Phase Transition In AgI Research Articles

Ab initio based interionic potential for silver iodide

In this paper, a new interionic potential is derived for silver iodide (AgI) via the Chen-Möbius lattice inversion and ab initio calculation. The accuracy of the proposed potential is checked by comparing the molecular dynamics simulation results on the static properties, structural stability, disordered states, mean-squared displacement and phase transition of AgI with experimental data. The simulation results are consistent with experimental data and density functional theory (DFT) calculations, indicating that the proposed interionic potential is valid over a wide range of interionic separations and applicable for describing the major properties of AgI.

Temperature hysteresis of AgI phase transition in AgI–chalcogenide glass nanolayered films

Using a method of laser ablation, a number of AgI–chalcogenide glass nanolayered films has been obtained with different thicknesses of the layers (10, 25, 50, and 100 nm). In order to study α ⇆ β phase transition in AgI, X-ray phase analysis has been carried out in the temperature range from 30 to 200°C. A correlation between the layer thickness and the temperature of the α → β phase transition during the lowering of the temperature is found. An explanation of the correlation is proposed.

Preparation, characterization, electrical and dielectric properties of (1−x) (PbI2–Ag2CrO4)–xTiO2 composite solid electrolytes

A series of novel composite solid electrolyte was prepared and characterized by various techniques such as X-ray diffraction, differential thermal analysis and scanning electron microscopy. The AC impedance spectroscopy revealed that the ionic conductivity was arisen by the simultaneous contribution of both the grain and grain boundary effects. The ionic conductivity of the composite was boost up in the low temperature phase (β phase) with the increase of x. The composition x=0.3 shows the maximum conductivity σ=0.00065Scm−1 at 30°C as compared to the other compositions. The temperature dependent conductivity and dielectric constant shows the sharp increase at 423K due to the β→α phase transition of AgI. The value of dielectric constant was increased with increase in temperature and it decreases with increase in frequency. The modulus value shows the increasing trend irrespective of increasing applied frequency.

Superionic phase transition in AgI embedded in molecular sieves

Superionic AgI particles embedded in MCM-41 and SBA-15 molecular sieves with differentpore sizes were studied using NMR and dielectric methods. The measurements confirmedthat the structure of the confined particles did not change noticeably compared tothat in bulk AgI. A regular increase in the temperature of the superionic phasetransition with decreasing pore size was observed and treated on the basis of theLandau theory of thermodynamic size effects. The maximum increase found uponwarming and cooling was about 10 and 8 K, respectively. The ionic conductivity inβ-AgI under confinement was shown to be due to silver mobility on the particlesurface.

EPR OF SUPERIONIC PHASE TRANSITION IN Cu-STABILIZED NANOCRYSTALLINE γ-AgI

EPR probed the zincblende (γ) to cubic (α) AgI structural phase transition in AgI through two intrinsic paramagnetic centers: an Ag 2+ based hole center (signal "A") and an Ag 0 based conduction electron center (signal "B") associated with AgI nanocrystallites. A sudden drop in intensity ( I PP), 〈g〉, and Δ H PP was observed at 423 K for pure AgI nanocrystals. Cu in AgI increases the thermal stability of the cation sublattice from 423 K (undoped AgI ) to 453 K ( Ag 0.75 Cu 0.25 I ) through the strengthened Ag-I bond. A sudden jump in the reciprocal susceptibility observed at well above the phase transition reflects the local bonding configuration in the γ -AgI structure.

Intrinsic paramagnetic defects probe superionic phase transition in Ag 1−xCu xI ( 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 Ag 2+-based hole center (signal ‘A’) and an Ag 0-based conduction electron center (signal ‘B’) associated with AgI nanocrystallites. Sudden drops in intensity ( I PP), < g>, and Δ H 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.

A study of formation of solid ionic conductors in the mixed system (CdI2)b[(Ag2O)·(CrO3)]100-b (20≤b≤80)

Using Wagner's polarization technique and EMF method the joinic and silver ionic transport number measurements of various compositions of the mixed system CdI2-Ag2O-CrO3 have been made. The presence of AgI in these materials has been inferred from the typical β → α phase transition of AgI, which is characterized by an endothermic peak at around 420 K in the DSC traces of these specimens. While the structural analysis performed by means of powder X-ray diffraction has revealed the formation of ionic phases involving polycrystalline compounds, the Fourier transform infrared (FTIR) spectroscopic results have indicated the presence of ionic species thus confirming the ionic nature of the products. The complex impedance studies carried out in the frequency range 20 Hz - 1 MHz and over the temperature range 294 – 442 K have revealed that the best conducting composition, namely 55%(CdI2) – 45%(Ag2O·CrO3), would exhibit a silver ionic conductivity of 1.3×10−5Scm−1 at 294 K.

