄ stretching vibrational modes predisposed by the Fe ion presence in the mineral. The organic inclusions are present in the whole volume of the investigated gem. Organic inclusion content appears to be versatile. Raman spectra confirm the presence of hydrogen sulphide (2610 cm⁻¹), methane (2912 cm⁻¹), ethane (2950 cm⁻¹), and propane (2895 cm⁻¹). In the corresponding bending vibrations region apart from CH₂, CH₃ vibrations and C–O stretching (CO₂) at (1280, 1390 cm⁻¹), characteristic bands are also observed at ca. 1340, 1600 and 1660 cm⁻¹. The last ones possibly mark the evolution of graphitization process occurring from fluid saturated inclusions within condensed ring aromatics (1660 cm⁻¹ band), towards slightly more organized, although still amorphous forms of carbonaceous matter (1340 and 1600 cm⁻¹).     

Mechanochemical synthesis and physical–chemical properties of carbon–fluorocarbon nanocomposition materials. A review

There are the described novel class of porous carbon–fluorocarbon nanocomposition materials “C–CF_1+x”, prepared via the mechanochemical activation (MA) in the heterogeneous mixed systems “nano-C–nano-CF_1+x”, having an atomic ratio C:F as 1.14–4.0. As nano-C it was used thermally expanded graphite TEG and mesoporous carbon material NUMS (free porosity 45–95% and specific area 25–400 m²/g). As nano-CF_1+x these were used a superstoichiometric fluorocarbons FS and FT (CF_1.18–1.25) having the coherent diffraction area (CDA) ∼20–25 Å, free porosity 60–70%, sum O + H₂O in mixtures ∼0.1–0.5 wt.%, and metals sum <0.01 wt.%.Prepared nanocomposites “C–CF_1+x” were studied by FTIR, Raman, XPS C1s, O1s, F1s, X-ray diffraction and by chemical C, H, F-analyses. It was shown, that decrease of weight in systems “C–CF_1+x” does not exceed 0.5 wt.% and stated two main features of the temporary dynamics in changes for all MA-products. These are monotonous changes in bulk properties, such as decrease of C-nanophase relative amounts, confirmed with XRD. Simultaneously, a decrease of sp³-C–F and sp³-CF₂-groups at 1200 and 1320 cm⁻¹ and an origination of sp³-C–F-groups at 1080–1120 cm⁻¹, typical for C₂F-like structures are observed. Decrease of specific surface is corresponding to decrease in CDA sizes and dencity for all MA-products.O-containing admixtures in starting materials have a key influence to interactions in nano-“C–CF_1+x” systems during MA-processing, despite to their low content. The main O-contained participant is H₂O and it is interaction with sp³-C–F–bonds is leading to primary hydrolytic substitution of F onto OH with the origin of surface sp³-C–OH-bonds and their subsequent transformations into edged sp²>CO or/and into ester bridges C–O–C among basal and edged nano-C and nano-fluorocarbon blocks. The presence of basal sp³-C–OH and edged sp²>CO (sp²>COOH)-groups is confirmed by FTIR and XPS C1s and O1s spectra for all MA-nanocomposites in “C–CF_1+x” systems. Changes in the surface properties of prepared MA-“C–CF_1+x” nanocomposites are corresponded to the origin of extrema in properties for MA-time 6–10 min, such as content of surface C-nanophases and surface F with the simultaneous appearance of extrema in the specific electro conductivities and capacitances. The nature of observed phenomena is explained by the origin of chemical carbon nanosized contacts on particle surface and it is possible to use for practical applications. The main difference among MA-“C–CF_1+x” nanocomposites is that the “FS–TEG” systems are dominated by sp²-C–sp³-C–F electroconductive bridges, whereas in “NUMS–FT” systems the character of conductivity is determined by contribution of ester bridges C–O–C.     

Synthesis and physico-chemical characterization of Au/TiO2 nanostructures formed by novel “cold” and “hot” nanosoldering of Au and TiO2 nanoparticles dispersed in water

A novel approach to synthesize Au/TiO₂ nanostructures with interesting optical properties is presented and discussed. It is based on the nanoparticle “cold” or “hot” nanosoldering occurring when two water suspensions of Au and TiO₂ nanoparticles are merely mixed at room temperature or laser irradiated after mixing.Thanks to the high fraction and mutual reactivity of surface species, immediately after the mixing process, the encounters between Au and TiO₂ nanoparticles in liquid phase are enough for “cold” nanosoldering of gold nanoparticles onto TiO₂ nanoparticles to occur. The optical characterizations show that this fast process (timescale less than 1 min) is followed by a slower process, attributable to some change of the Au nanoparticles. This latter process is significantly accelerated by the 532 nm laser light illumination. The structural and optical properties of “cold” and “hot” nanosoldered Au-TiO₂ nanoparticles were investigated by TEM, UV-vis and fluorescence spectroscopies.Interesting optical limiting response was detected at laser fluences above 0.8 J/cm². The nature of the nonlinear effect was investigated by the Z-scan technique, determining both the nonlinear absorption coefficient and the refraction index. Such interesting non-linear optical properties are worth to be tailored for specific applications.     

