Brillouin Scattering from the Soft Optical Phonon in the Orthorhombic Form of a K_3Co_<1-x>Fe_x(CN)_6 (r≤0.03) Crystal

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The soft optical phonon in the two-layer orthorhombic form of a K_3Co_<1-x>Fe_x(CN)_6 (x ≤ 0.03) single crystal has been reinvestigated using a (3+3)-pass tandem Fabry-Perot interferometer around the structural phase transition temperature. The soft phonon response can be well described using the phonon-relaxator coupled model. The soft phonon frequency ω_∞/2π obeys the |T - T^±_c|^<1/2> law except in the vicinity of T_c = 82.5±0.1 K; (ω__∞/2π)^2 = 1.20×10^3(T-82.1) (GHz^2) for T > T_c and 4.55×10^3(83.0-T) (GHz^2) for T < T_c. The soft phonon-relaxator coupling constant increases linearly above the T_c as the temperature decreases and the slope becomes steeper below T_c. The relaxation time is longer than 40 ps and independent of temperature above T_c, but becomes one order of magnitude shorter below T_c. The Fe^<3+> defects seem to be responsible for these unusual temperature dependences.
社団法人応用物理学会の論文
1997-05-30