Pressure-induced phase transition(s) in KMnF3 and the importance of the excess volume for phase transitions in perovskite structures

摘要

We report a pressure-dependent investigation of KMnF_3 by x-ray diffraction up to 30GPa. The results are discussed in the framework of Landau theory and in relation to the isostructural phase transition in SrTiO _3. The phase transition temperature near 186K in KMnF_3 shifts to room temperature at a critical pressure of P_c=3.4GPa; the pressure dependence of the transition point follows ΔP_c/ ΔT_c=0.0315GPaK~1. The transition becomes second order under high pressure, close to the tricritical point. The phase transition is determined by the rotation of MnF_6 octahedra with their simultaneous expansion along the rotation axis. The rotation angle was found to increase to 10.5°at 24GPa. An additional anomaly was observed at higher pressure around 25GPa, suggesting a further phase transition.