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.     

Development of the diffusive gradient in thin films technique for the measurement of labile mercury species in waters

The diffusive gradients in thin films (DGT) technique, utilizing resin gel with ion-exchange resin Duolite GT73 and new ion-exchange resin Ambersep GT74, was investigated for the accumulation of four mercury species (Hg²⁺, CH₃Hg⁺, C₂H₅Hg⁺, C₆H₅Hg⁺). The diffusion coefficients of mercury species in agarose gel calculated on the basis of Fick’s Law were mercury species-specific. The diffusion coefficients of Hg²⁺ and CH₃Hg⁺ at 25 °C (9.07 ± 0.23 × 10⁻⁶ cm² s⁻¹ and 9.06 ± 0.30 × 10⁻⁶ cm² s⁻¹, respectively) were very similar, but the diffusion coefficients of C₂H₅Hg⁺ (6.87 ± 0.23 × 10⁻⁶ cm² s⁻¹) and C₆H₅Hg⁺ (3.86 ± 0.19 × 10⁻⁶ cm² s⁻¹) were significantly lower. Influence of experimental conditions (pH, selected cations, chlorides and humic substance) on mercury species accumulation by DGT was studied. The DGT technique was applied to river water spiked with mercury species.     

Foreign gas broadening and shift of the strongly “forbidden” lead line at 1278.9 nm

The collisional broadening and shift rate coefficients of the “forbidden“ 6p^{2 3}P₀ → 6p^{2 3}P₁ transition in lead were determined by diode laser absorption measurements performed simultaneously in two resistively heated hot-pipes. One hot-pipe contained Pb vapor and noble gas (Ar or He) at low pressure, while the other was filled with Pb and noble gas at variable pressure. The measurements were performed at temperatures of 1220 K and 1290 K, i.e., lead number densities of 4.8 × 10¹⁵ cm^− 3 and 1.2 × 10¹⁶ cm^− 3. The broadening rates were obtained by fitting the experimental collisionally broadened absorption line shapes to theoretical Voigt profiles. The shift rates were determined by measuring the difference between the peak absorption positions in the spectra measured simultaneously in the heat pipe filled with noble gas at reference pressure and the one with noble gas at variable pressure. The following data for the broadening and shift rate coefficients due to collisions with Ar and He were obtained: γ_B^Ar = (3.4 ± 0.1) × 10^− 10 cm³ s^− 1, γ_B^He = (3.8 ± 0.1) × 10^− 10 cm³ s^− 1, γ_S^Ar = (− 7.3 ± 0.8) × 10^− 11 cm³ s^− 1, γ_S^He = (− 6.5 ± 0.7) × 10^− 11 cm³ s^− 1.     

Structure and high-temperature thermoelectric properties of SrAl2Si2

Single crystals of SrAl₂Si₂ were synthesized by reaction of the elements in an aluminum flux at 1000 °C. SrAl₂Si₂ is isostructural to CaAl₂Si₂ and crystallizes in the hexagonal space group P-3m1 (90 K, a=4.1834 (2), c=7.4104 (2) Å, Z=1, R1=0.0156, wR2=0.0308). Thermal analysis shows that the compound melts at ∼1020 °C. Low-temperature resistivity on single crystals along the c-axis shows metallic behavior with room temperature resistivity value of ∼7.5 mΩ cm. High-temperature Seebeck, resistivity, and thermal conductivity measurements were made on hot-pressed pellets. The Seebeck coefficient shows negative values in entire temperature range decreasing from ∼−78 μV K⁻¹ at room temperature to −34 μV K⁻¹ at 1173 K. Seebeck coefficients are negative indicating n-type behavior; however, the temperature dependence is consistent with contribution from minority p-type carriers as well. The lattice contribution to the thermal conductivity is higher than for clathrate structures containing Al and Si, approximately 50 mW cm⁻¹ K, and contributes to the overall low zT for this compound.     

