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 Γ25 phonon mode at the [111] zone boundary. The overall characteristics of the Γ25 mode are similar to those that have been observed for this same mode in SrTiO3near 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 Pbnm D (formula omited). 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. © 1969, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.