From composites to solid solutions: modeling of ionic conductivity in the CaF2-BaF2 system

By using calcium fluorite and barium fluorite as test materials, we demonstrated that homovalent "dopants" can greatly affect ionic conductivity through locally changing the defect density. Whilst this doping is a state-of-the-art effect in the case of dopants that replace native ions of different charge (heterovalent dopants), it is a rather surprising effect at a first glance for substitutional dopants of the same charge; here, the phenomenon is not electrostatic, but elastic in nature. As a consequence of size mismatch, the smaller Ca atoms in the BaF(2) lattice favored the formation of interstitial sites that were located close to the Ca atoms, whilst doping larger Ba species into the CaF(2) phase favored vacancy formation. In terms of conductivity, and in agreement with the different mobilities, the first doping effect was favorable, whilst the other decreased conductivity. The concentration effects were formalized by a heterogeneous Frenkel reaction that was distinguished from the mean Frenkel reaction by additional (elastic) trapping that became more pronounced the lower the temperature. It was very revealing to relate this phenomenon to CaF(2)-BaF(2) multilayers and composites. In very general terms, these effects in the solid solutions were understood as being the atomistic limit of the interfacial charge-transfer that occurred at the hetero-interface of the crystallites or films, and reflected the transition from heterogeneous doping (higher-dimensional doping) to homogeneous doping (zero-dimensional doping).     

Propriétés électriques des solutions solides à structure fluorine excédentaire en anions Na0.5−xY0.5+xF2+2x

A study by impedance spectroscopy has been made of the ac conductivity of the Na_0.5−xY_0.5+xF_2+2x solid solutions which have a fluorite defect structure stabilized by doping. Although connected with the fluorine motion, the conductivity is almost independent of the anion excess. This behavior is different from that of CaF₂ or from that of βPbF₂ doped with trivalent cations.     

Effect of CuO Doping on the Electrical Behavior of ZrO2

Electrical conductivity has been measured at different temperatures for ZrO2 doped with various molar ratios of CuO. The conductivity increases due to migration of vacancies, created by doping. The conductivity was found to increase with increase in temperature till 220℃ and thereafter decrease due to collapse of the fluorite framework. A second rise in conductivity around 500 ℃ was observed due to phase transition of ZrO2. X-ray powder diffraction, DTA and IR studies were carried out for confirming doping effect and phase transition in ZrO2.     

Influence of a Second Cation (M = Ca2+, Mg2+) on the Phase Evolution of (BaxM1–x)F2 Starting from Amorphous Deposits

A second type of cation (Mg²⁺, Ca²⁺) was introduced into BaF₂ by low‐temperature atomic beam deposition. The structure evolution from low‐temperature (–150 °C) amorphous deposits to high‐temperature (< 1000 °C) annealed crystalline phases was studied by in‐situ transmission electron microscopy and X‐ray diffraction. Amorphous (Ba_0.5, Ca_0.5)F₂ crystallizes in a first step to metastable solid solution phase (fluorite‐type), which then decomposes into the pure phases of CaF₂ and BaF₂ at higher temperature. The crystallization behavior of amorphous (Ba_xMg_1–x)F₂ is completely different. When the Mg/Ba atomic ratio is around 1:1, the mixture transforms to the ternary compound BaMgF₄ at annealing, and no decomposition occurs by further heating up to 1000 °C. When the Ba concentration is below 15 % in atomic ratio (x < 0.15), the mixture forms a solid solution phase (rutile type) with the lattice expanded by +1 % compared to rutile type MgF₂. The difference between the phase evolutions of the two mixture systems is discussed.     

Secondary doping in poly(3,4‐ethylenedioxythiophene):Poly(4‐styrenesulfonate) thin films

The electrical and structural properties of poly(3,4‐ethylenedioxythiophene):poly(4‐styrenesulfonate) (PEDOT:PSS) thin films deposited from aqueous dispersion using different concentrations of selected secondary dopants are studied in detail. An improvement of the electrical conductivity by three orders of magnitude is achieved for dimethyl sulfoxide, sorbitol, ethylene glycol, and N,N‐dimethylformamide, and the secondary dopant concentration dependence of the conductivity exhibits almost identical behavior for all investigated secondary dopants. Detailed analysis of the surface morphology and Raman spectra reveals no presence of the secondary dopant in fabricated films, and thus the dopants are truly causing the secondary doping effect. Although the ratio of benzenoid and quinoid vibrations in Raman spectra is unaffected by doping, the phase transition in PEDOT:PSS films owing to doping is confirmed. Further analysis of temperature‐dependent conductivity reveals 1D variable range hopping (VRH) charge transport for undoped PEDOT:PSS, whereas highly conductive doped PEDOT:PSS films exhibit 3D VRH charge transport. We demonstrate that the charge ‐ hopping dimensionality change should be a fundamental reason for the conductivity enhancement. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1139–1146     

