Soft Phonon Modes in KMnF₃

Neutron inelastic scattering experiments show that the phase transition of KMnF ₃ at 184°K is a result of the softening of the \(\varGamma_{25}\) phonon mode at the [111] zone boundary. The over-all characteristics of the \(\varGamma_{25}\) mode are similar to those that have been observed for this same mode in SrTiO ₃ near its 110°K phase transition. We also observe that the M ₃ 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 KMnF ₃ below 184°K is the orthorhombic P b n m D ( _{2 h} ¹⁶ ). This orthorhombic phase is consistent with our observation that the \(\varGamma_{25}\) 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 \(\varGamma_{25}\) condensing at the transition temperature.