Mobile Ag Ions Research Articles

Excitation Light Energy Dependence of Silver Photodiffusion into Amorphous Germanium Sulfide: Neutron and X‐Ray Reflectivity and X‐Ray Diffraction

Silver photodiffusion into amorphous chalcogenides is the photoinduced phenomenon, in which mobile Ag ions are injected into semiconductor films, and is applicable to nonvolatile memory devices. To understand the role of light illumination in the silver diffusion into a chalcogenide layer, the excitation light energy dependence of silver photodiffusion in Ag/a‐Ge20S80/Si substrate stacks is investigated by neutron reflectivity, X‐ray reflectivity, and X‐ray diffraction. The measurements reveal that there is an energy threshold to induce silver photodiffusion, which corresponds to the optical gap of amorphous Ge20S80. The excitation of the lone‐pair electrons to the antibonding states by the light illumination with greater energy than the optical gap leads to the bond breaking, and the Ge–S network structure is reorganized involving Ag ions.

Velocity correlation between mobile Ag ions in Ag β-alumina by MD calculation

The diffusional motion of mobile Ag ions was studied by molecular dynamics (MD) simulation in the super ionic conductor Ag β-alumina. The diffusion coefficient of Ag ion was estimated from the mean square displacement and the velocity correlation function, especially from the cross-correlation among the Ag ions. The velocity auto-correlation and cross-correlation functions exhibited approximately similar contribution to the Ag ionic diffusion at 300 K. The result of cross-correlation occurring cooperative ionic motion provided a qualitative explanation for the super ionic conduction.

Memristive devices from ZnO nanowire bundles and meshes

We report two types of memristive devices made of ZnO nanowire assemblies and Ag electrodes: nanowire-bundle and nanowire-mesh memristors. Although constructed with the same materials, these devices exhibit different characteristics. Nanowire-bundle memristors have small On/Off ratios and feature stable hysteresis under X-ray irradiation. Nanowire-mesh memristors show large On/Off ratios and multiple distinct states. We attribute the switching in bundle nanowires to the modification of the Schottky barrier by the mobile Ag ions and the stability of hysteresis to the ability of the bundles to retain Ag in the alleys between nanowires, as confirmed by first-principles calculations and energy dispersive x-ray measurements. For nanowire-mesh memristors, the high On/Off ratio leads us to attribute the switching mechanism to the formation and dissolution of Ag bridges in the nano-gaps at the intersections of nanowires.

Structure of silver bromide doped chalcogenide glasses

High-energy X-ray diffraction, neutron diffraction and extended X-ray absorption fine structure experiments have been carried out for (AgBr)x(As2Se3)1-x glasses. The addition of a large amount of AgBr does not significantly affect short-range orderings of the host network matrix. The interatomic distance of Ag-Br pairs in the present glass is much shorter than that of crystalline AgBr. Moreover, the coordination number of Ag ions is also different from the case of crystalline AgBr, and it decreases to be nearly four at x = 0.6. These results suggest that the basic structural motif around mobile Ag ions in the (AgBr)x(As2Se3)1-x glass is tetrahedral AgBr4 units as same as in the AgI-doped chalcogenide glass system AgI-As2Se3. It would be predictable that the structure model for AgBr-As2Se3 glasses is proposed to be a pseudo-binary mixture of the As(Se1/2)3 network matrix and AgBr-related conduction pathways.

Chemical bond and density of states of silver and copper halides based on DV-Xα method

We have calculated the electronic states of AgX and CuX (X = Cl, Br, I) by using the DV-Xα method. The density of states (DOS) and the bond overlap populations (BOP) of AgX and CuX are calculated by assuming the several model clusters with the nearest, second, and third-neighbor atoms. The cluster-size variations of DOS and BOP are discussed from the viewpoint of the chemical bond between the neighboring ions. We found that the BOP of AgI and CuX are larger than those of AgCl and AgBr. This result is consistent with the Phillips ionicity for ANB8-N compounds. Furthermore, we have investigated the variation of the BOP between Ag and I ions along the diffusion path of the mobile Ag ions in the model cluster for α-AgI.

不規則性をもつ結晶，超イオン導電体，非晶質，ガラス，融体の構造解析―不規則系・ランダム系の結晶学―１．周期系における不規則性の解析 回折法とＸＡＦＳ法を併用した不規則構造解析―α‐ＡｇＩ型構造の超イオン導電機構―

The Debye-Waller factors determined by diffraction and XAFS experiments provide important information about the thermal vibration and disordered structure. The statistical distribution of Ag in α-AgI and supersonic conduction mechanism have been discussed based on the effective one-particle potential, probability density function and effective pair potential. The disordered nature of α-AgI is due to the thermal motion and the probability of finding mobile Ag ions at saddle point is a few percent in α-AgI.

Dynamics ofAg+ions in binary tellurite glasses

We have reported the ionic relaxation for different compositions of $x{\mathrm{Ag}}_{2}\mathrm{O}\ensuremath{-}(1\ensuremath{-}x)\mathrm{Te}{\mathrm{O}}_{2}$ conducting glasses in wide temperature and frequency ranges. We have explained the composition dependence of the conductivity and the activation energy in terms of composition-dependent network structure of these glasses. We have compared the results for silver tellurite glasses with those for the alkali (Li,Na) tellurite glasses and observed that the migration of ${\mathrm{Ag}}^{+}$ ions in glasses, in general, appears to be anomalous. We have analyzed the frequency-dependent electrical data in the framework of the power-law conductivity formalism. We have explained the decreasing trend of the relaxation time with increasing ${\mathrm{Ag}}_{2}\mathrm{O}$ content in terms of structural transformation of tellurite network. Interestingly, we have observed that the concentration of mobile Ag ions is less than the concentration of total Ag ions and decreases with the increase of silver oxide content in the compositions, while the conductivity increases. We have shown that the scaling of the conductivity spectra is independent of temperature, but depends on composition.