Recherche de matériauxa`conductivitéionique ame´liore´e de´rivant des hydrurofluorures de calcium CaF2−xHx: conductivite´ ionique des phases Ca1−yNay(F2−xHx)1−y/2

The doping of CaF_2?xH_x hydridefluorides by aliovalent ions is studied, the aim being the preparation of materials of improved ionic conductivity. It is shown that doping by monovalent Na⁺ ions is possible. Three hydrogen-rich phases, formulated Na_yCa_1?y(F_2?xH_x)_1?y/2 have been studied. Their conductivity is mainly ionic, but, compared with the same doping in CaF₂, the conductivity enhancement is low. This result is interpreted from energetic and structural considerations.     

Nanoporous Graphene with Single‐Atom Nickel Dopants: An Efficient and Stable Catalyst for Electrochemical Hydrogen Production

Single‐atom nickel dopants anchored to three‐dimensional nanoporous graphene can be used as catalysts of the hydrogen evolution reaction (HER) in acidic solutions. In contrast to conventional nickel‐based catalysts and graphene, this material shows superior HER catalysis with a low overpotential of approximately 50 mV and a Tafel slope of 45 mV dec^?1 in 0.5 M H₂SO₄ solution, together with excellent cycling stability. Experimental and theoretical investigations suggest that the unusual catalytic performance of this catalyst is due to sp–d orbital charge transfer between the Ni dopants and the surrounding carbon atoms. The resultant local structure with empty C–Ni hybrid orbitals is catalytically active and electrochemically stable.     

Influence de l'oxygène sur la conductivité électrique du fluorure de baryum monocristallin

The influence of oxygen on the electrical properties of single crystals of BaF₂ was investigated in the temperature range 450–1150°C. Two types of studies have been undertaken: total ac conductivity and cation transport measurements. The results confirm an anion Frenkel disorder model in MeF₂. Substitution of the fluorine atoms by oxygen in the lattice supposes the creation of the vacancies V^·_F. The evaluation of intrinsic, extrinsic and association domains as a function of oxygen partial pressure has been studied. Values for the energy of formation of the intrinsic defects, the motion energies of V^·_F-vacancies and association or precipitation energy have been deduced from the slopes of the conductivity curves.     

Comparison of surface and bulk doping levels in chemical polypyrroles of low,medium and high conductivity

Chemical polypyrroles (PPys) of low (σ < 75 S/cm), medium (75 < σ < 200 S/cm) and high electrical conductivity (σ > 200 S/cm) having chloride dopants have been investigated by XPS, chloride ion‐selective electrode (ISE) measurements and high‐resolution termogravimetric analysis (TGA). The average surface doping level in these PPys was 0.28 ± 0.03 as determined from deconvoluted XPS N 1s, Cl 2p and C 1s spectra by using the well‐established N⁺/N and Cl^?/N atomic ratios as well as a new ratio denoted as Cα*/Cα_Total. This new ratio provides an estimation of the relative amount of α‐C atoms in charged pyrrole units per total α‐C atoms. The average bulk doping level in these materials was 0.31 ± 0.02 from direct chloride ISE measurements. High‐resolution TGA was employed for the first time in the determination of the amount of hydrogen chloride evolved from the PPy samples during degradation at high temperatures. The resulting average bulk doping level by TGA was 0.30 ± 0.04 for these PPys, in very good agreement with the ISE results. Since surface and bulk doping levels are almost identical for the PPys of low, medium and high conductivity, the differences in conductivity between samples have been attributed to differences in conjugation length among them. For PPy of high conductivity (σ = 288 S/cm), a conjugation length 2.6 times higher than that of PPy of low conductivity (σ = 29 S/cm) has been calculated. Copyright © 2006 John Wiley & Sons, Ltd.     

Kristall‐ und elektronische Strukturen von AIr2P2 (A: Ca — Ba)

Crystal and Electronic Structures of AIr₂P₂ (A: Ca — Ba) Single crystals of CaIr₂P₂ (a = 6.610(3), c = 7.031(3)Å) were prepared by reaction of the elements in a lead flux and investigated by X‐ray methods. The compound crystallizes with the EuIr₂P₂ type (P3₂21; Z = 3) just detected in the case of SrIr₂P₂. In the structure all the P atoms and half of the Ir atoms build a three‐dimensional framework with Ca and the remaining Ir atoms in the cavities. The latter atoms form threefold screws along [001] with relatively short Ir‐Ir distances and they are connected with the framework by Ir‐P bonds. LMTO band structure calculations suggested that the compounds with Ca, Sr, and Eu should be semiconductors. For EuIr₂P₂ this was confirmed by conductivity measurements. BaIr₂P₂ (a = 3.946(1), c = 12.572(2)Å) synthesized by heating the elements at 1050 °C for a long time crystallizes with the ThCr₂Si₂ type structure (I4/mmm; Z = 2). Due to the rigid layers of IrP₄ tetrahedra and the atomic size of barium the P‐P distance between the layers with a value of 3.71Å is very long.     

