Silver Borate Research Articles

Silver ion dynamics in silver borate glasses: spectra and multiple-time correlation functions from 109Ag-NMR

The motion of silver ions in (AgI) x –(Ag 2O–B 2O 3) 1− x glasses with AgI concentrations of x = 0.5 and 0.7 was studied using 109Ag-NMR. The NMR spectra were analyzed in terms of a superposition of two different contributions. These are associated with Ag ions, which hop on vastly different time scales. The existence of dynamic heterogeneity, i.e. the existence of slow and of fast contributions to the hopping dynamics was directly demonstrated using a four-time stimulated-echo experiment. The results are compatible with an intrinsically exponential response. From measurements of the spectra and of two-time correlation functions a Gaussian distribution of energy barriers, hindering the ionic motion, could be mapped out.

X-ray absorption fine structure: characterization of thermal and structural disorder in non-crystalline solids

A great progress has been made in the last years in the XAFS studies of thermal and structural disorder, due to the increased quality of measurements allowed by the intense third generation synchrotron radiation sources, and the refinement of the analysis techniques. A description of XAFS strengths and limits is given and some paradigmatic investigations on thermal and structural characterization of crystalline and non-crystalline solids are reported. In particular, the environments of the nearest neighbors of selected species in some germanium based compounds and silver borate glasses are described in terms of radial distribution functions and their main parameters, such as mean distance, variance and asymmetry. The determination of local thermal expansion coefficients in crystalline and non-crystalline solids by temperature dependent studies is shown.

Ionic Conductivity and Relaxation Dynamics in Fast Ion Conducting Silver Borate Glasses

Dynamics of Ag + ions in AgI-Ag 2 O-B 2 O 3 glasses has been studied in the frequency range 10 Hz to 2 MHz and in the temperature range from 323 K to 93 K. The frequency dependent electrical data have been analyzed in the framework of conductivity and modulus formalisms. Both of these formalisms have provided for qualitative similarities in the compositional variation of the relaxation times and the dc conductivity. The results were compared with specific assumptions in the various structureal models of the glasses. We have obtained the mobile ion concentration and the power law exponent from the analysis of the conductivity spectra. We have observed that the concentration of Ag + ions is independent of temperature and the conductivity is primarily determined by the mobility. A fraction of the Ag + ions in the glass is involved in the dynamic process. We have also shown that the power law exponent is independent of temperature and demonstrated the existence of three dimensional conduction pathways of the glasses. The results are also supported by the temperature and composition independence of the scaling of the conductivity spectra. The stretched exponential parameter β remains almost invariant with temperature and composition and its small value for all the compositions signify that, the conductivity relaxation is highly non-exponential. Conductivity relaxation in these glasses was observed to be independent of temperature as that of conductivity spectra.

Low-frequency Raman scattering of silver borate glasses

The vibrational dynamics of Ag2O–B2O3 glasses has been investigated by Raman spectroscopy in the frequency region below 200 cm−1. The spectra show the presence of the boson peak (BP), whose shape and frequency location are almost independent of the silver concentration. The increase in the sound velocity with increasing Ag+ content testifies to the progressive stiffening of the borate skeleton and refers to the existence of acoustic modes with increasing frequency. Consequently it is believed that the vibrational modes mainly contributing to the BP have a localized character and are decoupled from the acoustic phonons. An interpretation of these experimental observations ascribes a significant role in the evolution of the low-frequency vibrational dynamics of these glasses to the local vibrations of Ag+ ions.

Ordered and disordered models of local structure around Ag cations in silver borate glasses based on x-ray absorption near-edge structure spectroscopy

The local coordination of Ag cations in silver borate glasses ${\mathrm{Ag}}_{2}\mathrm{O}\ensuremath{\cdot}\mathrm{n}{\mathrm{B}}_{2}{\mathrm{O}}_{3}$ has been studied by comparing the experimental x-ray absorption near edge structure (XANES) at the Ag K edge with results of theoretical simulations. We demonstrate that simple models which describe the local structure around Ag with a single geometric configuration cannot be reconciled with experimental XANES spectra. In order to obtain a satisfactory agreement between theory and experiment, it is necessary to include the disorder also at the short range. Structural information is extracted from XANES data using an empirical approach, which is based on the presence of a multiplicity of atomic structural configurations around photoabsorbing atoms. This approach is particularly suited for describing the local environment of atomic species belonging to glass network modifiers.

