Charge transport properties in microcrystalline KDyFe(CN)6

Microcrystalline solid dysprosium(III) hexacyanoferrate(II) was synthesized by co-precipitation in aqueous solution. The resulting solid has been studied by Fourier transform infrared spectroscopy, X-ray analysis and solid state electrochemistry. The use of a cavity microelectrode was necessary to explore a wide range of time scale and minimize the (undesired) capacitive currents. Cyclic voltametric experiments were very helpful to understand the kinetic of charge transfer in such microstructure. A structure-properties relationship has been established from the crystallographic and the electrochemical properties. A square-scheme is presented to explain the unique electrochemical behavior of hexacyanoferrate containing dysprosium since this compound exhibits a second redox system. The solid presents an open channel-like morphology in which the motion of charged species occurs during the redox processes. Precisely, the electronic transfer is accompanied by a cation diffusion inside the microcrystalline structure. The size of these channels strongly suggests that the kinetic of charge transfer is limited by the cation transport into these structures.     

Magnetoresistance and magnetization anomalies in CeB6

High precision magnetoresistance (MR) Δρ/ρ(H,T) and magnetization M(H,T) measurements have been carried out for well known and typical strongly correlated electron system—cerium hexaboride. The detailed measurements have been fulfilled on single crystalline samples of CeB₆ over a wide temperature range in magnetic fields up to 70 kOe. It was shown that the MR anomalies in the magnetic heavy fermion compound under investigation can be consistently interpreted in the frameworks of a simple relation between resistivity and magnetization—Δρ/ρ∼M² obtained by Yosida [Phys. Rev. 107(1957)396]. A local magnetic susceptibility χ_loc(T,H)=(1/H^*(d(Δρ/ρ)/dH))^1/2 was deduced directly from the MR Δρ(H,T) measurements and compared with the experimental data of magnetization M(H,T). The magnetic susceptibility dependences χ_loc(T,H) and χ(T,H) obtained in this study for CeB₆ allow us to analyze the complicated H-T magnetic phase diagram of this so-called dense Kondo-system.     

Photogeneration and transport of charge carriers in polysilylenes

The attachment of π-conjugated chromophores that absorb the radiation with long wavelengths to poly(methylphenylsilylene) ( 1 ) via reactions of its formylated derivative is described. Some of the polymers obtained show improved photostability and higher quantum photogeneration efficiency in comparison with the parent polymer. Photoconductive ultra-thin layers can be prepared from polar derivatives of ( 1 ) by the Langmuir–Blodgett technique.     

Optical properties of CeBO3 and CeB3O6 compounds: first-principles calculations and experimental results

The cerium borates o-CeBO₃, m-CeBO₃ and CeB₃O₆ have been shown to be isostructural to their lanthanum derivatives. From diffuse reflectance, electron energy loss spectroscopy (EELS) and band structure calculations, it has been evidenced that a Ce³⁺ 4f-5d transition is responsible for weak absorption peaks around 3.5 eV while the O2p-Ce5d charge transfer gives rise to a strong absorption around 7 eV. Starting from self-consistent full potential LAPW calculations, the dielectric tensors of the three compounds were computed and compared to experimental data. It results in a satisfactory fit between the observed and the calculated extinction coefficient k and the index of refraction n.     

Structure and basic magnetic properties of the honeycomb lattice compounds Na2Co2TeO6 and Na3Co2SbO6

The synthesis, structure, and basic magnetic properties of Na₂Co₂TeO₆ and Na₃Co₂SbO₆ are reported. The crystal structures were determined by neutron powder diffraction. Na₂Co₂TeO₆ has a two-layer hexagonal structure (space group P6₃22) while Na₃Co₂SbO₆ has a single-layer monoclinic structure (space group C2/m). The Co, Te, and Sb ions are in octahedral coordination, and the edge sharing octahedra form planes interleaved by sodium ions. Both compounds have full ordering of the Co²⁺ and Te⁶⁺/Sb⁵⁺ ions in the ab plane such that the Co²⁺ ions form a honeycomb array. The stacking of the honeycomb arrays differ in the two compounds. Both Na₂Co₂TeO₆ and Na₃Co₂SbO₆ display magnetic ordering at low temperatures, with what appears to be a spin-flop transition found in Na₃Co₂SbO₆.     

