Phonon-Assisted Indirect Transitions in Cadmium Sulfate Crystals

Abstract

The fundamental absorption edge of “suprapur” cadmium sulphate (3CdSO4 · 8H2O) crystal occurs in the region below 200 nm. The power-law dependence of the optical density on photon energy suggests indirect electronic transitions made possible by the emission or absorption of 0.067 eV phonons. The indirect band gap (Eg′) at 300 K is 6.09 eV. The phonon energy of 0.067 eV is found to arise from one of the Raman-active modes (E ) of the sulphate ion. The band edge is found to shift monotonically towards higher energies at a rate of - 2.3 meV per K between 300 and 77 K. The laser Raman spectrum of the 3CdSO4 · 8H2O is also reported.