TEMPERATURE-DEPENDENCE OF THE VIBRATIONAL PROPERTIES OF PBI2

Infrared reflectivity measurements are made from 20 to 600 cm-1 at tem peratures ranging from 5 to 575 K for the layered semiconductor PbI2. The reflection spectra are analysed by means of a damped Lorentzian-os cillator model and the calculated optical properties are presented. Th e infrared reflectivity of PbI2 shows a broad reststrahlen band, point ing to the significantly ionic character of this compound. Optical pro perties are compared with those of other layered semiconductors. The o ptical constants, long-wavelength optical-phonon frequencies (both TO and LO), and their half-widths are obtained as functions of temperatur e. The temperature dependence is discussed using an anharmonic couplin g model with a two-phonon decay. The general behaviour of anharmonic e ffects in PbI2 agrees qualitatively with the theory of Maradudin and F ein.