Neutron powder diffraction, quasi elastic and inelastic scattering measurements were performed to examine the mechanism of phase transition and protonic conduction of K
3H (SeO
4)
2. SeO
4 tetrahedra in the high-temperature phase (phase I) were rotationally displaced with the occupancy 1/3 and the crystal structure in the room temperature phase (phase II) remained locally. Proton distribution maps obtained from MEM describe that 13.4% of protons exist at inter-layer space. Proton diffusion constants were estimated as 1.9 (1) × 10
-10 m
2/s at 393 K and 2.1 (3) × 10
-10 m
2/s at 413 K respectively. A rotational mode of SeO
4 tetrahedra causes change from
R3
m to
C2/
c. No soft mode was observed both of transversal acoustic and zone boundary optical modes, however, a diffusive spectrum was observed around 0 meV at the L-point (0.5 2 0) above
Tc as an over-dumped phonon mode. The rotational mode of SeO
4 tetrahedra drives the improper ferroelastic phase transition, and assists the disconnection and reconstruction of hydrogen bonds in the phase I.
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