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

The structural behavior of the three silver (I) halides AgCl, AgBr, and AgI has been investigated to pressures $p\ensuremath{\sim}13--16\mathrm{GPa}$ using angle-dispersive x-ray diffraction with an image-plate device. Principal attention has been paid to the structural characterization of the phase transitions in the pressure range $p\ensuremath{\sim}7--13\mathrm{GPa}$ observed previously by optical and resistivity techniques. In AgCl, the ambient-pressure rocksalt-structured phase transforms at $p=6.6(6)\mathrm{GPa}$ to a monoclinic structure with a KOH-type arrangement and then to an orthorhombic TlI-type configuration at $p=10.8(8)\mathrm{GPa}.$ Both these structures can be considered to be more densely packed arrangements derived from rocksalt and possess ionic environments intermediate between the octahedrally coordinated rocksalt structure and an eightfold coordinated CsCl-type one adopted at higher pressure. AgBr and AgI both undergo $\mathrm{rocksal}\stackrel{\ensuremath{\rightarrow}}{t}\mathrm{KOH}$-type structural transitions, at $p=7.9(4)\mathrm{GPa}$ and $p=11.3(2)\mathrm{GPa},$ respectively. The nature of the pressure-induced $\mathrm{rocksal}\stackrel{\ensuremath{\rightarrow}}{t}\mathrm{CsCl}$ 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 calculations.