Structural transitions in fluorine-based perovskites: A molecular-dynamics study ofKMnF3

Constant-pressure molecular dynamics is used to investigate the structure of the fluorine-based perovskite ${\mathrm{KMnF}}_3$. On cooling from the high-temperature cubic phase, the system undergoes two structural transitions, first to a tetragonal, then to a base-centered orthorhombic structure. These transitions are accompanied by cooperative antiferrodistortive rearrangements of the ${\mathrm{MnF}}_6$ octahedra corresponding, respectively, to the condensation of an R[010] phonon mode, and to the simultaneous condensation of an M[010] and an R[101] mode. No evidence is found for the experimentally observed ground-state structure, suggesting that magnetism is responsible for this phase in real crystals.