Effect of Pressure upon the Optical Absorption Spectra of Silver Chloride and Silver Bromide

The effect of quasihydrostatic pressure up to 50 kbar upon the optical absorption spectrum of AgCl and AgBr at 295°K was measured in the spectral region 500-220 mμ. At 50 kbar the absorption edge of AgCl (AgBr) shifted to the red 350 ${\mathrm{cm}}^{-1\mathrm{}}$ (200 ${\mathrm{cm}}^{-1\mathrm{}}$), whereas the direct exciton absorption exhibited a blue shift of 1900 ${\mathrm{cm}}^{-1\mathrm{}}$ (1950 ${\mathrm{cm}}^{-1\mathrm{}}$); an isostatic point was observed at 25 800 ${\mathrm{cm}}^{-1\mathrm{}}$ (21 700 ${\mathrm{cm}}^{-1\mathrm{}}$). The shift of the direct exciton absorption was the same for both halides when expressed in terms of the fractional volume change of the solid. The data are discussed in terms of pressure-induced changes in the structure of the conduction and valence bands. The energy of the conduction-band minimum was found to be a linear function of the unit-cell volume for volumes ranging from 193 ${\mathrm{\AA }}^3$ (AgBr at 0 kbar) to 155 ${\mathrm{\AA }}^3$ (AgCl at 50 kbar).