Anomalous octahedral distortion and multiple phase transitions in the metal-ordered manganite YBaMn2O6

By means of powder X-ray diffraction, powder neutron diffraction and transmission electron microscopy (TEM), we determined the crystal structures of a metal-ordered manganite YBaMn₂O₆ which undergoes successive phase transitions. A high-temperature metallic phase (T_c1=520 K<T) crystallizes in a triclinic P1 with the following unit cell: Z=2, a=5.4948(15) Å, b=5.4920(14) Å, c=7.7174(4) Å, α=89.804(20)°, β=90.173(20)°, γ=91.160(4)°. The MnO₆ octahedral tilting is approximately written as a⁰b⁻c⁻, leading to a significant structural anisotropy within the ab plane. The structure for T_c2<T<T_c1 is a monoclinic P2 (Z=2, a=5.5181(4) Å, b=5.5142(4) Å, c=7.6443(3) Å, β=90.267(4)°) with an a⁻b⁻c⁻ tilting. The structural features suggest a d_x²−y² orbital ordering (OO). Below T_c2=480 K, crystallographically inequivalent two octahedra show distinct volume difference, due to the Mn³⁺/Mn⁴⁺ charge ordering. The TEM study furthermore revealed a unique d_3x²−r²/d_3y²−r² OO with a modified CE structure. It was found that the obtained crystal structures are strongly correlated to the unusual physical properties. In particular, the extremely high temperature at which charge degree of freedom freezes, T_c2, should be caused by the absence of the structural disorder and by heavily distorted MnO₆ octahedra.     

Ferromagnetic ordering in ThSi2 type CeAu0.28Ge1.72

The compound CeAu_0.28Ge_1.72 crystallizes in the ThSi₂ structure type in the tetragonal space group I4₁/amd with lattice parameters a=b=4.2415(6) Å c=14.640(3) Å. CeAu_0.28Ge_1.72 is a polar intermetallic compound having a three-dimensional Ge/Au polyanion sub-network filled with Ce atoms. The magnetic susceptibility data show Curie-Weiss law behavior above 50 K. The compound orders ferromagnetically at ∼8 K with estimated magnetic moment of 2.48 μ_B/Ce. The ferromagnetic ordering is confirmed by the heat capacity data which show a rise at ∼8 K. The electronic specific heat coefficient (γ) value obtained from the paramagnetic temperature range 15-25 K is∼124(5) mJ/ mol K². The entropy change due to the ferromagnetic transition is ∼4.2 J/mol K which is appreciably reduced compared to the value of R ln(2) expected for a crystal-field-split doublet ground state and/or Kondo exchange interactions.     

Hexyl substitution of pentathienoacene toward a significant improvement in charge transport

The introduction of hexyl chains endows the semiconductor with two or three orders of magnitudes enhancement in carrier mobility or current on/off ratio respectively.     

Combined powder neutron and X-ray diffraction study of charge and orbital order in Bi0.75Sr0.25MnO3

The room temperature structure of Bi_0.75Sr_0.25MnO₃ has been fitted to high-resolution synchrotron X-ray and time-of-flight neutron powder diffraction data. Constrained structural models were refined using a Pn11 supercell (, , , and α=89.894(1)°) of the underlying Pnma perovskite structure. The best-fit model evidences a 3:1 Mn³⁺/Mn⁴⁺charge ordering with only 30% of the ideal separation of bond valence sums. An ordered intergrowth of antiferro-orbitally ordered (LaMnO₃ type) and charge and ferro-orbitally ordered (YBaMn₂O₆ type) blocks is observed. Off-centre Bi/Sr displacements are ferroelectrically ordered in this model.     

A new type of charge compensating mechanism in Ca5(PO4)3F:Eu phosphor

The X-ray powder diffraction, reflectance, photoluminescence, photoluminescence excitation and ESR spectra of Ca₅(PO₄)₃F:Eu³⁺ phosphor have been studied. Three distinct variants of calcium substitutional Eu³⁺-sites have been observed in this host and the charge compensating species related to each of these sites has been identified. It is noted that the host related trace impurities those have prospects of acting as charge compensator, and the reaction environment that exists during the preparation of the material, greatly influence the preferential substitution of different Ca²⁺-sites by the Eu³⁺ ions. It is also noted that the charge compensating species in a suitable case, takes part in the photophysical process of luminescence of the Eu³⁺.     

The actual structure of K6(VO)2(V2O3)2(PO4)4(P2O7): Ordered distribution of P2O7 groups and charge ordering of vanadium

The actual structure of the vanadium phosphate K₆(VO)₂(V₂O₃)₂(PO₄)₄(P₂O₇) has been determined, using a much larger single crystal than previously used for the isostructural Rb-phase. The actual supercell is four times larger than the corresponding orthorhombic subcell with , , , α=β=γ=90°. The structure resolution, performed in the triclinic space group C-1, shows that the P₂O₇ groups alone are responsible for the superstructure, all the other atoms keeping the atomic positions of the orthorhombic subcell. This structural study shows a perfect ordering of the P₂O₇ groups in the actual structure, in contrast to the results obtained from the subcell. Concomitantly, the V⁴⁺ and V⁵⁺ are found to be ordered in the form of [110] stripes.     

