Optical absorption of small polarons in semiconducting NiO and CoO in the near and far infra-red

Abstract

The optical absorption associated with charge carriers in NiO and CoO has been obtained from transmission measurements on pure and Li-doped crystals in the wavelength region 0·5 to 700 μm, at temperatures between 100 and 500°K. The absorption increases smoothly with frequency to a maximum near 1 ev and then shows a further increase extending to 2 ev and higher. In the region from 0·5 to about 4 μm the absorption increases slowly with temperature and correlates with the Li concentration. In the far infra-red the absorption σFI is independent of frequency and, for T <or= 300-400 °K, σFI > σdc. The activation energy associated with σFI is smaller than the d.c. value.