Kinetics of the CH3 + HCl/DCl → CH4/CH3D + Cl and CD3 + HCl/DCl → CD3H/CD4 + Cl reactions: An experimental H atom tunneling investigation

The kinetics of the radical reactions of CH₃ with HCl or DCl and CD₃ with HCl or DCl have been investigated in a temperature controlled tubular reactor coupled to a photoionization mass spectrometer. The CH₃ (or CD₃) radical, R, was produced homogeneously in the reactor by a pulsed 193 nm exciplex laser photolysis of CH₃COCH₃ (or CD₃COCD₃). The decay of CH₃/CD₃ was monitored as a function of HCl/DCl concentration under pseudo-first-order conditions to determine the rate constants as a function of temperature, typically from 188 to 500 K. The rate constants of the CH₃ and CD₃ reactions with HCl had strong non-Arrhenius behavior at low temperatures. The rate constants were fitted to a modified Arrhenius expression k = QA exp (−E_a/RT) (error limits stated are 1σ + Students t values, units in cm³ molecule⁻¹ s⁻¹): k(CH₃ + HCl) = [1.004 + 85.64 exp (−0.02438 × T/K)] × (3.3 ± 1.3) × 10⁻¹³ exp [−(4.8 ± 0.6) kJ mol⁻¹/RT] and k(CD₃ + HCl) = [1.002 + 73.31 exp (−0.02505 × T/K)] × (2.7 ± 1.2) × 10⁻¹³ exp [−(3.5 ± 0.5) kJ mol⁻¹/RT]. The radical reactions with DCl were studied separately over a wide ranges of temperatures and in these temperature ranges the rate constants determined were fitted to a conventional Arrhenius expression k = A exp (−E_a/RT) (error limits stated are 1σ + Students t values, units in cm³ molecule⁻¹ s⁻¹): k(CH₃ + DCl) = (2.4 ± 1.6) × 10⁻¹³ exp [−(7.8 ± 1.4) kJ mol⁻¹/RT] and k(CD₃ + DCl) = (1.2 ± 0.4) × 10⁻¹³ exp [−(5.2 ± 0.2) kJ mol⁻¹/RT] cm³ molecule⁻¹ s⁻¹.     

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⁺.     

Influence of hole mobility on the response characteristics of p-type nickel oxide thin film based glucose biosensor

RF sputtered p-type nickel oxide (NiO) thin film exhibiting tunable semiconductor character which in turns enhanced its functional properties. NiO thin film with high hole mobility is developed as a potential matrix for the realization of glucose biosensor. NiO thin film prepared under the optimized deposition conditions offer good electrical conductivity (1.5 × 10⁻³ Ω⁻¹-cm⁻¹) with high hole mobility (2.8 cm² V⁻¹ s⁻¹). The bioelectrode (GO_x/NiO/ITO/glass) exhibits a low value of Michaelis–Menten constant (K_m = 1.05 mM), indicating high affinity of the immobilized GO_x toward the analyte (glucose). Due to the high surface coverage (2.32 × 10⁻⁷ mol cm⁻²) of the immobilized enzyme on to the NiO matrix and its high electrocatalytic activity, the prepared biosensor exhibits a high sensitivity of 0.1 mA (mM⁻¹-cm⁻²) and a good linearity from 25 to 300 mg dL⁻¹ of glucose concentration with fast response time of 5 s. Various functional properties of the material (mobility, crystallinity and stress) are found to influence the charge communication feature of NiO thin film matrix to a great extent, resulting in enhanced sensing response characteristics.     

Transport properties of undoped and Br-doped PbTe sintered at high-temperature and pressure ≥4.0 GPa

The thermoelectric properties of nominally undoped PbTe and Br doped PbTe materials sintered at high-pressure and high-temperature (HPHT) have been studied. All samples show n-type semiconducting behavior with negative thermopower. For undoped PbTe, four different HPHT treatments were performed at pressures between 4.0 and 6.5 GPa. PbTe doped with Br at 0.5, 1.0, 2.0, 3.0×10¹⁹ cm⁻³ was HPHT treated at 4.0 GPa and 1045 °C. As the dopant concentration increases, the absolute thermopower decreases, thermal conductivity increases, and electrical resistivity decreases. At a nominal dopant concentration of 1.0×10¹⁹ cm⁻³, carrier mobility of 1165 cm²/V s and dimensionless thermoelectric figure-of-merit, ZT, of around 0.27 at 300 K were obtained. These results demonstrate that HPHT post-processing is a viable and controllable way of tuning the thermoelectric properties of PbTe-based materials.     

