Soft Phonon Modes in KMnF₃

Abstract

Neutron inelastic scattering experiments show that the phase transition of KMnF₃ at 184°K is a result of the softening of the Γ₂₅ phonon mode at the [111] zone boundary. The over-all characteristics of the Γ₂₅ mode are similar to those that have been observed for this same mode in SrTiO₃ near its 110°K phase transition. We also observe that the M₃ mode at the [110] zone boundary has the very low energy of 3.1±0.2 MeV at room temperature, and condenses not at 184°K but rather in the vicinity of its Néel temperature, 88°K. The X-ray studies by Beckman and Knox indicate that the space group of KMnF₃ below 184°K is the orthorhombic PbnmD(_2h¹⁶). This orthorhombic phase is consistent with our observation that the Γ₂₅ mode is responsible for the 184°K transition in that the dominant contribution to the rotation of the fluorine octahedra is a superposition of two of the three triply degenerate modes of Γ₂₅ condensing at the transition temperature.