Temperature-controlled change of charge transport mechanisms in nonionic water-in-oil microemulsions

 The temperature-controlled transition from the Stokes charge transport of aqueous nanodroplets to the intrinsic conduction of nanodroplet clusters in nonionic microemulsions was studied. Two different charge transport processes apparent from a minimum value of the conductance have been simulated based on straightforward physical models. Their predictions compare favourably with the observations. Received: 4 September 2001 Accepted: 20 September 2001     

Electrochemistry of solid-phase reactions: Phase formation in the In2O3-WO3 system. Processes at the WO3|In2O3 and WO3|In6WO12 interfaces

Reactions of interactions at the WO₃|In₂O₃ and WO₃|In₆WO₁₂ heterophase reaction interfaces, whose main product is In₂(WO₄)₃, are studied by electrochemical methods for the first time ever. Due to a far greater n type conductance inherent in the initial substances, the reactions are a model object for the development of methodology of the electrochemical approach. Both reactions are discovered to proceed at the expense of the transport of components of WO₃ and no evidence is discovered for the contribution of In³⁺ into diffusion and migration. Consisted data are obtained between the polarity of a spontaneously generated reaction difference of potentials and the direction of the field that accelerates the reaction: the current that is passed through electrochemical cells accelerates the reactions exclusively at the (−)-potential of a brick of WO₃. A difference is discovered between the charge and mass transport paths—spontaneous and field-induced mass transport of WO₃ or its components occurs via heterophase interfaces and adjacent areas and does not touch upon the In₂(WO₄)₃ grains. Shown is the antibatic character of the behavior exhibited by dependences of identical properties of cells (potential drop across a cell) following a change in the dc polarity. A possible role of a reactionless electrosurface transport of WO₃ in the mechanism of reaction and evolution of electrochemical properties of model electrochemical cells is demonstrated. The obtained data may or may not testify in favor of a hypothesis that presumes a prevailing role of the {WO₄}²⁻ mobility in the In₂(WO₄)₃ structure. Original Russian Text ? A.Ya. Neiman, T.E. Kulikova, 2007, published in Elektrokhimiya, 2007, Vol. 43, No. 6, pp. 714–726. Based on the report delivered at the 8th International Meeting on Fundamental Problems of Solid-State Ionics, Chernogolovka (Russia), 2006.     

The AMn5(SO4)6 family of triangular lattice, ferrimagnetic sulfates

A new family of anhydrous sulfates, A²⁺Mn₅(SO₄)₆ (A=Pb, Ba, Sr) is reported. The crystal structures of PbMn₅(SO₄)₆ and SrMn₅(SO₄)₆ are solved by powder X-ray and neutron diffraction. BaMn₅(SO₄)₆ is isostructural. PbMn₅(SO₄)₆ crystallizes with symmetry and unit cell parameters of a=14.551(1) Å and c=7.535(1) Å. The structure has rich features, including dimers of face-sharing MnO₆ octahedra, and two complementary triangular layers of Mn atoms. All compounds undergo a magnetic ordering transition at 10 K, below which, the magnetic susceptibility of the compounds varies systematically with the radius of the non-magnetic cation. Low temperature neutron diffraction shows that the complementary triangular layers result in a ferrimagnet with a net moment corresponding to one high spin Mn²⁺ per unit cell, correlating well with the magnetization data. The non-magnetic variant PbMg₅(SO₄)₆ is also reported.     

Lattice dynamics in the Kondo insulator YbB12

The phonon dispersion in the Kondo-insulator YbB₁₂ and its structure analogue LuB₁₂ has been studied in a wide energy range (up to 55 meV) by means of inelastic neutron scattering. The specific shape of phonon dispersion curves for low-frequency lattice vibrations could be described on the basis of a strong hierarchy suggested for the interactions between boron and rare-earth (RE) atoms: B-B?B-RE?RE-RE.     

锌掺杂对TiO2染料敏化电池光阳极中电荷俘获态分布及电子复合过程的影响

基于瞬态光电压和瞬态光电流技术研究了锌掺杂的TiO2染料敏化太阳能电池中电子复合及传输的动力学行为.通过实验获得了不同阳极掺杂条件下的电子复合时间常数与电子收集时间常数,考察了锌掺杂对电池阳极材料导带能级和电子俘获态的影响.研究结果表明,锌的掺杂在提高TiO2导带能级的同时延长了俘获态电子的复合时间常数,从而大大提高了电池的开路电压.     