High-temperature crystal structure and transport properties of the layered cuprates Ln2CuO4, Ln=Pr, Nd and Sm

High-temperature crystal structure of the layered cuprates Ln₂CuO₄, Ln=Pr, Nd and Sm with tetragonal T′-structure was refined using X-ray powder diffraction data. Substantial anisotropy of the thermal expansion behavior was observed in their crystal structures with thermal expansion coefficients (TEC) along a- and c-axis changing from TEC(a)/TEC(c)≈1.37 (Pr) to 0.89 (Nd) and 0.72 (Sm). Temperature dependence of the interatomic distances in Ln₂CuO₄ shows significantly lower expansion rate of the chemical bond between Pr and oxygen atoms (O1) belonging to CuO₂-planes (TEC(Pr-O1)=11.7 ppm K⁻¹) in comparison with other cuprates: TEC (Nd-O1)=15.2 ppm K⁻¹ and TEC (Sm-O1)=15.1 ppm K⁻¹. High-temperature electrical conductivity of Pr₂CuO₄ is the highest one in the whole studied temperature range (298-1173 K): 0.1-108 S/cm for Pr₂CuO₄, 0.07-23 S/cm for Nd₂CuO₄ and 2×10⁻⁴-9 S/cm for Sm₂CuO₄. The trace diffusion coefficient (D_T) of oxygen for Pr₂CuO₄ determined by isotopic exchange depth profile (IEDP) technique using secondary ion mass spectrometry (SIMS) varies in the range 7.2×10⁻¹³ cm²/s (973 K) and 3.8×10⁻¹⁰ cm²/s (1173 K) which are in between those observed for the manganese and cobalt-based perovskites.     

Two new coordination polymers of Robson-type macrocycle bridged by perchlorate anions: Structure,electrochemistry and magnetism

Two new coordination polymers of Robson-type macrocycles, [Cu₂L¹(μ-ClO₄)₂]_∞ (1) and [Cu₂L²(μ-ClO₄)₂]_∞ (2) (where H₂L¹and H₂L² are the [2+2] condensation products of 2,6-diformyl-4-flurophenol with 1,3-diaminopropane and 2-hydroxy-1,3-diaminopropane, respectively), have been synthesized and characterized. The intriguing feature is that intermolecular perchlorato bridges occur between adjacent copper(II) centers. The cyclic voltammograms of the complexes show that each complex undergoes two pseudo-reversible processes with the half wave potentials, −0.361 V and −0.729 V for 1, and −0.372 V and −0.744 V for 2, respectively. Magnetic susceptibility was measured for 1 and 2 over a temperature range of 2–300 K. The optimized magnetic data were J = −359.6 cm⁻¹, j′ = −30 cm⁻¹ and R = 6.8 × 10⁻⁸ for 1 and J = −411 cm⁻¹, j′ = −26 cm⁻¹ and R = 2.4 × 10⁻⁷ for 2, respectively. The data reveal antiferromagnetic couplings between the copper(II) ions of intra- and intermolecular units.     

Lithium conductivity in an Li-bearing double-ring silicate mineral, sogdianite

The crystal structure of an Li-bearing double-ring silicate mineral, sogdianite ((Zr_1.18Fe³⁺_0.55Ti_0.24Al_0.03)(?_1.64,Na_0.36)K_0.85[Li₃Si₁₂O₃₀], P6/mcc, a≈10.06 Å, c≈14.30 Å, Z=2), was investigated by neutron powder diffraction from 300 up to 1273 K. Rietveld refinements of displacement parameters revealed high anisotropic Li motions perpendicular to the crystallographic c-axis, indicating an exchange process between tetrahedral T2 and octahedral A sites. AC impedance spectra of a sogdianite single crystal (0.04×0.09×0.25 cm³) show that the material is an ionic conductor with conductivity values of σ=4.1×10⁻⁵ S cm⁻¹ at 923 K and 1.2×10⁻³ S cm⁻¹ at 1219 K perpendicular to the c-axis, involving two relaxation processes with activation energies of 1.26(3) and 1.08(3) eV, respectively.     

Binuclear ruthenium and osmium mixed-valence complexes containing fused and flexible polypyridyl bridging ligands

{Os(bpy)₂}²⁺ and {Ru(CN)₄}²⁻ mononuclear and binuclear complexes with ligands 2,3-di-(2-pyridyl)quinoxaline (dpq) and dipyrido[2,3-a:3′,2′-c]phenazine (ppb) have been prepared. For the binuclear complexes a splitting in oxidation potentials is observed consistent with the formation of mixed-valence species with comproportionation constants (K_com) ranging from 2.5 × 10⁴ to 1.8 × 10⁶. The electronic absorption spectra of the mixed-valence species reveal IVCT transitions in the near infrared region. The absorption maximum for the IVCT band ranges from 5800 to 9980 cm⁻¹ and the extinction coefficients from 80 to 6300 M⁻¹ cm⁻¹. In general the {Os(bpy)₂}²⁺ complexes show larger K_com values and more intense IVCT bands than the corresponding {Ru(CN)₄}²⁻ complexes.     

