Silver-halide valence and conduction states: Temperature-dependent ultraviolet-photoemission studies

Abstract

The electronic states of AgBr, AgCl, and $\ensuremath{\beta}\ensuremath{-}\mathrm{AgI}$ were studied using photoemission spectroscopy at temperatures $T$ between 80 and 295\ifmmode^\circ\else\textdegree\fi{}K for photon energies $h\ensuremath{\nu}$ from threshold (7.15, 7.55, and 6.6 \ifmmode\pm\else\textpm\fi{} 0.1 eV, respectively) through 21.2 eV. Without using theoretical band-structure information, the atomic origin and in some cases Brillouin-zone location are determined by the $h\ensuremath{\nu}$ and $T$ variations of energy-distribution-curve structure. We unambiguously identify the filled states of almost pure $4{d}_{\frac{5}{2}}$ symmetry at -3.7, -3.3, and -4.4 \ifmmode\pm\else\textpm\fi{} 0.1 eV below the valence-band maximum in AgBr, AgCl, and $\ensuremath{\beta}\ensuremath{-}\mathrm{AgI}$, respectively. The halogen $p$-derived valence states are all highly hybridized with Ag $\mathrm{Ag} 4d$ states and are characterized by two large density-of-states regions at -0.85 and 2.9; -0.8 and -2.65; -0.45 and -1.7 eV in AgBr, AgCl, and $\ensuremath{\beta}\ensuremath{-}\mathrm{AgI}$, respectively. The halogen peak closest to the valence-band maximum probably has a symmetry other than $\ensuremath{\Gamma}$. A secondary peak of mainly $d$ character is identified in the density of AgBr valence states at -1.95 eV. Using Mason's x-ray photoemission results, we locate similar structure at -1.6 eV in AgCl and -1.1 eV in $\ensuremath{\beta}\ensuremath{-}\mathrm{AgI}$. In addition, the lower $\mathrm{Ag} (4{d}_{\frac{3}{2}})$ derived hybridized states are located at -5.0 eV in AgBr and result in a 6.0-eV valence-band width. The photoemission clearly shows the $\ensuremath{\Gamma}$ nature of the $\ensuremath{\beta}\ensuremath{-}\mathrm{AgI}$ valence-band maximum; the band gap is direct only in $\ensuremath{\beta}\ensuremath{-}\mathrm{AgI}$, as expected from its wurtzite rather than rock-salt crystal structure. We report the first measurement of silver-halide conduction-state features. Large density-of-states regions are found at 7.3, 8.1, and 7.1, and 7.8 eV above the valence-band maximum in AgBr, AgCl, and $\ensuremath{\beta}\ensuremath{-}\mathrm{AgI}$, respectively; these are probably derived from the halogen $d$ states. The $5p$ conduction states have a large density at 8.8 and 9.3 eV in AgBr and 9.85 eV in AgCl. Strong atomic and $k$-conservation selection rules dominate the optical excitation process. These are used to identify other conduction-state features; for example, in $\ensuremath{\beta}\ensuremath{-}\mathrm{AgI}$ the $I(5d) {\ensuremath{\Gamma}}_{{25}^{\ensuremath{'}}}$ state occurs at 8.0 \ifmmode\pm\else\textpm\fi{} 0.2 eV and the $\mathrm{Ag}(5p) {\ensuremath{\Gamma}}_{{2}^{\ensuremath{'}}}$ level is at 10.1 \ifmmode\pm\else\textpm\fi{} 0.2 eV. Comparison is made to other measurements and to band-structure calculations.