Order-disorder and electronic transitions in Ag2+ delta S single crystals studied by photoemission spectroscopy.

Abstract

The electronic structure of ${\mathrm{Ag}}_{2+\ensuremath{\delta}}\mathrm{S}$ single crystals is investigated as a function of temperature and nonstoichiometry, $\ensuremath{\delta}$. It is shown that the stoichiometric, low-temperature $\ensuremath{\alpha}$-${\mathrm{Ag}}_{2}$S 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 $\ensuremath{\delta}$ from the stoichiometric ($\ensuremath{\delta}=0$) phase in the high-temperature $\ensuremath{\beta}\ensuremath{-}{\mathrm{Ag}}_{2+\ensuremath{\delta}}\mathrm{S}$ 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 $\ensuremath{\alpha}\ensuremath{-}\ensuremath{\beta}$ phase transformation in the stoichiometric ${\mathrm{Ag}}_{2}$S is suggested to arise from the order-disorder transition and the consequent redistribution of states near the top of the valence band.