Structural Phase Transitions and Magnetic Superexchange in M ^I Ag ^II F ₃ Perovskites at High Pressure**

Pressure-induced phase transitions of MIAgIIF3 perovskites (M=K, Rb, Cs) have been predicted theoretically for the first time for pressures up to 100 GPa. The sequence of phase transitions for M=K and Rb consists of a transition from orthorhombic to monoclinic and back to orthorhombic, associated with progressive bending of infinite chains of corner-sharing [AgF6]4− octahedra and their mutual approach through secondary Ag⋅⋅⋅F contacts. In stark contrast, only a single phase transition (tetragonal→triclinic) is predicted for CsAgF3; this is associated with substantial deformation of the Jahn–Teller-distorted first coordination sphere of AgII and association of the infinite [AgF6]4− chains into a polymeric sublattice. The phase transitions markedly decrease the coupling strength of intra-chain antiferromagnetic superexchange in MAgF3 hosts lattices.