Complex impedance and structural analyses of the mixed system 40(Cu 1− xAg xI)–30(Ag 2O)–30(SeO 2), (0.05≤ x≤0.25)

Using complex impedance analysis, temperature-dependent electrical conductivity studies of various compositions of the mixed system 40(Cu 1− x Ag x I)–30(Ag 2O)–30(SeO 2), (0.05≤ x≤0.25) over the temperature region 295–445 K have been carried out. All the chosen compositions of the system exhibit Arrhenius type of conductivity as a function of temperature and their activation energies for conduction lie in the range 0.27–0.42 eV. The values of ionic transport number of these materials are greater than 0.96. The presence of AgI in these materials has been inferred from the β→α phase transition of AgI, which is characterized by an endothermic peak at around 416 K in the DSC traces of these specimens. While the structural analysis carried out by means of X-ray diffraction has revealed the formation of AgI, Ag 2SeO 4 and new phases involving glassy and crystalline compounds, the Fourier transform infrared (FTIR) spectroscopic results have indicated the presence of SeO 4 2−, SeO 3 2− and Se 2O 7 2− species thus confirming the ionic nature of the phases.

A phase transition of AgI at 150 K

Anomalies in the heat capacity curves of AgI have been detected near the temperature of 150 K. Powder X-ray diffraction experiments have been performed in the temperature range 23–299 K. These anomalies were observed for the specimens which experienced the α- to β/γ-phase transition (heated up to 773 K and then quenched to room temperature) but became unobservable in the subsequent heating cycles. The various values (1.5, 13.0 and 4.1 Jmol −1) of the enthalpy of transition obtained depend on the thermal history. It can be considered that the anomaly is a transformation from the intermediate (high-pressure) phase to the β/γ phase.

Disorder Entropy at the Phase Transition of AgI and Ag2S

AgI and Ag 2 S are well studied superionic conductors which undergo a first order phase transition. By comparing the theoretical and experimental values of the discontinuous entropy change at the transition temperature it is possible to find the nature of correlated motions of Ag ions in the conductive phase right above the transition temperature

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 α-AgI and the α⇆β 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.

Use of computer simulation technique to study atomic migration in solids

Nature of ionic motions in superionic conductors AgI and Ag 2S are studied using molecular-dynamics (MD) technique. Our model of these superionic conductors is based on the use of effective pair-potentials. To determine the constants in these potentials, cohesive energy and bulk modulus are used as input; in addition one uses notions of ionic size based on the known crystal structures. The α⇄β structural phase transition in AgI is studied with the use of the new molecular dynamics (NMD) technique which allows for a dynamical variation of the shape and size of the cell. In the present model, upon heating the βAgI, the iodine ions undergo a hcp→bcc transformation and silver ions became mobile, whereas the reverse transformation is observed on cooling of αAgI. The calculated α⇄β transition temperature, and structural and dynamical properties are in good agreement with experiments. It has also become possible to observe crystal growth of α and β phases when a system of molten AgI is cooled to different temperatures. Transition temperatures determined from the studies of α⇄β structural transitions and from crystal growth studies are in good agreement. The calculated transition temperatures are also in good accord with the experimental results. A model of superionic conductor Ag 2S is also studied. The silver density map and its temperature dependence is calculated. We have also calculated the x-ray and neutron structure factors which explain the anomalous intensity at and near Q=(1.6, 1, 0) observed in diffuse scattering experiments. A study of low-energy excitations in Ag 2S is in progress.

Bulk interaction between ionic conductor AGI and dispersed second phase

The bulk interaction between two phases in AgI(ν-Al2O3 or α-Fe2O3) has been investigated by means of electrical conductivity, DTA and Mossbauer spectra. It shows that both α->β phase transition of AgI and Morin transition of α-Fe2O3 influence each other.

Structural Transitions in Superionic Conductors

The $\ensuremath{\alpha}\ensuremath{\rightleftarrows}\ensuremath{\beta}$ phase transition in AgI is studied with use of the new molecular-dynamics technique which allows for a dynamical variation of the shape and size of the cell. In the present model, upon heating of $\ensuremath{\beta}$-AgI, the iodine ions undergo a hep \ensuremath{\rightarrow} bee transformation and silver ions become mobile, whereas the reverse transformation is observed on cooling of $\ensuremath{\alpha}$-AgI. The calculated $\ensuremath{\alpha}\ensuremath{\rightleftarrows}\ensuremath{\beta}$ transition temperature and structural and dynamical properties are in good agreement with experiments.

Thermal Shifts in Energy Level Separations in Some Rubidium and Silver Halides Studied by ESCA

Energy level separations and their temperature dependences have been measured for some rubidium and silver halides. The observed thermal shifts agree quite well with the shifts in Madelung energy due to thermal expansion for the outer levels in the rubidium halides and the core levels in RbCl and AgCl. The experimental results on the bromides, however, indicate that differences in repulsive effects between different core levels are significant. The phase transition in AgI at 147°C is shown to influence the energy separation between the Ag 3d and I 3d levels. Valence band results for the silver halides at different temperatures are presented.