Soft Phonon Modes in KMnF_3

概要

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Neutron inelastic scattering experiments show that the phase transition of KMnF_3 at 184°K is a result of the softening of the Γ_<25> phonon mode at the [111] zone boundary. The over-all characteristics of the Γ_<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 vincinity of its Neel 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 PbnmD(^<16>_<2h>). This orthorhombic phase is consistent with our observation that the Γ_<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 Γ_<25> condensing at the transition temperature.
社団法人日本物理学会の論文
1969-03-05