二价金属元素掺杂对LiCoO2体系电子输运性质的影响

为了解释Ca掺杂与Mg掺杂在影响锂离子二次电池正极材料LiCoO₂体系电子输运性质方面的不同效应，采用基于密度泛函理论的第一性原理方法研究了该体系的电子结构.计算结果表明，虽然在LiCoO₂体系中用Ca或Mg替代Co都会在费米能级附近产生部分占据的受主带，但两者对应的电子态都具有明显的局域化特征；此外，与Mg掺杂体系明显不同的是，Ca掺杂体系的受主带与价带之间存在清晰的带隙.这一带隙的存在正是Ca掺杂不能明显提高LiCoO₂体系电导率的主要原因.此外，Ca²⁺与Mg²⁺离子半径的较大差别也是造成这两个掺杂体系的电导率存在明显差异的一个重要因素.     

Crystal structure,dielectric and piezoelectric properties of Ta/W codoped Bi3TiNbO9 Aurivillius phase ceramics

Aurivillius phase Bi₃Ti_1−xTa_xNb_1−xW_xO₁₂ high temperature piezoceramics were prepared by a conventional solid state reaction method. The crystal structure, dielectric, electrical conduction and piezoelectric properties were systematically studied. Pure or modified Bi₃TiNbO₉ ceramics revealed the presence of only two-layered Aurivillius phase, indicating that Ta/W doping entered into the B-site of pseudo-perovskite structure and formed solid solutions. The Curie temperature had a strong reliance on the structural distortion. Furthermore, Ta/W dopants act as a donor doping, decrease the number of oxygen vacancies and facilitate the domain wall motion. As a result, Ta/W modifications significantly increase the DC resistivity and piezoelectric properties. Bi₃Ti_0.98Ta_0.02Nb_0.98W_0.02O₁₂ ceramics possess the optimum d₃₃ value (∼12.5 pC/N) together with a high T_C point (∼893 °C). Moreover, the resonance–antiresonance spectra demonstrate that the Ta/W-BTN ceramics are indeed piezoelectric in nature at 600 °C. The d₃₃ value of BTTNW-2 ceramic remains ∼12.2 pC/N after annealing at 700 °C. These factors suggest that the BTTNW-based ceramic is a promising candidate for ultra-high temperature sensor applications.     

Crystal structure, electronic and transport properties of AgSbSe2 and AgSbTe2

Undoped and p- and n-doped AgSbX₂ (X=Se and Te) materials were synthesized by direct fusion technique. The structural properties were investigated by X-ray diffraction and SEM microscopy. The electrical conductivity, thermal conductivity and Seebeck coefficient have been measured as a function of temperature in the range from 300 to 600 K.To enlighten electron transport behaviours observed in AgSbSe₂ and AgSbTe₂ compounds, electronic structure calculations have been performed by the Korringa-Kohn-Rostoker method as well as KKR with coherent potential approximation (KKR-CPA) for ordered (hypothetical AgX and SbX as well as AgSbX₂ approximates) and disordered systems (Ag_1−xSb_xX), respectively. The calculated density of states in the considered structural cases shows apparent tendencies to opening the energy gap near the Fermi level for the stoichiometric AgSbX₂ compositions, but a small overlap between valence and conduction bands is still present. Such electronic structure behaviour well agrees with the semimetallic properties of the analyzed samples.     

Defect formation and water incorporation in Y2O3

The molecular statics method is used to study the formation of defects and water incorporation in Y₂O₃. The crystal structure, the isothermal compressibility, and the formation enthalpy of Y₂O₃ calculated with the chosen interaction potentials are in good agreement with the experimental data. The formation energies of intrinsic and impurity defects are evaluated. The binding energy of protons and oxygen vacancies with an acceptor impurity at different distances is calculated. Various water incorporation reactions in the oxide are examined, including the mechanisms involving oxygen interstitial sites and oxygen vacancies produced by the acceptor doping. It is shown that the water incorporation in pure Y₂O₃ is energetically less favorable than in the acceptor doped oxide.     

