The electrical conductivity and photo-decomposition of small crystals of lead iodide

Abstract

This paper describes measurements of the electrical conductivity of small single crystal platelets of lead iodide, heated in visible light to temperatures varying between 0°c and 350°c. The crystals show three distinct conducting processes. These are interpreted as photo-conductivity, extrinsic ionic conductivity (structure sensitive) and intrinsic ionic conductivity. There might also be some surface conductivity but this has not been identified separately. An anomalous decrease in conductivity occurs at temperatures near 170°c and this has been correlated with a precipitation of finely divided particles of lead appearing in the crystals at the same temperature. This is clearly related to photo-decomposition. The decrease in overall conductivity follows the extinction of photo-conductivity and both the magnitude of the decrease and the density of precipitate depend on the intensity of light incident on the crystals. This photo-effect also depends on the wavelength of the incident light but the discussion of this and related observations is reserved for the second paper in this series. The ionic components of the conductivity are discussed in relation to earlier measurements made by Seith, together with the values of transport numbers determined by Tubandt and co-workers. It is concluded that, whereas interstitial Pb⁺⁺ ions dominate the conductivity at high temperatures (above 275°c), the movement of I⁻ ion vacancies controls the conductivity at lower temperatures. Both the extinction of photo-conductivity and the photo-decomposition indicate a rapid increase in the rate of trapping of photo-electrons at 170°c. This is discussed briefly in connection with the movement and aggregation of structural defects to form stable traps. The paper concludes with the suggestion that the measurement of electrical conductivity, which so clearly follows the events leading to photo-decomposition in this case, might be used to study the primary processes involved in other forms of decomposition, and particularly those which occur during electron irradiation.