Dielectric properties of PbI2 single crystals

Dielectric constant (K), loss (tan delta), and conductivity (sigma) of single crystals of PbI2 are measured (E parallel c-axis) as a function of frequency in the range 102 to 107 Hz and at temperatures between 77 to 560 K. At room temperature (approximately 303 K), the K-value at 106 Hz is 6.7. The dielectric constant of this crystal increases slowly with temperature up to about 300 K (this increase being practically frequency-independent) beyond which the K increase is fast and frequency-dependent; this later stage of increase of K is attributed to an appreciable space charge polarisation (due to crystal defects) at such temperatures. The activation energy for intrinsic conduction (beyond 450 K) is calculated to be 0.44 eV from the lg sigma versus 1/T plots at different frequencies. The lg (ΔK) versus 1/T plots at different frequencies for this crystal (where ΔK is the difference between the value of the dielectric constant at any temperature T and that value at room temperature) are straight lines from about 400 K, having the same slope for all frequencies; the activation energy is 0.41 eV. This agreement between the two activation energy values suggests that the same charge carriers are responsible for the two processes. (author)