Brillouin scattering study of the structural phase transition in the two-layer orthorhombic form of K3Co(CN)6 single crystals

The second-order structural phase transition in the two-layer orthorhombic form of K3Co(CN)6 single crystals (D2h14-C2h5; Tc approximately=83.5 K) is investigated through Brillouin scattering. A very-low-frequency soft optical phonon, which is the lowest soft optical phonon ever seen by light scattering techniques, has been observed. The soft phonon exhibits relaxational character as well as oscillatory character in the vicinity of the phase transition. Since the soft phonon belongs to the B3g irreducible representation of the D2h class, the C44 elastic constant is also expected to show softening. However, scattering from the C44 acoustic phonon is too weak to observe even near the phase transition. The (100) longitudinal acoustic phonon frequency shows a weak and continuous change through the phase transition. The behaviour can be reasonably explained by the Landau-Khalatnikov process with the dynamic susceptibility of the soft optical phonon.