Electronic structure of oligoaniline doped by inorganic and organic acids

The electronic structure of doped‐oligoaniline with various dopants is investigated by means of DFT method. After doping by hydrochloric acid (HCl) and camphorsulfonic acid (HCSA), the alternation of bond‐lengths is decreased and the co‐planarity of adjacent aromatic rings is increased. The π‐conjugating effect is increased in the electronic nature of Ph‐N system because the electrons can be delocalized along the backbone of oligoaniline where the hydrogen bonds as a bridge transfer the electrons. The electronic structure of polaron and bipolaron conformation and their relative stability is discussed, indicating that the preferable conformation is dependant on various dopants. The calculation results reveal that there is a relatively stronger interaction between the organic dopant of HCSA and N atoms of PANI, and more charge transfer between PANI and HCSA is a reason for the fact that the conductivity of HCSA‐doped PANI is higher than that of HCl‐doped PANI. The doping mechanism is proposed based on the calculation results. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Defect and dopant properties of MgTa2O6

Atomistic computer simulation techniques have been used, for the first time, to reproduce the crystal structure of MgTa₂O₆ and to investigate the defect chemistry and dopant properties of this material. The calculated defect energetics suggest that the concentration of intrinsic atomic defects in this phase is insignificant and that the system is probably stable to both oxidation and reduction. Dopant solution energy versus ion size trends are found for both isovalent and aliovalent dopant incorporation at Mg and Ta sites. Divalent dopants (e.g. Ca, Cu) preferentially occupy the Mg site whereas dopants with higher charge (e.g. Sc, Zr, Nb) are more favorable on the Ta site. High migration activation energies (>2 eV) predict limited ionic conductivity in this material.     

Direct Imaging of Light Emission Centers in Two‐Dimensional Crystals and Their Luminescence and Photocatalytic Properties

To fully understand the fundamental properties of light‐energy‐converting materials, it is important to determine the local atomic configuration of photofunctional centers. In this study, direct imaging of one‐ and two‐Tb‐atom emission centers in a two‐dimensional Tb‐doped Ca₂Ta₃O₁₀ nanocrystal was carried. The emission centers were located at the Ca sites in the perovskite structure, and no concentration‐based quenching was observed even when the emission centers were in close proximity to each other. The relative photoluminescence efficiency for green emission of the nanosheet suspension was 38.1 %. Furthermore, the Tb‐doped Ca₂Ta₃O₁₀ nanocrystal deposited co‐catalyst showed high photocatalytic activity for hydrogen production from water (quantum efficiency: 71 % at 270 nm). Tb³⁺ dopants in the two‐dimensional crystal might have the potential to stabilize the charge separation state.     

Vanadium‐Doped WS2 Nanosheets Grown on Carbon Cloth as a Highly Efficient Electrocatalyst for the Hydrogen Evolution Reaction

Two‐dimensional transition‐metal dichalcogenides have been widely studied as electrocatalysts for the hydrogen evolution reaction (HER). However, limited active sites and poor conductivity hinder their application. To solve these disadvantages, heteroatom doping has attracted wide attention because it can not only increase the active sites but also affect the intrinsic catalytic properties of the electrocatalyst. Herein, we grew vanadium‐doped WS₂ nanosheets on carbon cloth (V‐WS₂/CC) as an electrocatalyst for HER under acidic and alkaline conditions. With a proper vanadium doping concentration, the electrochemical surface areas of V_0.065‐WS₂/CC were 9.6 and 2.6 times as large as that of pure WS₂ electrocatalyst under acidic and alkaline conditions, respectively. In addition, the charge‐transfer resistance also decreased with moderate vanadium doping. Based on this, the synthesized vanadium‐doped WS₂ nanosheets exhibited good stability with high HER catalytic activity and could reach a current density of 10 mA cm⁻² at overpotentials of 148 and 134 mV in 0.5 m H₂SO₄ and 1 m KOH, respectively. The corresponding Tafel slopes were 71 and 85 mV dec⁻¹. Therefore, our synthesized vanadium‐doped WS₂ nanosheets can be a promising electrocatalyst for the production of hydrogen over a wide pH range.     