Strain engineering of ion migration in LiCoO2

Strain engineering is a powerful approach for tuning various properties of functional materials. The influences of lattice strain on the Li-ion migration energy barrier of lithium-ions in layered LiCoO₂ have been systemically studied using lattice dynamics simulations, analytical function and neural network method. We have identified two Li-ion migration paths, oxygen dumbbell hop (ODH), and tetrahedral site hop (TSH) with different concentrations of local defects. We found that Li-ion migration energy barriers increased with the increase of pressure for both ODH and TSH cases, while decreased significantly with applied tensile uniaxial c-axis strain for ODH and TSH cases or compressive in-plane strain for TSH case. Our work provides the complete strain-map for enhancing the diffusivity of Li-ion in LiCoO₂, and therefore, indicates a new way to achieve better rate performance through strain engineering.     

Defect properties of CuInSe2 and CuGaSe2

Using first-principles electronic structure theory, we have calculated defect formation energies and defect transition levels in CuInSe₂ and CuGaSe₂. We show that (i) it is easy to form Cu vacancies in CuInSe₂, and (ii) it is also relatively easy to form cation antisite defects (e.g. In_Cu) for this ternary compound. Consequently, defect pairs such as (2V_Cu+In_Cu) have a remarkably low formation enthalpy. As a result, the formation of a series of Cu-poor compounds (CPCs) such as CuIn₅Se₈ and CuIn₃Se₅, is explained as a repeat of (2V_Cu+In_Cu) pairs in CuInSe₂. The very efficient p-type self-doping ability of CuInSe₂ is explained by the easy formation of the shallow Cu vacancies. The electrically benign character of the natural defect in CuInSe₂ is explained in terms of an electronic passivation of the by . For CuGaSe₂, we find that (i) the native acceptor formation energies and transition energy levels are similar to that in CuInSe₂, but the donor formation energy is larger in CuGaSe₂. (ii) The Ga_Cu donor level in CuGaSe₂ is deeper than In_Cu donor level in CuInSe₂, therefore, Ga_Cu behaves as an electron trap in CuGaSe₂, even when it is passivated by V_Cu. We have also calculated the band alignment between the CPCs and CuInSe₂, showing that it could have significant effect on the solar cell performance.     

Electrical conductivity and defect structure of Mg-doped Cr2O3

The electrical conductivity of Cr₂O₃ nominally doped with 2 mol% MgO has been studied by the four point a.c. technique as a function of the oxygen activity (O₂ + Ar, CO + CO₂ and H₂ + H₂O) in the temperature range 400–1200 °C. It is concluded that Cr₂O₃ doped with MgO is an extrinsic conductor and that the dissolved Mg-dopant is compensated by the formation of electron holes at near atmospheric oxygen pressures and by oxygen vacancies (or possibly interstitial chromium ions) at highly reduced oxygen activities (in CO + CO₂ and H₂ + H₂O gas mixtures). In H₂ + H₂O mixtures Mg-doped chromia also dissolves hydrogen as protons and significantly affects the defect structure and electrical conductivity. The defect structure of the oxide under various conditions is discussed.     

Growth,structure and physical properties of gadolinium doped Sr2IrO4 single crystal

A series of Gd-doped Sr₂IrO₄ single crystals were grown using a flux method. Analysis of the temperature-dependent resistance of these crystals reveals that these samples show two-dimensional weak localization at 150 to 300 K, while three-dimensional variable range hopping (VRH) behavior is observed at temperatures lower than 150 K. Two localization lengths are observed in the VRH behavior, with a transition temperature of around 88 K. Correspondingly, temperature-dependent magnetization observations along the ab-plane reveal magnetic anomalies at both 150 and 85 K. This work demonstrates the correlation between the electrical and magnetic properties of 5d transition-metal compounds.     

