Absence of a space-charge-derived enhancement of ionic conductivity in β|γ- heterostructured 7H- and 9R-AgI

Extreme room temperature conductivity enhancements have been reported for nanocrystalline AgI of up to ×?10(4) relative to bulk β-AgI (Guo et?al 2005 Adv.?Mater. 17 2815-9). These samples were identified as possessing 7H and 9R polytype structures, which can be considered as heterostructures composed of thin, commensurate layers in the β (wurtzite) and γ (zincblende) phases. It has been proposed that space-charge layer formation at β|γ-interfaces causes near complete disordering of the Ag(+)?sublattice in these polytypes, resulting in a massive intrinsic enhancement of ionic conductivity. We have performed molecular dynamics simulations of β- and γ-AgI and mixed β|γ?superlattices, to study the effect of heterostructuring on intrinsic defect populations and Ag(+) transport. The ionic conductivities and Ag(+)?diffusion coefficients vary as β?>?7H?≈?9R?≈?10L?>?γ. The β|γ-heterostructured polytypes show no enhancement in defect populations or Ag(+)?mobilities relative to the β-AgI phase, and instead behave as simple composites of β- and γ-AgI. This contradicts the proposal that the extreme conductivity enhancement observed for 7H and 9R polytypes is explained by extensive space-charge formation.     

Pseudopotential bandstructure calculations for β-AgI and γ-AgI

The pseudopotential calculations of Cohen and Bergstresser on the zinc-blende structures of Sn, InSb and CdTe, are extended to the I–VII compound, AgI. The resulting bandstructure for the metastable phase with the zinc-blende structure, denoted γ-AgI, is found to be in good agreement with that derived by a previous tight-binding calculation.The same pseudopotential form factors are used to calculate the bandstructure for β-AgI which is the stable low temperature phase having the wurtzite structure. This bandstructure is shown to be consistent with the predictions of the earlier tight-binding analysis.Taken together, these bandstructures and pseudopotential form factors provide a suitable basis for calculating the electronic properties of these two silver iodide polytypes.     

含分散第二相粒子的离子导体AgI的电学性质

测量了纯AgI和AgI含分散第二相粒子(简称DSPP)η-Al₂O₃(0.05μm)和γ-Fe₂O₃(0.6μm)的电导率随成分和温度的变化。β-AgI的电导率随加入第二相粒子含量的增加而明显增加,大约在40—50mol％时电导出现极大值。室温下含DSPP的电导率比纯AgI增加二到三个数量级。含DSPP的α-AgI电导率降低,激活能增加。电导率测量和差热分析(DTA)表明第二相的加入明显地影响AgI从α相到β相的相转变温度,AgI(η-Al₂O₃,50mol％)α相到β相的转变温度比纯AgI低17℃。室温下对AgI(γ-Fe₂O₃,50mol％)的直流极化实验表明电子电导是n型导体,电子电导率与总电导率相比可以忽略。 关键词：     

Solution calorimetric investigation of AgCl-AgI ionic conductor composites at 298 K: observation of metastable AgI modifications

The enthalpies of solution of pure silver halides AgCl and AgI and a composite material with molar composition 0.5 AgCl-0.5 AgI were measured at 298 K in a mixture of Na₂S₂O₃ (1 M) and NH₄OH (1 M). X-ray diffraction patterns showed that the composite material contained the metastable γ-AgI phase; different mechanisms for its stabilization were discussed. The phase transition enthalpies of AgI modifications and the enthalpy of formation of the composite material were deduced from the measurements. The latter could be related to a change of interfacial enthalpies.     

Dielectric and NMR Studies of the superionic conductor AgI embedded in mesoporous silicate matrices

This paper reports on the results of studies of small particles of the superionic conductor AgI embedded in MCM-41 and SBA-15 molecular sieves with different sizes of pores (channels). The studies are performed by the dielectric and nuclear magnetic resonance techniques. The temperature of the superionic phase transition is shown to increase monotonically with decreasing pore size. The enthalpy of activation of the ion motion in β-AgI in restricted geometry is determined. It is shown that introduction of AgI into pores does not noticeably affect its structure.     

