Order-disorder and electronic transitions in Ag_2+δ S single crystals studied by photoemission spectroscopy

Abstract

The electronic structure of in Ag_2+δS single crystals is investigated as a function of temperature and nonstoichiometry, δ. It is shown that the stoichiometric, low-temperature α-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 inδ from the stoichiometric (δ=0) phase in the high-temperature β-Ag_2+δS 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 α-β 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.