Environmental assessment of CFC alternatives: Rate constants for the reactions of OH radicals with fluorinated compounds

The rate constants for the reactions of OH radicals with CF₃OCHFCF₃, and CF₃CHFCF₃ have been measured over the temperature range 250-430 K. Kinetic measurements have been carried out using the flash photolysis, and laser photolysis methods combined, respectively, with the laser induced fluorescence technique. The influence of impurities in the samples has been investigated by using gas chromatography. No sizable effect of impurities was found on the measured rate constants of these fluorinated compounds, if the purified samples were used in the measurements. The following Arrhenius expressions were determined: k(CF₃OCHFCF₃) = (4.39 ± 1.38) × 10⁻¹³ exp[−(1780 ± 100)/T] cm³ molecule⁻¹ s⁻¹, and k(CF₃CHFCF₃) = (6.19 ± 2.07) × 10⁻¹³ exp[−(1830 ± 100)/T] cm³ molecule⁻¹ s⁻¹.     

Cu22Bi12S21Cl16—A mixed conductor with fast one-dimensional copper(I) ion transport

Melting reactions of Cu, CuCl, S, and Bi₂S₃ yield black, shiny needles of Cu₂₂₍₁₎Bi₁₂S₂₁₍₁₎Cl₁₆₍₁₎. The compound decomposes peritectically at 649(5) K. Oxidation state +I of the copper atoms is supported by Cu-K-XANES. The compound crystallizes in the hexagonal space group P6/m with a=2116.7(7) pm and c=395.17(5) pm. Seven anions coordinate each of the two independent bismuth cations in the shape of mono-capped trigonal prisms. These polyhedra share edges and faces to form trigonal and hexagonal tubes running along [0 0 1]. The hexagonal tubes are centered by chloride ions, which are surrounded by disordered copper cations. The majority of copper cations are distributed over numerous sites between the tubes. The Joint Probability Density Function (JPDF) reveals a continuous pathway along [0 0 1]. The high mobility of the copper cations along [0 0 1] was demonstrated by impedance spectroscopy and DC polarization measurements on single crystals. The ionic conductivity at 450 K is about σ_ion=0.06 S cm⁻¹, and the activation energy for Cu⁺ ion conduction is E_a=0.44 eV. The chemical diffusion coefficient of copper is in the order of D_cu^δ=10¹⁹ cm⁻³ at 420 K. The electronic band gap (p-type conductor) was determined as E_g=0.06 eV. At room temperature the thermal conductivity of a pressed pellet is about κ=0.3 W K⁻¹ m⁻¹ and the Seebeck coefficient is S=43 μV K⁻¹.     

Electrosorption of Os(III)-complex at single-wall carbon nanotubes immobilized on a glassy carbon electrode: application to nanomolar detection of bromate, periodate and iodate

A simple procedure was developed to prepare a glassy carbon electrode modified with single-wall carbon nanotubes (SWCNTs) and Os(III)-complex. The glassy carbon (GC) electrode modified with CNTs was immersed into Os(III)-complex solution (direct deposition) for a short period of time (60 s). 1,4,8,12-Tetraazacyclotetradecane osmium(III) chloride, (Os(III)LCl₂)·ClO₄, irreversibly and strongly adsorbed on SWCNTs immobilized on the surface of GC electrode. Cyclic voltammograms of the Os(III)-complex-incorporated-SWCNTs indicate a pair of well defined and nearly reversible redox couple with surface confined characteristic at wide pH range (1-8). The surface coverage (Γ) and charge transfer rate constant (k_s) of the immobilized Os-complex on SWCNTs were 3.07 × 10⁻⁹ mol cm⁻², 5.5 (±0.2) s⁻¹, 2.94 × 10⁻⁹ mol cm⁻², 7.3 (±0.3) s⁻¹ at buffer solution with pH 2 and 7, respectively, indicate high loading ability of SWCNTs for Os(III) complex and great facilitation of the electron transfer between electroactive redox center and carbon nanotubes immobilized on the electrode surface. Modified electrodes showed higher electrocatalytic activity toward reduction of BrO₃⁻, IO₃⁻ and IO₄⁻ in acidic solutions. The catalytic rate constants for catalytic reduction bromate, periodate and iodate were 3.79 (±0.2) × 10³, 7.32 (±0.2) × 10³ and 1.75 (±0.2) × 10³ M⁻¹ s ⁻¹, respectively. The hydrodynamic amperometry of rotating modified electrode at constant potential (0.3 V) was used for nanomolar detection of selected analytes. Excellent electrochemical reversibility of the redox couple, good reproducibility, high stability, low detection limit, long life time, fast amperometric response time, wide linear concentration range, technical simplicity and possibility of rapid preparation are great advantage of this sensor.     