AgI nanostructure development in sputter-disordered and Al-doped Ag films probed by XRD,SEM, optical absorption and photoluminescence

By use of thermal evaporation and rf magnetron sputtering 300 and 500 Å thick Ag and Ag (Al) films were prepared. γ-AgInanoparticles were formed during (a) short time (2–5 min) iodization of undoped thermally evaporated Ag films, (b) longer time (12 h) iodization of undoped rf sputtered Ag films and (c) short time (2–15 min) iodization of thermally evaporated Ag_0.95Al_0.05 and Ag_0.90Al_0.10 films of 500 and 300 Å thickness respectively. Both rf sputtered and Al doped Ag films yielded ～20 to ～60 nm sized γ-AgI particles upon iodization. Optical absorption spectra reveal Z_1,2 and Z₃ exciton transitions with increased broadening of γ-AgI nanoparticles, suggesting the effect of disorder produced during film formation. Blue shift observed with increasing film thickness could be the effect of decreasing particle size, thereby increasing the quantum confinement effects. Photoluminescence studies show that the donor-acceptor recombination rate, enhanced by 25% for Ag_0.95Al_0.05I film relative to that of undoped AgI, is due to the tight binding of Al to surface defect sites.     

pH响应的包覆超顺磁性纳米颗粒的γ-聚谷氨酸二级结构研究

用傅里叶红外光谱法(FTIR)定量研究了pH对包覆超顺磁性纳米颗粒的γ-聚谷氨酸(γ-PGA)二级结构变化的影响。结合傅里叶去卷积技术和二阶导数法对原始谱带(酰胺Ⅰ带)进行高斯拟合,计算了二级结构的相对百分含量。红外结果显示:pH变化影响γ-PGA的二级结构。γ-聚谷氨酸磁性纳米微球中γ-PGA的β-折叠和β-转角的总含量很大,达65％以上,而α-螺旋和无规卷曲的含量则比较少。随着pH值的增大,β-折叠的含量逐渐减少相反β-转角的含量逐渐增大。γ-PGA二级结构变化与γ-聚谷氨酸磁性纳米微球在水溶液的稳定性有关。考察了γ-聚谷氨酸纳米微球的zeta电位随pH的变化。结果表明,pH为10.2时zeta电位出现极小值,其绝对值最大,颗粒稳定性最好。     

(NaZSM-5)-AgI主体-客体纳米复合材料的研究

用热扩散法成功地把AgI组装于NaZSM5的孔道中,对制备的样品(NaZSM5)AgI进行了化学分析、粉末XRD分析、化学吸附、红外光谱表征,研究了所制备的样品固体扩散漫反射吸收光谱及发光性质.化学分析表明,AgI已进入NaZSM5主体中.粉末XRD分析显示了组装AgI后NaZSM5沸石骨架依然存在,红外光谱表明了(NaZSM5)AgI样品骨架振动与NaZSM5的骨架振动有细微差别,这主要是由组装了AgI引起的.吸附研究说明了AgI已进入NaZSM5的孔道.固体扩散漫反射吸收光谱表明,主体NaZSM5对制备的主体客体样品光吸收几乎没有影响,(NaZSM5)AgI的吸收光谱受AgI尺寸大小的影响.发光研究表明,制备的样品能带隙很高,辐射过程很强,(NaZSM5)AgI样品系具有发光功能的复合材料.     

Intrinsic paramagnetic defects probe superionic phase transition in Ag1−xCuxI (x=0, 0.05, 0.15 and 0.50) nanocrystals

EPR probed the zincblende (γ) to cubic (α) AgI structural phase transition in AgI at 423 K through two intrinsic paramagnetic centers: an Ag²⁺-based hole center (signal ‘A’) and an Ag⁰-based conduction electron center (signal ‘B’) associated with AgI nanocrystallites. Sudden drops in intensity (I_PP), <g>, and ΔH_PP observed at 423 K for pure AgI nanocrystals. Addition of Cu in AgI increases the thermal stability of the cation sublattice as seen from the increase in the transition temperature from 423 K (undoped AgI) to 453 K. Abrupt jumps in the number of spins (N) and reciprocal susceptibility (1/χ) observed at increased phase transition temperatures in Cu-substituted AgI relative to that in undoped AgI reflects progressively strengthened local bonding configuration of γ-AgI structure induced by Cu.     

Ionic conductivity and interfacial interaction in penton/AgI composites

New nanostructured composite system (1???x)penton/xAgI (where 0?< x?<?1) was synthesized by a solution-based technique, which involves the process of modification of polymer surface. Samples of the composite system were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and electrical impedance spectroscopy. It was shown that AgI nanoparticles form a continuous layer at the surface of penton particles and consist mainly of cubic γ-phase with a small amount of hexagonal β-phase. Maximum conductivity enhancement at almost one order of magnitude higher compared to the pristine AgI has been observed for the sample with x?=?0.5. The overall activation energy for conduction varies from 0.23 to 0.38 eV, depending on the content of γ-phase of silver iodide in the samples. Two percolation thresholds has been also recorded at the points x?=?0.1 and x?=?0.3.     

