Structural Phase Transitions and Magnetic Superexchange in M Ag F Perovskites at High Pressure.

Pressure-induced phase transitions of M Ag F 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 [AgF ] octahedra and their mutual approach through secondary Ag⋅⋅⋅F contacts. In stark contrast, only a single phase transition (tetragonal→triclinic) is predicted for CsAgF ; this is associated with substantial deformation of the Jahn-Teller-distorted first coordination sphere of Ag and association of the infinite [AgF ] chains into a polymeric sublattice. The phase transitions markedly decrease the coupling strength of intra-chain antiferromagnetic superexchange in MAgF hosts lattices.