Diffusion Mechanism of Ag ions in Superionic Conductor Ag2Se from Ab Initio Molecular-Dynamics Simulations

The dynamic properties of mobile Ag ions in the superionic conductor Ag 2 Se are studied by ab initio molecular-dynamics simulations. The population analysis including the gross charge and the overlap population shows that the bonding properties of Ag ions change accompanied with the diffusive motion. By investigating the displacement-correlation functions between Ag ions, it is clarified that Ag ions exhibit a collective motion with their neighbor Ag ions when they migrate between neighboring tetrahedral sites.

P– d hybridization, layered structure, and tetrahedral analysis in superionic conductors

The electronic states of silver halides and sodium halides are calculated by the DV-Xα cluster method to get microscopic evidence for the p–d hybridization in noble metal halides. It is found that both components of antibonding and bonding exist in the diagram of overlap population for AgX (X=halogen) and these two components are made up of the 4d band of Ag + and the p band of halogen ion, which form the p–d hybridization. Next molecular-dynamics studies have been carried out on a model material which is composed of accumulating two different fluoride conductors: ⋯BaF 2–CaF 2–BaF 2–CaF 2⋯. The diffusion coefficient and ionic conductivity of F ions in the layered fluoride conductors increases with decreasing periods. These dependences coincide with experimental results. Lastly, the dynamical correlation between diffusing cations and tetrahedrons (THs) of the anion bcc sublattice in α-AgI is studied using molecular-dynamics simulations. When a Ag ion approaches the boundary surface of an empty marked TH, in which the Ag ion is not included, the area of boundary plane of the TH becomes larger in order for the Ag ion to go easily into the TH. THs repeat contraction and expansion with motion of mobile Ag ions.

Low-temperature phases (phase II) of ionic conductors Ag 7TaS 6 and Ag 7NbS 6

The crystal structures of low-temperature forms of argyrodite-type superionic conductors, namely Ag 7TaS 6 II (between approx. 280 K and approx. 170 K) and Ag 7NbS 6 II (between approx. 280 K and approx. 140 K), analyzed based on powder X-ray diffraction data have been compared with each other. The crystal structure of Ag 7TaS 6 II could be described on the basis of a space group Pc with the lattice constants a=7.453, b=7.403, c=12.806 Å and β=124.62°. In the Rietveld analysis of Ag 7NbS 6 II, a commensurately modulated structure approach was adopted to obtain a smooth convergence. The crystal data of Ag 7NbS 6 II are expressed as a 1=7.368, a 2=14.770, a 3=12.768 Å, σ=(0.25 0 0), β=124.28° in a four-dimensional formalism with a superspace-group Pc( α 1/2 γ), that is A=29.47, B=14.770, C=12.768 Å, β=124.28° in a three-dimensional formalism with a space group Cc. In Ag 7TaS 6 II and Ag 7NbS 6 II, almost a half of Ag ions are in distorted tetrahedrons and the other Ag ions are in triangular faces shared by two tetrahedrons. A split model of mobile Ag ions in the room-temperature phase I is discussed in connection with Ag sites in the low-temperature phase II.

Powder X-ray diffraction of low-temperature phases of argyrodite-family compound Ag 7TaS 6

Low-temperature X-ray diffraction studies revealed the existence of two low-temperature phases of an argyrodite-family compound Ag 7TaS 6 that contains mobile Ag ions at room temperature, the phase II (between about 280 and 170 K) and the phase III (between about 170 and 10 K). The powder pattern of the phase II could be indexed on the basis of an orthorhombic unit cell, A = 7.442, B = 7.403 and C = 10.531 A ̊ . A probable assignment on the basis of tetragonal superstructure model was proposed for the powder pattern of the phase III.

Phonon spectra in superionic and normal states of the layered crystal AgCrSe 2

We measured far-infrared spectra of layered AgCrSe 2 through the transition temperature T c. The observed frequency shifts for the E- and A 1-modes at the highest frequency showed the knee at temperatures corresponding to the change of activation energy. This result suggests a small possibility for direct coupling of mobile Ag ions with those phonons, and is also consistent with the already proposed mechanism.

(AgI)x(Ag2S·GeS2)1−x glasses studied by 109Ag NMR

The 109 Ag spin-spin and spin-lattice relaxation times for x =0.375 and x =0.5 have been measured in order to elucidate the Ag diffusion dynamics. The results are consistent with the fact that the conductivity rises with increasing x and that the GeS 2 glasses are better conductors than the corresponding B 2 O 3 glasses. Although the spin-lattice relaxation cannot be explained by the simple BPP formula or a Cole-Davidson distribution of correlation times, the data allow to estimate the number of mobile Ag ions. On the NMR time scale all ions are mobile provi---one assumes that the Ag ions “hop” from one iodine ion to the other.

The interdependence of mobile and stationary ions in RbAg4I5— An NMR investigation

Previous NMR studies in RbAg4I5 have been supplemented by additional measurements on the 87Rb nuclei and extended to the mobile Ag ion. The thermally activated relaxation rates of the stationary and the mobile ions show remarkable differences in the overall temperature behavior as well as at the 208 K phase transition. A possible interpretation is given in terms of “trial and error” hopping and configurational response of the mobile Ag ions.