Thermal behavior of holes in lead-chloride crystal

Temperature dependence of emission spectra in PbCl2 has been measured under steady optical excitation in the region above the band gap energy. Emission spectra below 85 K are made up of a broad band (BG band) locating at 2.60 eV, which is attributed to luminescence due to electron-hole recombination between the (Pb-2)(3+) self-trapped electron (STEL) center and some hole trapping center. With raising temperature, two emission bands with their peaks at 2.79 and 2.08 eV appear newly, growing in intensity at the expense of the BG emission band. The two emission bands are not produced by optical or thermal stimulation in PbCl2 crystals irradiated with UV-light. This result suggests that most of holes are not trapped individually at high temperatures. The change from the BG emission band to the two emission bands will be explained in terms of the change in the electronic state of holes.