Optical spectra of the layered Cd2P2S6 and Cd2P2Se6 compounds in the region from 1.6 to 5.5 eV

Abstract

Room temperature absorption and reflectivity data are presented for Cd2P2S6 and Cd2P2Se6 crystals in the range of photon energies between 1.6 and 5.5 eV. For both compounds the characteristic feature of the optical absorption is that their absorption edge appears to correspond to an indirect allowed transition with an optical energy gap of 3.06 eV for Cd2P2S6 and 2.29 eV for Cd2P2Se6. An associated phonon energy of 74 meV for Cd2P2S6 and 50 meV for Cd2P2Se6 is also deduced. The Cd2P2X6 (with X=S or Se) reflectivity spectra are compared with the early reflectivity spectrum of Zn2P2S6. As observed in previous reflectivity spectra of the Hg2P2X6 compounds, near the fundamental absorption edges the Cd2P2X6 reflectivity spectra show a pronounced rise in reflectivity, a feature common to most layered compounds. For photon energies greater than their absorption edges, the reflectivity spectra of both materials look similar to one another and to that of Zn2P2S6. On the basis of these similarities we have adopted the so-called transition metal weakly interacting model and interpreted the spectral structures as direct allowed interband transitions. For both compounds, these electronic transitions are described by a simplified energy-band scheme.