Highly phosphonated polypentafluorostyrene: Characterization and blends with polybenzimidazole

In this study we present results of the conductivity and resistance to thermooxidative and condensation reactions of a highly phosphonated poly(pentafluorostyrene) (PWN2010) and of its blends with poly(benzimidazole)s (PBI). This polymer, which combines both: (i) a high degree of phosphonation (above 90%) and (ii) a relatively high acidity (pK_a (–PO₃H₂ ↔ –PO₃H⁻) ∼ 0.5) due to the fluorine neighbors, is designed for low humidity operating fuel cell. This was confirmed by the conductivity measurements for PWN2010 reaching σ = 5 × 10⁻⁴ S cm⁻¹ at 150 °C in dry N₂ and σ = 1 × 10⁻³ S cm⁻¹ at 150 °C (λ = 0.75). Furthermore, this polymer showed only 48% of anhydride formation when annealing it at T = 250 °C for 5 h and only 2% weight loss during a 96 h Fenton test. These properties combined with the ability of the PWN2010 to form homogeneous blends with polybenzimidazoles resulting in stable and flexible polymer films, makes PWN2010 a very promising candidate as a polymer electrolyte for intermediate- and high-temperature fuel cell applications.     

The Heusler phases LiRh2Si and LiRh2Ge: Synthesis, structure and properties

The isostructural Heusler phases LiRh₂Si and LiRh₂Ge have been synthesized from the elements and an excess of lithium at 1000 °C. Both materials adopt the CuMn₂Al crystal structure, space group Fm−3m (No. 225) with the room temperature lattice parameter a=5.747(1) Å [Vol=189.866(1) Å³] and a=5.847(1) Å [Vol=199.88(6) Å³] for LiRh₂Si and LiRh₂Ge, respectively. X-ray analyses suggest mixed site occupancy of the form Li_1−xRh₂Si_1+x (x<0.4), but not for LiRh₂Ge. Both materials are diamagnetic, χ_mol(LiRh₂Si)=−6×10⁻⁵ cm³(mole)⁻¹ and χ_mol(LiRh₂Ge)=−10×10⁻⁵ cm³(mole)⁻¹ and metallic with room temperature resistivities of approximately 19 and 32 μΩ cm, respectively. These properties are consistent with the calculated electronic structure.     

Anomalous charge transport in CeB6

The comprehensive study of conductivity σ, Hall coefficient R_H and Seebeck coefficient S has been carried out on high-quality single crystals of CeB₆ in a wide range of temperatures 1.8-300 K. An anomalous behavior of all transport characteristics (σ, R_H, S) was found for the first time in the vicinity of T^*≈80 K. The strong decrease of conductivity σ as well as the unusual asymptotic behavior of Seebeck coefficient S(T)∼−ln T observed below T^* allowed us to conclude in favor of crossover between different regimes of charge transport in CeB₆. The pronounced change of Hall mobility μ_H, which diminishes from the maximum value of 20 cm²/(V s) at T^* to the values of ∼6 cm²/(V s) at T∼10 K, seems to be attributed to the strong enhancement of charge carriers scattering due to fast spin fluctuations on Ce-sites. The low-temperature anomalies of the charge transport characteristics are compared with the predictions of the Kondo-lattice model.     

A novel gel polymer electrolyte based on poly ionic liquid 1-ethyl 3-(2-methacryloyloxy ethyl) imidazolium iodide

Poly ionic liquid 1-ethyl 3-(2-methacryloyloxy ethyl) imidazolium iodide (PEMEImI) as a single-ion conductor was designed and synthesized. When appropriate amount of suitable plasticizers, I₂ and polyacrylonitrile (PAN) were incorporated into it, the complex formed gel polymer electrolyte. Chemical structure, thermal behavior and ionic conductive properties of the gel polymer electrolyte were investigated by Raman spectra, UV-Vis spectra, differential scanning calorimetry (DSC), and complex impedance analysis, respectively. For the new gel polymer electrolyte, the ionic conductivity of about 1 × 10⁻³ S cm⁻¹ at room temperature was achieved.