BRILLOUIN-SCATTERING FROM THE SOFT OPTICAL PHONON IN THE ORTHORHOMBICFORM OF A K3CO1-XFEX(CN)(6) (X-LESS-THAN-OR-EQUAL-TO-0.03) CRYSTAL

The soft optical phonon in the two-layer orthorhombic form of a K3Co1- xFex(CN)(6) (x less than or equal to 0.03) single crystal has been rei nvestigated using a (3 + 3)-pass tandem Fabry-Perot interferometer aro und the structural phase transition temperature. The soft phonon respo nse can be well described using the phonon-relaxator coupled model. Th e soft phonon frequency w(infinity)/2 pi obeys the \ T - Tc+/-\(1/2) l aw except in the vicinity of T-c = 82.5 +/- 0.1K; (w(infinity)/2 pi)(2 ) = 1.20 x 10(3)(T - 82.1) (GHz(2)) for T > T-c and 4.55 x 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 4 0 ps and independent of temperature above T-c, but becomes one order o f magnitude shorter below T-c. The Fe3+ defects seem to be responsible for these unusual temperature dependences.