HNC calculations of the partial structure factors of molten AgI

We have calculated the pair distribution functions and partial structure factors of molten AgI within the hypernetted chain (HNC) approximation. We have assumed that the effective interionic pair potentials are the same in the liquid and solid states, and have used the potentials constructed by Parrinello et al., which they used to describe both -AgI and the phase transition in AgI. The structure of molten AgI exhibits features similar to those of molten CuCl. Both of these salts melts from a superionic phase.     

AgI(α-Fe2O3)复合离子导体相转变温度相互影响的研究

通过测量AgI(α-Fe₂O₃)复合离子导体的直流和交流电导率,发现α-Fe₂O₃的Morin转变温度随着AgI含量的增加而降低,电导率测量的结果与磁化率测量和M?ssbauer谱测量结果定性的符合。α-Fe₂O₃也使AgI的α→β相转变温度显著下降。结果表明复合离子导体二相材料各自存在的相转变温度彼此产生影响。 关键词：     

Phonon dispersion and β→α transition in silver iodide

The phonon dispersion of β-AgI single crystals has been measured by inelastic neutron scattering at room temperature. The most interesting feature is a low lying optical branch at 2 meV. This mode shows a flat dispersion and has an unusually high amplitude of vibration favouring the promotion of a cation to an interstitial site. It is shown that this mode is essential for the occurence of a cation order-disorder transition into a highly ionic conducting phase (α-AgI) at 147°C.     

Phase states and magnetic properties of iron nanoparticles in carbon nanotube channels

The structure, phase composition, and magnetic properties of carbon nanotubes filled with iron nanoparticles and obtained by thermolysis of a mixture of ferrocene and C₆₀ fullerene or ferrocene and orthoxylene at a temperature of 800°C are investigated. Electron microscopy, X-ray diffraction, and Mössbauer spectroscopy data lead to the conclusion that carbon nanotubes are multilayer systems partially filled with iron nanoparticles and/or nanorods. Metallic inclusions in nanotube channels form α-Fe, γ-Fe, and Fe₃C phases. The concentration of each phase in the samples is determined. It is shown that 10–20-nm iron clusters in nanotubes exhibit magnetic properties typical of bulk phases of iron. High elasticity of carbon nanotube walls facilitates stabilization of the high-temperature γ-Fe phase; the relative concentration of this phase in a sample can be increased by lowering the concentration of ferrocene in the initial reaction mixture.     

Ab-initio study of structural phase transitions and optoelectronic properties in BeH2 at increasing pressure

The structural stability, electronic and optical properties of BeH₂ under high pressure have been studied using the density functional theory (DFT) employing full potential-linearized augmented plane wave (FP-LAPW) method. The exchange correlation functional has been solved using the generalized gradient approximation. The calculations show that BeH₂ becomes unstable upon application of pressure. At a pressure of 29.40 GPa the ground state α-BeH₂ transforms to hypothetical phase β-BeH₂ and further at a pressure of 53.77 GPa (with respect to the ground state α-BeH₂) β-BeH₂ transforms to γ-BeH₂. In α-BeH₂ phase it remains as an insulator while in β-BeH₂ phase its behavior becomes metallic. But upon further increase in pressure it becomes a semiconductor in γ-BeH₂ phase. Hence the possibility of obtaining high-pressure phases with superconducting properties cannot be ruled out. There occurs a huge equilibrium volume collapse at α- to β-phase transition and relatively smaller volume changes at β- to γ-phase transition. Our obtained value of dielectric constant (3.0) for α-BeH₂ is in excellent agreement with earlier reported value (3.1). Also BeH₂ shows anisotropic behavior in all three studied phases.     

Structural studies of the P-T phase diagram of sodium niobate

The crystal structure of sodium niobate (NaNbO₃) has been investigated by energy-dispersive X-ray diffraction at high pressures (up to 4.3 GPa) in the temperature range 300–1050 K. At normal conditions, NaNbO₃ has an orthorhombic structure with Pbcm symmetry (antiferroelectric P phase). Upon heating, sodium niobate undergoes a series of consecutive transitions between structural modulated phases P-R-S-T(1)-T(2)-U; these transitions manifest themselves as anomalies in the temperature dependences of the positions and widths of diffraction peaks. Application of high pressure leads to a decrease in the temperatures of the structural transitions to the R, S, T(1), T(2), and U phases with different baric coefficients. A phase diagram for sodium niobate has been build in the pressure range 0–4.3 GPa and the temperature range 300–1050 K. The dependences of the unit-cell parameters and volume on pressure and temperature have been obtained. The bulk modulus and the volume coefficients of thermal expansion have been calculated for different structural modulated phases of sodium niobate. A phase transition (presumably, from the antiferroelectric orthorhombic P phase to the ferroelectric rhombohedral N phase) has been observed at high pressure (P = 1.6 GPa) and room temperature.     