A DFT study of lithium battery materials: application to the β-VOXO4 systems (X=P, As, S)

VOXO₄ systems have been considered as potential lithium battery electrodes. They mainly present two distinct structural types: the tetragonal “α” type with a two-dimensional framework, and the three-dimensional orthorhombic “β”. DFT calculations were performed on this latter system for several β-Li_xVOXO₄ compounds (x=0, 1; X=P, As, S). They allowed to propose structural models for VOAsO₄ and LiVOSO₄, not fully crystallographically well described yet. Based on an experimental model of two-phase processes, these calculations led also to a good simulation of electrochemical potential values. A density of states analysis put in evidence the “inductive effect” and the role played by (XO₄)ⁿ⁻ groups inside the host frameworks on these potentials.     

Implementation of an automatic standard addition method in a flow-batch system: application to copper determination in an alcoholic beverage by atomic absorption spectrometry

A novel strategy for implementing the automatic standard addition method (SAM) is described. By using a flow-batch system that presents the intrinsic favourable characteristics of the flow and batch techniques, the proposed strategy performs fast standard additions with sufficient flexibility and versatility and employs only one standard solution per analyte. To calculate the analyte concentration, a mathematical model based on a classical SAM and flow variables of the system was developed. The proposed flow-batch SAM was applied to copper determination by flame atomic absorption spectrometry (AAS) in sugar cane-made alcoholic beverages, known as “Cachaça”, available in Brazil. A SAM has been recommended for these analyses because “Cachaças” presents a significantly different composition causing matrix effects and copper determination by calibration using matrix-matching standards can yield inaccurate results. The results show good agreement between the obtained values with the proposed flow-batch SAM and a manual SAM. The mean relative errors and overall standard deviations were always <1.0% (n=6) and 0.2 mg l⁻¹, respectively, for 1.0-7.0 mg l⁻¹ Cu. By using five standard addition levels, the sample throughput was 70 h⁻¹ and the consumption of sample and standard solution were 1.5 and 0.5 ml per analysis, respectively.     

Quantitative analysis of synthetic calcium carbonate polymorphs using FT-IR spectroscopy

Fourier Transform Infrared Spectroscopy (FT-IR) was used successfully for the simultaneous quantitative analysis of calcium carbonate phases (calcite, aragonite, vaterite) in ternary mixtures. From the FT-IR spectra of pure calcite, aragonite and vaterite powders with KBr, the absorptivities, α, of the absorption bands at 713 cm⁻¹ for calcite, 745 cm⁻¹ for vaterite, 713 and 700 cm⁻¹ for aragonite, were determined. In order to overcome the absorption band overlapping a set of equations based on Beer's law was developed. The detection limits were also established and found to be 1.1×10⁻⁴ mg calcite per mm² of pellet at 713 cm⁻¹, 3.6×10⁻⁴ mg aragonite per mm² of pellet at 700 cm⁻¹, 1.8×10⁻⁴ mg aragonite per mm² of pellet at 713 cm⁻¹ and 3.1×10⁻⁴ mg vaterite per mm² of pellet at 745 cm⁻¹. Analysis of a known ternary mixture of calcium carbonate polymorphs tested the validity of the method.     

Chemistry of organophosphonate scale growth inhibitors: two-dimensional, layered polymeric networks in the structure of tetrasodium 2-hydroxyethyl-amino-bis(methylenephosphonate)

Aminomethylene phosphonates are important scale inhibitors applied in diverse areas of technology. This study adds to the existing body of information on this subject and reports the crystal and molecular structures of tetrasodium 2-hydroxyethylamino-bis(methylene-phosphonate) decahydrate ([Na₄(HOCH₂CH₂N(CH₂PO₃)₂)]·10H₂O, 1). The crystal structure of 1 could be described as two-dimensional polymeric layered structure hydrogen bonded into a 3D supramolecular polymeric network. The structure of the tetraanion consists of a “three-arm” backbone stemming from the N atom. Two “arms” are deprotonated methylene phosphonate (-CH₂PO₃²⁻) moieties and the third is a hydroxyethyl (-CH₂CH₂OH) moiety. One Na cation forms an intramolecular complex with two oxygens from separate phosphonate groups, a hydroxyl oxygen, the nitrogen and two lattice water molecules. The position of this Na cation points to a possible coordination site for Ca in a proposed Ca-HEABMP complex (HEABMP=2-hydroxyethylamino-bis(methylene-phosphonate).     

Determination of selenite and selenate in drinking water: a fully automatic on-line separation/pre-concentration system coupled to electrothermal atomic spectrometry with permanent chemical modifiers

A time-based flow injection (FI) separation pre-concentration system coupled to an electrothermal atomic absorption spectrometer (graphite furnace) has been developed for the direct ultra-trace determination of selenite and selenate in drinking water. The pre-concentration of both forms of selenium is carried out onto a micro-column packed with an anionic resin (Dowex 1X8) that is placed in the robotic arm of the autosampling device. Selenite and selenate are sequentially eluted with HCl 0.1 M and HCl 4 M, respectively. The interference of large quantities of chloride during selenium atomisation is prevented by using iridium as a “permanent” chemical modifier. The features of the pre-concentration separation system for both species are: 53% efficiency of retention and an enhancement factor of 82 for a pre-concentration time of 180 s (sample flow rate=3 ml min⁻¹) with HCl elution volumes of 100 μl. The detection limit (3 s) is 10 ng l⁻¹ for the two species and the relative standard deviation (n=10) at the 200 ng l⁻¹ level is 3.5% for selenite and 5.6% for selenate. The addition of selenite and selenate stock standard solutions to tap water samples yields a 97-103% recovery of both species.