Structural, vibrational and dielectric properties of new potassium hydrogen sulfate arsenate: K4(SO4)(HSO4)2(H3AsO4)

A new compound, K₄(SO₄)(HSO₄)₂(H₃AsO₄) was synthesized from water solution of KHSO₄/K₃H(SO₄)₂/H₃AsO₄. This compound crystallizes in the triclinic system with space group P1¯ and cell parameters: a=8.9076(2) Å, b=10.1258(2) Å, c=10.6785(3) Å; α=72.5250(14)°, β=66.3990(13)°, γ=65.5159(13)°, V=792.74(3) Å³, Z=2 and ρ_cal=2.466 g cm⁻³. The refinement of 3760 observed reflections (I>2σ(I)) leads to R₁=0.0394 and wR₂=0.0755. The structure is characterized by SO₄²⁻, HSO₄⁻ and H₃AsO₄ tetrahedra connected by hydrogen bridge to form two types of dimer (H(16)S(3)O₄⁻?S(1)O₄²⁻ and H(12)S(2)O₄⁻?H₃AsO₄). These dimers are interconnected along the [1¯ 1 0] direction by the hydrogen bonds O(3)-H(3)?O(6). They are also linked by the hydrogen bridge assured by the hydrogen atoms H(2), H(3) and H(4) of the H₃AsO₄ group to build the chain S(1)O₄?H₃AsO₄ which are parallel to the “a” direction. The potassium cations are coordinated by eight oxygen atoms with K-O distance ranging from 2.678(2) to 3.354(2) Å.Crystals of K₄(SO₄)(HSO₄)₂(H₃AsO₄) undergo one endothermic peak at 436 K. This transition detected by differential scanning calorimetry (DSC) is also analyzed by dielectric and conductivity measurements using the impedance spectroscopy techniques. The obtained results show that this transition is protonic by nature.     

Mg,Al掺杂对LiCoO2体系电子结构影响的第一原理研究

为了研究Mg, Al掺杂对锂二次电池正极材料LiCoO₂体系的电子结构的影响，进而揭示Mg掺杂的LiCoO₂具有高电导率的机理，对Li(Co, Al)O₂和Li(Co, Mg)O₂进行了基于密度泛函理论的第一原理研究. 通过对能带及态密度的分析，发现在Mg掺杂后价带出现电子态空穴，提高了电导，并且通过歧化效应（disproportionation）改变了Co-3d电子在各能级的分布，而Al掺杂则没有这些作用. O关键词： 2')" href="#">LiCoO₂ 电子结构 第一原理 电导     

Electrochemical behaviour of sputtered c-V2O5 and LiCoO2 thin films for solid state lithium microbatteries

Here are reported for the first time electrochemical data on all-solid-state lithium microbatteries using crystalline sputtered V₂O₅ thin films as cathode materials and LiPON as solid electrolyte. The stable specific capacity of 30 µAh/cm² found with a 2.4 µm thick film competes very well with the best values obtained for solid state microbatteries using amorphous films. With the challenge of decreasing the temperature of heat treatment for sputtered LiCoO₂ thin films, we show that a temperature of 500 °C combined with an optimized bias sputtering (-50 V) allows to get highly crystalline deposits, to minimize the presence of Co₃O₄ and to suppress any trace of the cubic phase. At the same time the theoretical specific capacity is reached in the 4.2 V-3 V range and a good cycling behaviour is achieved with a high capacity of 50 µAh/cm²/µm after 140 cycles at 10 µA.cm².     

Crystal structure, electrical and magnetic properties of La1 − xSrxCoO3 − y

The crystal structure and properties of La_{1 − x}Sr_xCoO_{3 − y} with strontium contents ranging from x = 0.1 to x = 0.7 have been studied. The lattice parameters were measured as a function of temperature (4.2–400 K) and the crystal structure was found to change from rhombohedral (at low temperatures and values of x) to cubic. While LaCoO₃ is paramagnetic the oxides in the composition range 0.2 < x < 0.6 are soft ferromagnets. The strontium additions are compensated by the formation of Co⁴⁺ (cobalt ions with one positive effective charge, Co_Co^.) and oxygen vacancies (V_o^..). From the results it is concluded that the relative importance of oxygen vacancies increases with increasing temperature and decreasing oxygen activity. As a result the concentration of electronic charge carriers — and the resultant electrical conductivity — decrease with increasing temperature. The defect structure is discussed and it is concluded that defect associations — probably between oxygen vacancies and strontium ions — and formation of microdomains of perovskite-related phases are important aspects of the overall structure of these perovskite phases.     