Thermoelectric power measurements in polycrystalline AgI: Cd system

Thermoelectric power of polycrystalline AgI: Cd system was measured as a function of temperature from 50 to 180°C. The heat of transport $\text{Q}{}^1$, intrinsic vacancy concentration c_o and ratio of interstitial to vacancy mobilities in the β phase were deduced under the assumption of no association. The heat of transport $\text{Q}{}^1$ was also temperature dependent in β-AgI: 0.27 eV at 80°C, 0.21 eV at 100°C and 0.17 eV at 120°C. It was estimated that the formation energy of the defect pair h_F was 0.66 ± 0.06 eV and the activation energies for motion of vacancies and interstitials were 0.55 ± 0.03 and 0.38 ± 0.03 eV, respectively. These approximately agree with data reported up to date in single and poly- crystal β-AgI. The heat of transport of vacancies was approximately equal to the activation energy of vacancy migration.     

Mn-Ga二元系的X射线研究

本文从44个缓冷和淬炼Mn-Ga合金摄取了德拜·谢乐照相,并配合了在富Ga部分的差热分析,初步画出了这个系统的相图。这个系统除纯Ga外共有十个相。Mn在Ga中的固溶度是几乎无可觉察的。α相是Ga在α-Mn中的原固溶体,在室温的固溶度为1.95at％Ga。β相在室温的均匀范围为8.6—19.2at％Ga,这是β-Mn结构,因此可看作是β-Mn的固溶体,由于Ga原子无规地替代了部分Mn原子而这个结构得在室温稳定存在。γ相可分成γ₁,γ₂,γ₃,三部分,γ₁是面心立方结构,γ₂是面心四方结构,γ₃是有序的面心四方结构,与Cu-Au系中的CuAuⅠ同型。在室温下稳定的是γ₃,均匀范围为37—45at％Ga,而在高温稳定的却总是γ₁。从γ₁变到γ₂,再从γ₂变到γ₃的变化是二级相变。有序度随Ga含量的递增而递增,随温度的递升而递降。整个γ相可看作是γ-Mn的固溶体,γ-Mn本身是不可能用淬炼的办法在室温获得的。δ相只存在于高温,可看作是δ-Mn的固溶体。由于Ga原子替代了部分Mn原子,因而δ一Mn结构产生了畸变而有序化。ε相是有序的六角密堆积结构,每个晶胞含8个原子,它是在约820℃从γ相同成份地转变而成的,在室温的均匀范围估计为27一30at％Ga。η相在室温约50—60at％Ga处有一宽广的均匀范围。从520到600℃,它经历一多型性变化,转变为λ相。λ相的相区随温度的递升而向富Mn的一边偏移。η和λ结构都很复杂。在富Ga的一边,存在着三个居间相χ,φ和ω,它们是由包析或包晶反应所形成的。ω相的化合式很可能相当于Mn₂Ga₉或MnGa₅,而φ相则与NiHg₄同型,在Mn_2.3Ga_7.7左右有一狭隘的均匀范围。在室温稳定存在的七个居间相中,β,ε,γ₃,X和φ是铁磁性的。铁磁性最强的是Ga含量较富的γ₃和φ相。我们测量了其中若干合金的饱和磁化强度与居里温度。 关键词：     

Hot stage X-ray diffractometer studies on Ag2PbI4 and Ag4PbI6

Mixtures of AgI and PbI₂ cooled from the melt result in the peritectic formation of a fast ion conducting phase centred about Ag₄PbI₆, which is face centred cubic with a = 6.33(5)A; this phase exhibits high electrical conductivity. On cooling to about 125°C, dissociation occurs to γAgI and PbI₂, accompanied by the transient formation of another phase, centred about Ag₂PbI₄. A modified form of the T-x section of the equilibrium phase diagram at AgI concentrations greater than 60 mole % and below 300°C is proposed.     

Variations of chemical bonds in silver halides under high pressures

ABSTRACT

Rocksalt structured AgI (rs-AgI), which appears under pressures between 0.4 and 11.3?GPa, shows high ionic conductivity as high as that in α-AgI, especially at high temperatures. Microscopic origins of ion conduction mechanisms have not been clarified until now and are therefore investigated using the discrete variational Xα (DV-Xα) cluster method. Comparable studies for AgCl and AgBr, which are known as high ionic conductors just below melting temperatures and form the rocksalt structure, are also done. Ionic interactions between a mobile Ag ion and remaining ions are almost the same between those under different pressures, while covalent interactions between the mobile Ag ion and the remaining Ag ions change drastically when the mobile Ag ion is migrating. Similar results are also obtained for AgCl and AgBr. The covalent interactions between the mobile Ag ion and the remaining Ag ions, which should affect the Ag ion migration, play important roles in not only rs-AgI but AgBr and AgCl.