Ultrasonic relaxations in borate glasses

Abstract The attenuation and velocity of ultrasonic waves of frequencies in the range from 10 to 70 MHz have been measured in M 2 O–B 2 O 3 borate glasses (M: Li or Ag) as a function of temperature between 15 and 350 K. The velocity of sound waves decreases with increasing temperature in all the glasses, the decrease as the temperature is increased is larger in glasses containing silver than in those with lithium. A broad relaxation peak characterises the attenuation behaviour of the lithium and silver borate glasses at temperatures below 100 K and is paralleled by a corresponding dispersive behaviour of the sound velocity. Above 100 K, the ultrasonic velocity shows a nearly linear behaviour regulated by the vibrational anharmonicity, which decreases with increasing content of modifier oxide and is smaller in lithium than in silver borates. These results suggest that the relaxation of structural defects and the anharmonicity of borate glasses are strongly affected by two parameters: the number of bridging bonds per network forming ion and the polarising power of network modifier ions which occupy sites in the existing interstices.

EXAFS studies of the local thermal expansion in borate glasses

Temperature-dependent EXAFS measurements have recently been made on the silver borate glass [Ag 2O·4B 2O 3], and the superionic AgI-doped glass [AgI] 0.75[3Ag 2O·1B 2O 3] 0.25. The Ag–O co-ordination has been studied in the first sample, the I–Ag in the second one. In both cases a significant reduction of the first shell inter-atomic distance has been observed as temperature increases from 15 to 300 K. Here a detailed analysis of the EXAFS results for the AgI-doped glass is presented. The distributions of first shell I–Ag distances reconstructed from EXAFS for the glass and c-AgI, and their leading statistical parameters, are compared and critically discussed. The local negative expansion in the glass is partially attributed to the relaxation of the a-AgI network within the host binary matrix.

Free volumes in glasses and their relation to compressibilities

Because of the disordered structure, glass has many voids in which no atoms exist. This is characterized by the structural-free volume. It is expected that the larger the structural-free volume, the larger would be the compressibility. This is verified for some alkali silicate and silver borate glasses; i.e., both the structural-free volume and the compressibility decrease monotonously with increasing alkali or silver content. Also found is that the compressibility is different for different alkali species. This implies the presence of another free volume that originates from the atoms in themselves. The atomic-free volume is defined for s-electron metals as alkalis and is related to the compressibility.

Infrared spectroscopy of AgI doped Ag 2S+B 2S 3 fast ion conducting thioborate glasses

The infrared (IR) spectra of binary silver thioborate xAg 2S + (1 − x)B 2S 3 glasses (0.3⩽ x⩽0.56) and ternary zAgI + (1 − z) [0.5Ag 2S + 0.5B 2S 3] glasses (0⩽ z⩽0.3) are reported. The structure of the binary glasses changes with an increase in modifier content as observed by a monotonic decrease in the fraction of tetrahedrally coordinated boron units and an increase in the fraction of orthothioborate units. This reduction in four coordinated boron and increase in tri-coordinated orthothioborate boron units has been seen in the alkali thioborates and analogous silver borate glasses. The structure of the ternary glasses shows little to no change in the IR spectra with an increase in AgI ( z) up to 30 mol%. This result agrees with previous studies performed on alkali thioborate and silver borate glasses.

Mixed cation effect in silver borate ion conducting glass

The partial replacement of silver iodide in ion conducting silver borate glass (AgI-Ag2O-B2O3) by sodium iodide has been carried out and its effect on the properties of the glass system has been studied. Conductivity studies show that with the simultaneous presence of two cations (Ag+ and Na+), mixed cation effect is observed. There is also lower solubility of AgI which crystallised out. The results obtained have been explained by data from X-ray diffraction, FTIR spectroscopy and conductivity vs. temperature studies.

Low-energy vibrational dynamics and connectivity in borate glasses

A study of the specific heat at low temperatures (1.5-25 K) of silver borate glasses has revealed the presence of an excess specific heat over the Debye contribution, which scales with the content. This observation has been related to the presence of non-bridging oxygens in the borate network, which are assumed to be a source for additional low-energy vibrations causing the excess . Furthermore, comparative measurements of and low-frequency Raman scattering (below ) at low temperatures in the same borate glass permit one to assess the frequency dependence of the photon-vibration coupling coefficient and the spectral density of low-energy vibrational modes . By comparison with the results obtained by the same procedure for vitreous , we show that a decreasing connectivity of the network leads to an increasing magnitude and softening of .

