The photoconductivity measurements of synthetic pyrite FeS2 single crystals at low temperatures are used to study the impurity and defect-related photoresponse peaks below the band edge. At temperatures below 100 K, several features on the low-energy side of the photoconductivity spectra are observed. By comparison of the results with those of the absorption and the photo-EPR measurements, the origin of the sharpest peak at 0.866 eV is identified. The nature of other weaker features is discussed. The existence of these features prevents us from determining the band gap of FeS2 by the Moss rule at low temperature. We propose that the energy gap of FeS 2 at low temperatures may be determined by adding the thermal ionization energy of the shallow acceptor states to the photo-ionization energy between the (SCl)2- states and the bottom of the conduction band.
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