  AgClO4 has been studied under compression by x-ray diffraction and density
functional theory calculations. Experimental evidence of a structural phase
transition from the tetragonal structure of AgClO4 to an orthorhombic
barite-type structure has been found at 5.1 GPa. The transition is supported by
total-energy calculations. In addition, a second transition to a monoclinic
structure is theoretically proposed to take place beyond 17 GPa. The equation
of state of the different phases is reported as well as the calculated
Raman-active phonons and their pressure evolution. Finally, we provide a
description of all the structures of AgClO4 and discuss their relationships.
The structures are also compared with those of AgCl in order to explain the
structural sequence determined for AgClO4.
