Proprietes electroniques et electrogalvaniques du sulfure d'argent α domaine d'existence

The study of electronic and electrogalvanic properties of α silver sulphide has been performed on samples whose non-stoichiometry is controlled by annealing under sulphur pressure or by coulometric titration with the cell Ag/AgI/Ag_2+εS/Pt.The data obtained lead to the adoption of an electronic model according to which the Frenkel defects are completly ionised. The homogeneity range has been drawn between 193° and 550°C. It extends to both sides of the stoichiometric composition.     

Range of homogeneity and defect model for Bi2S3

Sulphur pressure measurements inside and at the boundaries of the homogeneity range of Bi₂S₃ were performed as a function of composition and temperature. From the results and literature data a defect model of this substance is deduced. Important defects are singly charged sulphur vacancies and neutral sulphur atoms on bismut sites (antistructure defects). The range of homogeneity is calculated from this model. Bi₂S₃ exists with both sulphur deficit and sulphur excess of up to about 0.002 atoms sulphur per mole of Bi₂S₃. The phase diagram Bi-S is confirmed.     

Homogeneity range of cubic high temperature cuprous sulfide (digenite)

Within the homogeneity range of the cubic cuprous sulfide Cu_2?δS the sulphur fugacity was measured as a function of composition and temperature using direct (silica gauges) and indirect (H₂S/H₂) methods. The experiments were performed between 789 and 1321 K and in the whole field of compositions from copper-rich to the sulphur-rich boundary of the homogeneity range.The experimental data are interpreted by a combination of the thermodynamics of mixed phases and the point defect theory. The results show that Cu_2?δS is essentially a mixture of Cu₂S and CuS. Other defects than CuS dissolved in Cu₂S, e.g. copper interstitials, are present in small amounts at high temperatures and low sulphur fugacities. These results are in close agreement with data from the literature, derived from metallurgical, DTA, thermobalance and electrical conductivity measurements.     

Proprietes electroniques et electrogalvaniques du seleniure d'argent β: Domaine d'existence

A study of the electronic and electrogalvanic properties of β silver selenide has been performed on polycrystalline samples whose non-stoichiometry is controlled by eoulometric titration with the cell Ag/RbAg₄I₅/Ag_2+? Se/Pt. The adoption of an electronic model according to which the Frenkel defects of the silver sublattice are completely ionised allows us to compute independently the non-stoichiometry with the two formulaes δ = [Ag^o_i] ? [V'_Ag] and δ = n ? p. The homogeneity range has been drawn at positive temperatures and the formation enthalpy of the Frenkel defects is H'_F = 0,40 eV.     

Proprietes electroniques et electrogalvaniques dutellurure d'argent β domaine d'existence

A study of the electronic and electrogalvanic properties of β silver telluride has been performed on samples whose non-stoichiometry has been determined by coulometric titration with the cell Ag/RbAg₄I₅/Ag₂Te/Pt. The data lead to the adoption of a model of Frenkel defects on the silver sublattice that are fully ionised. Their formation enthalpy is H_F′ = 0,70 eV. On the tellurium side this model is completed by the association V_AgV_Ag^x. Th homogeneity range is reported from ?10°C to 132°C: it extends essentially on the tellurium side of the stoichiometric composition.     

Defect energetics and range of homogeneity of α-MnS

Sulphur pressure measurements between 10^?12 and 20 atm as a function of temperature (873–1364 K) and composition were performed on α-MnS samples. From the results and literature data on optical and electrical properties a quantitative defect structure model is derived. In the range investigated the deviation from stoiehiometry is caused by the formation of twice negatively charged manganese vacancies and, at high sulphur pressures, effectively neutral interstitial sulphur atoms (perhaps present as S″₂ on S″ sites). At low sulphur fugacities thermal generation of “holes” and “electrons” is important. These are both localized on manganese sites as Mn³⁺ and Mn¹⁺ ions, respectively. The boundary of the homogeneity range of α-MnS towards elemental sulphur is calculated from the model.     

Lifetime of Positrons in Nanostructured Nonstoichiometric Silver Sulfide Ag<Subscript>2–δ</Subscript>S

The lifetime of positrons in nanostructured nonstoichiometric silver sulfide Ag_2–δS samples with average nanoparticle sizes from 44 to 230 nm has been measured. It has been found that the lifetime spectra of Ag_2?δS nanoparticles include two components corresponding to the capture and annihilation of positrons in two types of defects: structural vacancies of the metallic sublattice and vacancy clusters at the interfaces between nanoparticles. The long surface component depends on the size of silver sulfide particles and indicates a difference in their defect structure.     

