Order-disorder and electronic transitions in Ag-2+delta single crystals studied by photoemission spectroscopy

The electronic structure of Ag2+deltaS single crystals is investigated as a function of temperature and nonstoichiometry, delta. It is shown that the stoichiometric, low-temperature alpha-Ag2S phase is a band insulator with correlation effects further increasing the band gap. The results suggest that the metal-insulator transition observed with increase in delta from the stoichiometric (delta=0) phase in the high-temperature beta-Ag2+deltaS is due to the tuning of the Fermi energy through the mobility edge. The increase in conductivity associated with the insulator-to-insulator transition across the a-p phase transformation in the stoichiometric Ag2S is suggested to arise from the order-disorder transition and the consequent redistribution of states near the top of the valence band.