Effect of Replacement of Glass-Modifier Ag2O by Na2O in Silver Borate Ion–Conducting Glass

Effect of Replacement of Glass-Modifier Ag2O by Na2O in Silver Borate Ion–Conducting Glass

Structure and cation dynamics in the systemAgI:Ag2MoO4:A109AgNMR study

The system $(\mathrm{AgI}{)}_{1\ensuremath{-}x}({\mathrm{Ag}}_{2}{\mathrm{MoO}}_{4}{)}_{x}$ is studied with ${}^{109}\mathrm{Ag}$ solid-state NMR and differential scanning calorimetry. Glasses are formed for $0.2lxl0.4.$ Outside the glass formation region a glassy phase of molar composition ${0.66\mathrm{}\mathrm{A}\mathrm{g}\mathrm{I}\ensuremath{-}0.33\mathrm{}\mathrm{Ag}}_{2}{\mathrm{MoO}}_{4}$ is always found, which is accompanied by \ensuremath{\beta}-AgI or ${\mathrm{Ag}}_{2}{\mathrm{MoO}}_{4}$ crystals. Upon thermal treatments above 120 \ifmmode^\circ\else\textdegree\fi{}C, a ${0.66\mathrm{}\mathrm{A}\mathrm{g}\mathrm{I}\ensuremath{-}0.33\mathrm{}\mathrm{Ag}}_{2}{\mathrm{MoO}}_{4}$ (2:1) phase crystallizes over the entire composition range. The chemical shift anisotropy tensor of this phase has been assigned. The anisotropy is \ensuremath{\sim}70 ppm, that is an order of magnitude lower than the isotropic chemical shift range observed in silver borate glasses. The addition of AgI to ${\mathrm{Ag}}_{2}{\mathrm{MoO}}_{4}$ causes the formation of oxy-iodide sites. The existence of AgI clusters can be ruled out. Spin-lattice relaxation inside the glass-formation region seems to be controlled by Ag-I scalar coupling of the first kind, rather than by chemical shift anisotropy. The existence of two maxima in the spin-lattice relaxation rate curve points towards ${\mathrm{Ag}}^{+}$ populations with different mobility.

X-ray analysis of the structure of Ag-tetraborate glasses (AgI) x (Ag 2O · 4B 2O 3) 1− x

New X-ray diffraction measurements on (AgI) x (Ag 2O · 4B 2O 3) 1− x glasses with AgI in a concentration range, x, from 0 to 0.55 are presented. The X-ray scattering diagram changes significantly when AgI is added to the tetraborate glass. The pre-peak, located at 1.12 Å −1 for Ag-tetraborate, moves to 1.0 Å −1 for x=0.1 and then shifts continuously until it reaches 0.91 Å −1 for x=0.55. At the same time, the peak amplitude increases. The radial electron density distribution curves obtained show that the peak corresponding to the first Ag–I coordination shell increases, and confirm that ordering of AgI units at greater distances is not present in these glasses. The analysis of these new data supports the already proposed structural mechanism for embedding AgI into silver borate glasses: in the Ag-tetraborate, as well as in the Ag-diborate, the AgI-rich regions start to organise to form a network surrounding the borate configurations. X-ray data confirm that, when the AgI content is increased, the borate configurations are separated by greater distances. The position of the smaller first scattering peak corresponds to the distance between the regions filled with borate configurations.

Temperature dependence of ionic carrier concentration and mobility in the silver borate glasses

The temperature dependence of conductivity σ and mobility μ (and hence the carrier concentration n) have been directly measured for silver-borate glass compositions (undoped and doped with PbO or SnO 2). The σ and μ increase while n decreases with the increase in temperature. The decrease in n has been assigned to Ion Association (Self and Network) as a result of formation of Non-Bridging Oxygen (NBO) functional groups. The PbO doping also increases the number of NBOs. A phenomenological description of the changes introduced in the potential energy experienced by the mobile cation is also given in terms of the formation of NBO's and/or Ion Association.