Energetics of defect formation and interaction in pyrrhotite Fe1−xS and its homogeneity range

Sulphur fugacities in equilibrium with Fe_1?xS (pyrrhotite) were measured at different temperatures in the range 820–1374 K by direct (silica gauges) and indirect (H₂S/H₂) methods. The results give direct evidence of the boundaries of the homogeneity range and can be interpreted in terms of iron vacancy formation and interaction and of an additional defect, e.g. iron on sulphur sites. The homogeneity range of Fe_1?xS is calculated as a function of temperature from the energetic parameters of the defects and the sulphur fugacities limiting the existence region of this phase.Fe_1?xS exists with positive values of x except at the eutectic with iron (1261 K) where x is 0±0.0001.     

Proprietes electroniques et electrogalvaniques du tellurure d'argent α domaine d'existence

A study of the electronic and electrogalvanic properties of α silver telluride has been performed on samples whose non-stoiehiometry has been determined by coulometric titration with the cell Ag/AgI/Ag₂Te/Pt. The data lead to the adoption of a model of Frenkel defects on the silver sublattice that are fully ionised. The homogeneity range is reported from 162 to 508°C: it extends essentially on the tellurium side of the stoichiometric composition. The free energies of formation of the α and β forms of the tellurides whose compositions are near Ag_1,6Te, Ag_1,9Te and Ag_2,0Te have been determined between 23°C and 360°C.     

A single crystal study of the initial stages of silver sulphidation: The chemisorption and reactivity of molecular sulphur (S2) on Ag(111)

Molecular sulphur undergoes rapid dissociative chemisorption on Ag(111) with an essentially constant sticking probability of unity up to the completion of the first layer of S atoms. At this stage a $\text{(√39 R 16.1° × √39}\text{R}\text{?}\text{16.1°)}$ structure is formed in which the S atom arrangement and spacing is similar to that in the (100) plane of γ-Ag₂S (the high temperature form of silver sulphide). Further dosing with S₂ leads to continued rapid uptake of sulphur and the appearance of a (√7 × √7) R 10.9° structure, the Auger, Δφ and thermal desorption data all indicate that fast formation of Ag₂S now occurs. Very well-ordered growth of γ-Ag₂S(111) is now observed, and low-temperature S₂ desorption spectra appear which show that the activation energy for S₂ desorption is ~175 kJ mol^?1 ; this value is in excellent agreement with that observed for the enthalpy of decomposition of bulk Ag₂S (2 Ag₂S(s) → 4 Ag(s) + S₂(g), ΔH = +179 kJmol^?1). All the properties of the Ag(111)-S system imply that the material characterised by the √39 structure (i.e. the first adsorbed layer of S) is very different from bulk Ag₂S. This is discussed and compared with the results of other studies on metal-sulphur systems.     

Proprietes electrogalvaniques et electroniques du sulfure d'argent β: Domaine d'existence

The study of electrogalvanic and electronic properties of the form β of silver sulfide has been performed on polycrystalline samples whose non stoichiometry is controled by coulometric titration with the cell Ag/RbAg₄I₅/Ag₂₊?S/Pt.The data obtained lead to the adoption of an electronic model according to which the Frenkel defects of the silver sublattice are completly ionized. The variation with temperature of the electronic properties near the stoichiometric composition between 23 and 176°C, the temperature of transition between the two forms β and α of Ag₂S, enables the forbidden gap E_i = (1,35?1,5 × 10^?3T) eV to be computed. The existence range has been drawn between 69°C and the α ? β transition temperature.The study of the ionic conductivity has made it possible to detect two ranges of preferential conduction and to determine the mechanism of migrations of the intersticial silver ions.     

Optical properties of small silver clusters supported at MgO

We present structural and optical properties of silver clusters Ag_n (n=2, 4, 6, 8) at two model support sites of MgO, stoichiometric MgO(100) and F_S-center defect, based on density functional theory and embedded cluster model. Our results provide the mechanism responsible for the absorption and emission patterns due to the specific interaction between the excitations within the cluster and the support site which is strongly cluster size and structure dependent. We propose Ag₄ at stoichiometric site as well as Ag₂, Ag₄ and Ag₆ at F_S-center defects as good candidates for the emissive centers in the visible regime.     

Specific features of the luminescence of silicate glasses with silver introduced by ion exchange

The luminescence spectra of silicate glasses with silver introduced by ion exchange have been investigated. It is shown that silver introduced into glass by ion exchange exists not only in the form of ions, but also as neutral atoms and charged and neutral molecular clusters, which provide intense luminescence in the visible spectral range. Cerium ions in glass facilitate the formation of neutral molecular silver clusters, due to which the luminescence intensity increases. It is shown that Ag _n -Ce ^x+ complexes can be formed in glass containing cerium ions and neutral molecular silver clusters.     

