Photoconductivity of the nickel dihalides and the nature of the energy gap

Measurements of the photoconductivity of single crystals of the nickel dihalides NiX₂ (X=Cl, Br, I) are reported. The photoconductivity energy gaps at 100 K are 4.7 eV (NiCl₂), 3.5 eV (NiBr₂), and 1.8 eV (NiI₂). It is shown that the energy gap in these materials is due to charge-transfer transitions from the anion p band to transition-metal d states. The photoconduction is due to holes in the anion p band.