Direct View of Cr Atoms Doped in Anatase TiO2(001) Thin Film

Imaging the doping elements is critical for understanding the photocatalytic activity of doped TiO₂ thin film. But it is still a challenge to characterize the interactions between the dopants and the TiO₂ lattice at the atomic level. Here, we use high angle annular dark-field/annular bright-field scanning transmission electron microscope (HAADF/ABF-STEM) combined with electron energy loss spectroscopy (EELS) to directly image the individual Cr atoms doped in anatase TiO₂(001) thin film from [100] direction. The Cr dopants, which are clearly imaged through the atomic-resolution EELS mappings while can not be seen by HADDF/ABF-STEM, occupy both the substitutional sites of Ti atoms and the interstitial sites of TiO₂ matrix. Most of them preferentially locate at the substitutional sites of Ti atoms. These results provide the direct evidence for the doping structure of Cr-doped A-TiO₂ thin film at the atomic level and also prove the EELS mapping is an excellent technique for characterizing the doped materials.     

Charge transfer in and conductivity of molecularly doped thiophene‐based copolymers

The electrical conductivity of organic semiconductors can be enhanced by orders of magnitude via doping with strong molecular electron acceptors or donors. Ground‐state integer charge transfer and charge‐transfer complex formation between organic semiconductors and molecular dopants have been suggested as the microscopic mechanisms causing these profound changes in electrical materials properties. Here, we study charge‐transfer interactions between the common molecular p‐dopant 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane and a systematic series of thiophene‐based copolymers by a combination of spectroscopic techniques and electrical measurements. Subtle variations in chemical structure are seen to significantly impact the nature of the charge‐transfer species and the efficiency of the doping process, underlining the need for a more detailed understanding of the microscopic doping mechanism in organic semiconductors to reliably guide targeted chemical design. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 58–63     

Electronic and magnetic properties of small RhnCa (n = 1–9) clusters: A DFT study

The equilibrium geometries, relative stabilities, electronic and magnetic properties of small Rh_nCa (n = 1–9) clusters have been investigated by DFT calculations. The obtained results show that the three‐dimensional geometries are adopted for the lowest‐energy Rh_nCa clusters, and the doped Ca atom prefers locating on the surface of the cluster. Based on the analysis of the second‐order difference of energies, fragmentation energies and the HOMO‐LUMO energy gaps, we identify that the Rh₄Ca, Rh₆Ca, and Rh₈Ca clusters are relatively more stable than their neighboring clusters, and the doping of Ca enhances the chemical reactivity of the pure Rh_n clusters, suggesting that the Rh_nCa clusters can be used as nanocatalysts in many catalytic reactions. The magnetic moment for these clusters is mostly localized on the Rh atoms, and the doping Ca atom has no effect on the total magnetic moment of Rh_nCa clusters. The partial density of states, VIP, VEA, and η of these clusters in their ground‐state structures were also calculated and discussed. © 2015 Wiley Periodicals, Inc.     

S-, N- and C-doped titanium dioxide nanoparticles: Synthesis, characterization and redox charge transfer study

Herein we report on the synthesis and characterization of TiO₂ nanomaterials doped with anions like sulfur, carbon and nitrogen. Upon doping, the absorption extends well into the visible region. This shift in the absorption edge is accompanied by a concomitant narrowing of band gap. The resulting anion-doped TiO₂ nanomaterials were characterized by XRD, XPS, elemental analysis, EDAX, TEM, UV-DRS, DC conductivity, AC impedance and cyclic voltammetric studies. XPS confirms the presence of the dopants and the elemental analysis determined the amount of dopants in TiO₂. Electrochemical characterization was carried out by cyclic voltammetry at pHs 2, 6.5 and 10. As against the response of undoped TiO₂, the doped samples show an active electrochemical response indicating an induced charge transfer across the titania/solution interface, thus forming two anodic peaks and a cathodic peak. This interesting and significant observation was understood in terms of band bending due to anion doping as well as to the pH changes in the experimental solutions.     

Structural Characterization of Doped Calcium Tartrate Tetrahydrate

Crystals of calcium tartrate tetrahydrate were grown in silica gel medium in pure form and with barium, strontium, cobalt, nickel, manganese, zinc, and cadmium as dopants. The crystals, with formula D_xCa_1-x(C₄H₄O₆).4H₂O (where D=doping atom), were characterized by X-ray diffraction, photoelectron spectroscopy, and Raman spectroscopy. Some of the doping atoms are located in the host lattice and others are distributed at random positions, especially on the surface of the crystals. In the host lattice the doping atom replaces Ca when an earth alkaline atoms is used but occupies an interstitial site when a transition metal atom is used. The dielectric permittivity is higher in doped compounds.