Pressure-induced phase transitions in AgCl, AgBr, and AgI

The structural behavior of the three silver (I) halides AgCI, AgBr, and AgI has been investigated to pressures p similar to 13-16 GPa using angle-disp ersive x-ray diffraction with an image-plate device. Principal attention ha s been paid to the structural characterization of the phase transitions in the pressure range p similar to 7-13 GPa observed previously by optical and resistivity techniques. In AgCl, the ambient-pressure rocksalt-structured phase transforms at p = 6.6(6) GPa to a monoclinic structure with a KOH-typ e arrangement and then to an orthorhombic TlI-type configuration at p = 10. 8(8) GPa. Both these structures can be considered to be more densely packed arrangements derived from rocksalt and possess ionic environments intermed iate between the octahedrally coordinated rocksalt structure and an eightfo ld coordinated CsCl-type one adopted at higher pressure. AgBr and AgI both undergo rocksalt-->KOH-type structural transitions, at p = 7.9(4) GPa and p = 11.3(2) GPa, respectively. The nature of the pressure-induced rocksalt-- >CsCl transformation in the three Ag(I) halides is discussed in relation to the continuous rhombohedral deformation model due to Buerger and to recent theoretical predictions derived from ab initio electronic structure calcul ations. [S0163-1829(99)02702-2].