Acoustic phonons and a central mode in the protonic conductor K₃ H (SeO₄) 2

The dynamical properties of protonic conductor K3 H (SeO4) 2 were examined by Brillouin scattering. Both longitudinal-acoustic modes (c33 and c11) and a transversal mode (c44) monotonously depend on temperature, and no soft mode was observed. A central mode with a width about 6 GHz was observed above phase-transition temperature, TC. The central mode has a symmetry Eg corresponding to the rotational mode of SeO4 tetrahedra. The Eg mode is compatible with a zone-boundary soft mode, L3, which accounts for the symmetrical change from R 3̄ m to C2/c. The intensity of the central mode declines in the vicinity of TC. This shows that chaotic rotational motion changes into a zone-boundary soft mode with decreasing temperature.