Band structure and photocatalytic properties of perovskite-type compound Ca2NiWO6 for water splitting

An oxide semiconductor Ca₂NiWO₆, with double-perovskite crystal structure, was synthesized by solid-state reaction method. The compound Ca₂NiWO₆ was characterized by X-ray diffraction, UV-visible diffuse reflectance, and photoluminescence. The photocatalytic properties of the compound for water splitting were investigated under UV and visible light irradiation. The results showed H₂ evolution was not observed over the compound under visible light irradiation (λ>420 nm) with a 300 W xenon arc lamp when using methanol (CH₃OH) as electron donor, although the compound was responsive to visible light region. Based on the experimental results, a possible band structure was proposed through theoretical calculation of the electronic structure by using the full potential-linearized augmented plane wave (F-LAPW). The band structure and photocatalytic properties were attributed to the special crystal and electronic structures. Due to the oxygen vacancies in the compound, which worked as electron-hole recombination centers, the photocatalytic activity of the compound was low.     

First-principles investigation of the intrinsic defects in Ti3SiC2

First-principles calculations have been carried out to investigate intrinsic defects including vacancies, interstitials, antisite defects, Frenkel and Schottky defects in the 312 MAX phase Ti₃SiC₂. The formation energies of defects are obtained according to the elemental chemical potentials which are determined by the phase stability conditions. The most stable self-interstitials are all found in the hexahedral position surrounded by two Ti(2) and three Si atoms. For the entire elemental chemical potential range considered, our results demonstrated that Si and C related defects, including vacancies, interstitials and Frenkel defects are the most dominant defects. Besides, the present calculations also reveal that the formation energies of C and Si Frenkel defects are much lower than those of all Schottky defects considered. In addition, the calculated profiles of densities of states for the defective Ti₃SiC₂ indicate that these defects should have great influence on its thermal and electrical properties.     

Crystal structure and magnetic properties of TbFe0.4Ge2 compound

Neutron diffraction and magnetic measurements were performed on polycrystalline TbFe_0.4Ge₂ which crystallizes with the orthorhombic structure of the CeNiSi₂-type. Despite the presence of some clear anomalies in the low-temperature magnetic susceptibility, the neutron diffraction experiment did not reveal any long-range magnetic ordering of the Tb magnetic moments down to 1.57 K.     

Influence of PbF2 and MoO3 on properties of PbWO4 crystals

The influence of doping (with F, Gd and Mo ions) and co-doping (with F, Gd or F, Gd, Mo ions) of PbWO₄ crystals was investigated. It is shown that even a small amount of specific impurities (especially F ions) causes an important redistribution of the point defects and changes the physical properties of the crystals. The cell parameters were refined from X-ray powder diffraction data. It was found that the cell parameters of F-doped PbWO₄ samples are larger than those of undoped samples. Photoluminescence and optical transmission spectra of pure and doped PbWO₄ were investigated.     

Electrical conductivity of (Er,U)O2+x and (Ce,U)O2+x

The electrical conductivity (σ) of (Er_yU_1−y)O_2+x (y=0.06, 0.20) and (Ce_yU_1−y)O_2+x (y=0.05, 0.15, 0.25) has been measured as a function of oxygen partial pressure in the temperature range of 1100≤T/°C≤1300 by a d.c. 4-probe method. Both of the oxides exhibited Po₂-regions where the electrical conductivity is independent of oxygen partial pressure, which indicates that doped Er and Ce exist as trivalent cations on uranium sites and fix the hole concentration by acting as electron acceptors, i.e. [h]=[Er′_U] and [h]=[Ce′_U], respectively. It is considered that strong oxidization tendency of uranium and reduction tendency of cerium simultaneously render the cerium ions exist exclusively as Ce³⁺ in the uranium dioxide. The electron-hole mobility of (Er_yU_1−y)O_2+x and (Ce_yU_1−y)O_2+x in the Po₂ region where σ is constant has been calculated by the combination of the electrical conductivity and charge carrier concentration; the activation energy (E_H) of each oxide has been obtained from the temperature dependence of the mobility. Small polaron hopping conduction mechanism was confirmed by small magnitude of the mobility (0.018-0.052 cm² V⁻¹ s⁻¹) and the activation energy (0.12-0.22 eV).     