Transport studies on a new fast silver ion conducting system: 0.7[0.75AgI: 0.25AgCl]: 0.3[yAg2O:(1 − y)B2O3

Abstract A silver borate system with the composition 0.7[0.75AgI: 0.25AgCl]: 0.3[ y Ag 2 O:(1 − y )B 2 O 3 ] has been prepared in which the host salt used is a “quenched [0.75AgI: 0.25AgCl] mixed system” instead of the commonly used host AgI. Three sample preparation procedures were adopted: (a) molten sample at ∼ 650°C dropped into neutral liquid bath at ∼ 10°C; (b) molten sample in silica ampule at ∼ 650°C left in air to cool by itself; (c) molten sample left in furnace and allowed to come to room temperature in ∼ 24 h. The highest room temperature conductivity σ ⋍ 2.23 × 10 −2 S cm −1 was obtained for the sample composition: 0.7[0.75AgI: 0.25AgCl]: 0.3[0.833Ag 2 O: 0.167B 2 O 3 ] prepared by procedure “a” and referred to as an “optimal composition”. The DTA, XRD (including linewidth measurement for determining Scherrer length/crystallite size), temperature dependence of σ (by Impedence Spectroscopy), mobility μ (by Transient Ionic Current technique), and number of charge carriers n are also reported. Solid state batteries with the cell configuration: Ag/ 0.7[0.75AgI: 0.25AgCl]: 0.3[0.833Ag 2 O: 0.167B 2 O 3 ]/ (C + I 2 ) were fabricated and the discharge characteristics were studied.

Small inhomogeneities in AgI—silver borate glasses and their relationship to sample composition

Low- Q neutron-scattering studies have been performed on some Agl silver borate glasses of compositions (AgI) x − (Ag 2O− nB 2O 3) 1− x -here n = ⅓. x = 0.65,0.6,0.4 and n = 1, x = 0.6,0.4,0.2. Analysis of the spectra have revealed the presence of inhomogeneities within the materials whose concentration increases with (1 − x)/ n x and whose size decreases as B 2O 3 content increases.

A neutron and x-ray diffraction investigation of intermediate range structure in silver borate glasses

The ion diffusion mechanism in ion conducting glasses has in many reports been suggested to be linked to the structure. However, in most cases the structures of the glasses are not very well known. We have investigated the structure of the glass system (AgI)x (Ag2O-2B2O3)1−x, with x=0, 0.6 by neutron and x-ray diffraction experiments. AgI doping produces an intense peak in the neutron data at an anomalously low Q-value (0.8 A−1) that indicates increased intermediate range ordering. Only a weak peak is observed at a similar Q-value in the x-ray structure factor. By comparing the different neutron and x-ray weighting factors the origin of the anomalous peak is unambiguously attributed to correlations within the B-O network. The experimental results have been used to model the glasses by Reverse Monte Carlo simulations to obtain more detailed views of the structures.

Structural studies of ionically conductive glasses by EXAFS

A good knowledge of the local structure is necessary for a better understanding of the high conductivity shown by some ions in oxide glasses. The ability of X-Ray Absorption Spectroscopy (XAS) to select a single atomic species makes this technique very useful in this field : XANES and EXAFS can give information not only on radial distribution function of atoms around the absorbing species but also on their geometrical arrangement. The experience of the last years shows that peculiar information can be extracted by studying multiple-scattering contributions and evaluating the influence of static and thermal disorder. The results of XAS studies on AgI-containing silver borate glasses are presented and discussed, with particular emphasis on the role of temperature dependent measurements in EXAFS. The comparison with results concerning other similar systems offers the possibility of discussing strengths and limits of XAS.

Structural study of AgI-Ag 2O-B 2O 3 glasses by X-ray absorption spectroscopy

After a review of the structural information obtained by room-temperature EXAFS spectroscopy on AgI-containing silver borate glasses, the first temperature-dependent EXAFS measurements on the glass (AgI) 0.55(Ag 2O·4B 2O 3) 0.45 in the range 77–300 K are presented. Low-temperature measurements and a more refined data reduction scheme allow a more accurate determination of the structural parameters. The absence of ordered AgI clusters is confirmed and discussed in terms of different structural models. The temperature dependence of EXAFS Debye-Waller factor of the first coordination shell of iodine allows the separation and evaluation for the first time of the contribution of static and thermal disorder. The thermal contribution is comparable with that of crystalline AgI.