Measurements are reported for the temperature variation of ionic conductivity in single crystals of hexagonal silver iodide. Both nominally pure and Cu 2+ doped crystals were studied and analysis of the measurements for conduction both parallel and normal to the c-axis leads to a formation energy for Frenkel defects of 0-60 eV, activation energies for diffusion of Ag + interstitials parallel and normal to the c-axis of 0.61 and 0.29 eV respectively and corresponding activation energies for Ag + vacancies of 0-50 and 0.39 eV. Similar measurements performed on thin films of AgI grown expitaxially on mica show varying behaviour but the activation energy for conduction is about 0.4 eV, which agrees with the value expected from the single-crystal results if the conductivity is vacancy controlled. Dielectric measurements in the frequency range 10-10~Hz at temperatures between 193 and 340~ show anomalous dispersion peaks at about 104 Hz for temperatures above 300~ which are tentatively interpreted in terms of trapped Frenkel defects and interfacial polarization. At temperatures below about 290"K a series of sharp resonant absorptions is observed whose origin is uncertain. The static dielectric constant approaches the value 6.8 +-0.2 at temperatures below-40~ * Based on a thesis submitted by G. Cochrane for the Ph.D. degree of the University of New England.
Transmission measurements on nominally pure single crystals of β-AgI have enabled the wavelength dependence (λ=437-750 nm) of the absorption constant and refractive index of this material to be determined in the region of the absorption tail. From an analysis of the absorption data, evidence is put forward in support of a phononassisted indirect transition between the valence and conduction bands. Experimentally it is found that the absorption edge shifts to longer wavelengths with increasing temperature, while calculations indicate that nearly all of the observed shift can be attributed to broadening of the energy bands brought about by lattice vibrations. Experimental evidence is presented to show that absorption in the long-wavelength tail (λ=650 nm) is related to the density of Frenkel defects, although the exact mechanism by which this absorption occurs is not known.The variation of photoconductivity with light intensity suggests that recombination is not by a simple monomolecular process. This conclusion is further supported by the lack of a simple exponential decay time for free carriers after the cessation of excitation.A measurement is described which enables the drift mobility of photo-excited electrons to be determined even when considerable trapping of the carriers occurs. A value of (31 ± 3) × 10−4 m2 V−1 s−1 is calculated for the drift mobility at 248 K.
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