Effect of impurity (Sb and Ag) incorporation on the a.c. conductivity and dielectric properties of a-Se70Te30 glassy alloy

The effect of additives (Sb and Ag) on a.c. conductivity and dielectric properties of Se₇₀Te₃₀ glassy alloy at temperature range 300-350 K and frequency range 1 kHz-5 MHz has been studied. Experimental results indicate that a.c. conductivity and dielectric parameters depend on temperature, frequency and the impurity incorporated in Se-Te glassy system. The a.c. conductivity in the aforesaid frequency range is found to obey the ω^s law. A strong dependence of a.c. conductivity and exponent s in the entire temperature and frequency range contradicts quantum-mechanical tunneling (QMT) model and can be interpreted in terms of the correlated barrier hopping (CBH) model. The temperature and frequency dependence of the dielectric parameters are also studied and it is found that the results agrees by the theory of hopping of charge carriers over potential barrier as suggested by Elliott in chalcogenide glasses. The change in the dielectric parameters with the opposite influence of the replacement of Te by Sb on the one hand, and by Ag, on the other hand is being correlated by the nature of covalent character of the studied composition and with the change in density of defect states.     

Electrochemical and structural properties of HT-LiCoO2 and LT-LiCoO2 prepared by the citrate sol-gel method

LT-LiCoO₂ has a slightly modified structure but significantly different electrochemical properties compared to LiCoO₂ prepared at high temperature. In this study, the structure and electrochemical properties of LT-LiCoO₂ have been evaluated using Raman spectroscopy, X-ray absorption spectroscopy (XANES and EXAFS), and electrochemical methods. According to the structural analysis, LT-LiCoO₂ has an intermediate structure between a layered and a spinel structure. The first discharge capacity of LT-LiCoO₂ is 80 mAh/g (100 μA/cm²).     

Phase transition, structural and electrical properties of Pb(Zn1/3Nb2/3)O3-doped Pb(Ni1/3Nb2/3)O3-PbTiO3 ceramics prepared by solid-state reaction method

Phase pure perovskite (1−x−y)Pb(Ni_1/3Nb_2/3)O₃-xPb(Zn_1/3Nb_2/3)O₃-yPbTiO₃ (PNN-PZN-PT) ferroelectric ceramics were prepared by conventional solid-state reaction method via a B-site oxide mixing route. The PNN-PZN-PT ceramics sintered at the optimized condition of 1185 °C for 2 h exhibit high relative density and rather homogenous microstructure. With the increase of PbTiO₃ (PT) content, crystal structure and electrical properties of the synthesized PNN-PZN-PT ceramics exhibit successive phase transformation. A morphotropic phase boundary (MPB) is supposed to form in (0.9−x)PNN-0.1PZN-xPT at a region of x=32-36 mol% confirmed by X-ray diffraction (XRD) measurement and dielectric measurement. The MPB composition can be pictured as providing a “bridge” connecting rhombohedral ferroelectric (FE) phase and tetragonal one since crystal structure of the MPB composition is similar to both the rhombohedral and tetragonal lattices. Dielectric response of the sintered PNN-PZN-PT ceramics also exhibits successive phase-transition character. 0.64PNN-0.1PZN-0.26PT exhibits broad, diffused and frequency dependent dielectric peaks indicating a character of diffused FE-paraelectric (PE) phase transition of relaxor ferroelectrics and 0.40PNN-0.1PZN-0.50PT exhibits narrow, sharp and frequency independent dielectric peaks indicating a character of first-order FE-PE phase transition of normal ferroelectrics. The FE-PE phase transition of 0.56PNN-0.1PZN-0.34PT is nearly first-order with some diffused character, which also exhibits the largest value of piezoelectric constant d₃₃ of 462pC/N.