Raman Scattering from Five Phases of Silver Iodide

The Raman scattering is investigated from five phases of AgI that can be reached with moderate pressure and temperature. Particularly noteworthy is the strong negative pressure coefficient of the 17 cm−1 E2 mode in the wurtzite phase. Its mode gamma is −7.5. The intensity of this mode decreases above 1 kbar and shows hysteresis during pressure cycles. A possible interpretation of this hysteresis in terms of a sluggish phase transition from wurtzite to zinc blende is discussed. In the narrow phase between 3 and 4 kbar (IV) two strong modes are observed. At low temperature the sluggish phase transition does not occur and wurtzite transforms directly to rock salt at 3 kbar. The rock salt phase shows a second order spectrum similar to AgBr. The Raman spectrum of the disordered phase shows strong broad Rayleigh like scattering extending out to 50 cm−1 and a broad lattice like mode at 100 cm−1.