All-solid-state BiVO4/ZnIn2S4 Z-scheme composite with efficient charge separations for improved visible light photocatalytic organics degradation

Constructing a Z-scheme is a significant approach to improve the separation of photogene rated carriers for effective organic pollutant degradation.Herein,a BiVO4/ZnIn2S4(BZ) Z-scheme composite was successfully synthesized,and applied to photodegrade methyl orange(MO) irradiated by a LED lamp.Anchoring the BiVO4 on the ZnIn2S4 nanoparticles promoted the separation of photogenerated electronholes and broadened the light response range.The detailed characterizations,including surface morphology,elements valence state,and photocurrent performance,demonstrated that the enhanced separation of photogenerated carriers was the pivotal reason for the enhanced photocatalysis reaction.Benefiting from the excellent photocatalytic characteristics,the 5% mass ratio of BZ composite presented the highest MO degradation rate of 0.00997 min^-1,which was 1.9 and 10.3 times greater than the virgin ZnIn2S4 and BiVO4,respectively.Furthermore,the BZ hybrid materials indicated a well photo-stability in the four recycling tests.     

Structural transformations in the Na4+xVO(PO4)2 vanadylphosphates

The crystal structures of new sodium vanadylphosphate, Na_4.35VO(PO₄)₂ (, , , Z=8, S.G. Ibam), and new (γ-) modification of Na₄VO(PO₄)₂ (, , , Z=8, S.G. Pbc2₁) have been investigated by X-ray single-crystal diffraction. Both structures contain isolated infinite chains of the corner-sharing VO₆ octahedra. The octahedra within the chains are additionally linked to each other by the tetrahedral PO₄ groups. Sodium atoms are situated in the positions between the chains. Depending on the conditions of synthesis, the number of sodium atoms in the unit cell of the Na_4+xVO(PO₄)₂ compounds may vary resulting in a change of the oxidation state of vanadium atoms and a change of their coordination environment. In Na_4.35VO(PO₄)₂ vanadium atoms have almost regular octahedral coordination with six close V-O separations and all chains in the structure are equivalent. The crystal structure of γ-Na₄VO(PO₄)₂ contains two non-equivalent chain types: the first one is similar to that found in Na_4.35VO(PO₄)₂ whereas the second one contains VO₆ octahedra with the short vanadyl bonds. The charge re-distribution was supposed in the new γ-modification of Na₄VO(PO₄)₂ where the V^4+δ and V^4−δ cations orderly occupy octahedral positions in different chains. The origin of this phenomena is discussed.     

Electrochemical electron transfer and its relation to charge transport in single molecule junctions

Ability to control charge transport at nanometer scale lies in the heart of design of fast reliable electronic devices. Molecular electronics thrive to use functional molecules for such transport. If the molecule contains redox center(s), a diode-like or transistor-like behavior can be easily explored by controlling not only the voltage difference between two metallic contacts of the molecular junction but also the potential of one of the contacting electrodes with respect to some reference. Thus, one needs to understand the relationship between electrochemical electron transfer and charge transport in metal–molecule–metal junctions. This review presents latest theoretical approaches toward understanding of such relationship and discusses pivotal experimental works to validate them. Tunneling and hopping pathways may operate in parallel (two-channel model), but experimental conditions dictate the channel preference.     

Structural phase transitions in BaV6O11

BaV₆O₁₁ was synthesized under high pressures and crystallizes in a structure closely related to magnetoplumbite. [V(1)O₆]-octahedra share common edges and form a Kagomé lattice normal to the hexagonal [0 0 1] direction. The layers are connected in the direction of c via trigonal [V(3)O₅]-bipyramids and [V(2)O₆]-octahedra, which share common faces. The Ba-atoms are incorporated into cavities of the vanadium oxide framework and are coordinated by 12 oxygen atoms in the shape of a dodecahedron.Three magnetic anomalies at approximately 250, 115 and 75 K were detected in this compound. All of them are accompanied by anomalies in the specific heat measurement. To characterize possible structural transitions and determine the response of the structure to the magnetic anomalies, single crystal X-ray diffraction studies were carried out in the temperature range from 293 to 80 K. At 250 K the compound undergoes a structural phase transition. The space group above the transition temperature is P6₃/mmc, at lower temperature the symmetry reduces to P6₃mc. For the refinements in P6₃mc an inversion twin model was used, this way accounting for the loss of the center of symmetry. The structural phase transition is characterized by a small displacement of the V(1)-atom (forming the Kagomé lattice) out of its central position in the octahedra. As a consequence part of the octahedral edges/angles are increased, while the opposite ones are decreased. One limiting surface of the octahedral sheet is corrugated, while the other one is smoothened with respect to the high-temperature structure. This deformation of the octahedral sheets leads to the corresponding geometrical changes in the other coordination polyhedra.The structural response to the magnetic anomaly at 115 K is weak and mainly observable in the geometric parameters concerning the [V(1)O₆]-octahedra and [V(3)O₅]-bipyramids. This may serve as a first indication that the corresponding central atoms play an important role in the mechanism of the magnetic phase transition.