Thermoelectric power of the vitreous electrolyte (AgI)0.79(Ag2O.B2O3)0.21

We have determined the thermoelectric power ? of the high ionic conductivity glass (AgI)_0.79(Ag₂O.B₂O₃)_0.21; ? is negative throughout the investigated T range, 320–500 K. The heat of transport of the mobile Ag⁺, Q_Ag, taken as the slope of the straight line fitting ? versus 1/T, is quite lower than the activation energy obtained from conductivity data, viz. Q_Ag = 2.81 kcal/mole^-1 < E_act = 4.34 kcalmole^-1. To circumvent this discrepancy, the analysis of the experimental data is carried out as follows: (i) it is supposed that Q_Ag = E_act in agreement with the free ion theory for solid electrolytes; (ii) the vibrational part of the silver ion entropy, S(Ag⁺, vib), is assumed to be equal to the entropy of silver, S(Ag); (iii) on the ground of a structural model for this kind of glasses, the ideal configurational entropy of the mobile Ag⁺, S(Ag⁺, conf)_id, is evaluated through a statistical approach. The ideal ionic entropy is defined as S(Ag⁺)_id = {S(Ag⁺, vib) + S(Ag⁺, conf)_id}; (iv) the difference {S(Ag⁺)_exp - S(Ag⁺)_id} is viewed as an excess entropy and is described according to the classical model of the regular solutions.     

Influence of UV irradiation and heat treatment on the luminescence of molecular silver clusters in photo-thermo-refractive glasses

The luminescence spectra of photo-thermo-refractive (PTR) glasses containing cerium, silver, and antimony ions before and after UV irradiation and after heat treatment have been thoroughly investigated for the first time. It is shown that silver is present in the initial PTR glass in the form of ions and positively charged molecular clusters. After UV irradiation into the absorption band of cerium ions, silver partially passes to the neutral state in the form of atoms and neutral molecular clusters Ag₂, Ag₃, and Ag₄, which exhibit bright luminescence in the visible spectral range. Subsequent heat treatment at a temperature below the glassformation temperature leads to an increase in the luminescence intensity due to the increase in the concentration of neutral molecular clusters. Heat treatment at a temperature above the glass-formation temperature leads to the formation of silver nanocrystals and luminescence quenching. It is proposed to use PTR glasses with molecular silver clusters as phosphors for converting UV radiation into the visible range for solar power engineering and white LEDs.     

Silver-tracer diffusion in Cu2O

The diffusion of¹¹⁰Ag in Cu₂O has been measured by a serial-sectioning technique as a function of temperature (700–1132°C) and oxygen partial pressure (6 × 10^?6 ?8 × 10^?2 atm). The data are fit to the defect model for Cu₂O developed by the authors in the preceding paper. Silver ions have a larger impurity-vacancy binding free energy and/or a larger jump frequency for the singly charged cation vacancies relative to that for the neutral cation vacancies. The activation enthalpies for the diffusion of copper and silver ions in Cu₂O are nearly equal, but the absolute value of D¹_Ag is about three times larger than D¹_Cu even though the silver ion is 31% larger than the copper ion.     

The chemical diffusion coefficient in (low temperature)α-Ag2S determined by an electrochemical relaxation method

An electrochemically probed relaxation method has been used to determine the chemical diffusion coefficient, D?, of α-Ag₂+δS. The experiments were performed at 168°C and span the homogeneity range of this phase, which at this temperature is of the order of 10^-5 in δ. D? is found to have values between 0.02 and 0.15 cm² s^-1 and, as a function of nonstoichiometry, to reflect correctly the composition dependence of the thermodynamic factor, d ln a_Ag/d ln c_Ag. Using previously determined data for the thermodynamic factor, the component diffusion coefficient of Ag⁺ at 168°C is found to be D_Ag⁺=(5.4 ± 0.4) × 10^-8cm²s^-1.     

Luminescence of molecular silver clusters in oxyfluoride glasses

Luminescence spectra of silver-containing oxyfluoride glasses have been investigated in the excitation spectral range of 360–480 nm. Analysis of the luminescence spectra shows that silver in oxyfluoride glasses that contain no strong reducers and were subjected to heat treatment affecting redox processes exists in the form of neutral atoms and neutral molecular Ag₂, Ag₃, and Ag₄ clusters.     

Propriétés électroniques et électrogalvaniques du séléniure d'argent α-domaine d'existence

The study of electronic and electrogalvanic properties of silver selenide has been performed on samples whose non-stoichiometry is controlled by annealing under selenium pressure or by coulometric titration with the cell Ag/AgI/Ag_2+?Se/Pt. The data obtained lead to the adoption of an electronic model according to which the Frenkel defects are completely ionised. The homogeneity range has been drawn between 139 and 500°C. It extends on both sides of the stoichiometric composition. The study of the Hall mobility shows that the electronic scattering is due to acoustic phonons.