Electronic conductivity of AgI using D.C. polarization and charge transfer techniques

The experimental conditions for Liesegang phenomenon of silver iodide in agar gel columns, and particularly the transition from revert to direct type of periodic precipitations, have been investigated. The theoretically derived Mathur's spacing law, which suits both revert and direct type of Liesegang's rings, has been experimentally verified. An attempt has been made to explain the mechanism of revert and direct type and of the transition of revert to direct type of periodic precipitation on the basis of adsorption on the precipitated silver iodide and of flocculation. The dependence of the transition point on the concentrations of the outer and inner electrolytes has been studied. The time law of Morse and Pierce has been verified. The dependence of the velocity constant ( K = x n t n 1 2 ) on the concentrations of the outer and inner electrolytes has been studied.

Revert and direct type Liesegang phenomenon of silver iodide

Silver halides are notable for exhibiting both ionic and electronic conductivity which is responsible for their widespread use in photography. Due to differences in ionicity and bonding, their properties vary from the highly ionic silver fluoride to the more covalent silver iodide. These fundamental properties such as defect formation energies, carrier mobilities and band-structures, particularly of silver iodide, have only recently been determined. One interesting finding is the presence of high surface fields, caused by differences between interstitial and vacancy formation energies, which results in separation of photo-generated electron hole pairs. This throws new light on the photographic process and on the suitability of silver bromide as a photographic material.a-AgI is also the first fast ion conductor to be discovered. Structural instability, low defect formation energy and optimum cation size are responsible for this phenomenon but existing models fail to explain all the experimental results. Recent microwave conductivity and neutron scattering experiments which provide fresh insight are discussed.

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Physics of silver halides and their applications

A photoinduced change in the ionic conductivity was measured for polycrystalline silver iodide (AgI) by complex impedance spectroscopy, and a three-order magnitude reduction (from gigaohms to megaohms) in the bulk resistance of β-AgI on photoexcitation was found at 77K. The bulk resistance gradually increased when the light was turned off. Reversible photoinduced switching (photoswitching) between low and high resistivity states was observed, depending on the time in the dark state. This gigantic photoinduced change in the ionic conductivity is explained in terms of distortion of the β-AgI lattice following the photoinduced generation of electron-hole pairs.

Gigantic Photoresponse and Reversible Photoswitching in Ionic Conductivity of Polycrystalline β-AgI

Tailoring of band gap, dielectric and antimicrobial properties of silver iodide nanoparticles through Cu doping

Excitation spectra and transient and steady-state photoconductivity have been studied in undoped and 0.8-mole% Cu-doped single-crystal β-AgI between 150 and 260°K. A single peak in the spectral response was found to occur in each case, at 2.88 eV for undoped and at 2.81 eV for copper-doped specimens at 260 K, the difference being due to a decrease in band gap. The anisotropy due to polarization of incident radiation parallel or perpendicular to the c direction, which is a measure of the energy difference between the Γ9 and Γ7 levels in the valence band, was found to be 0.010 eV. Transient-photoconductivity experiments showed that the hole lifetime was 6 μ sec at 300°K, an order of magnitude larger than the electron lifetime. The hole drift mobility was found to be 12±2 cm2/ V sec at 300°K and limited by traps at a depth of 0.51±0.01 eV with concentration (3-5)×109/cm3 and capture cross section 10-11 cm2. The study of photoconductivity decay versus temperature revealed the presence of shallow hole traps at 0.14±0.02 eV with concentration greater than 1016/cm3 and capture cross section 10-19 cm2. The steady-state photoconductivity was determined by the deep hole traps at 0.51 eV, and showed the presence of shallow electron traps at a depth of 0.28 eV. The trap distribution was found to be substantially the same in pure and copper-doped specimens, indicating the monovalent substitutional role of copper. The effects of iodine annealing, cadmium doping, and heating above the transition temperature were also studied. The possible nature of the traps is discussed.

PA Govindacharyulu

Papers

Photoconductivity in β-AgI