Neutron Scattering Study of the Phase Transition in KD3(SeO3)2

Abstract

The phase transition of KD 3 (SeO 3 ) 2 has been investigated by means of neutron scattering. The transverse acoustic phonon mode corresponding to the C 44 stiffness constant (C 44 TA mode) was measured along (0, 0, ξ) from ξ=0.025 to the zone boundary ξ=0.5. The spectral density of the C 44 TA mode is anomalous for 0.05<ξ<0.2. Above this region, the phonon is unrenormalized and is well defined. Below the anomalous region, the TA mode shows soft phonon behavior which agrees well with the ultrasonic and the Brillouin scattering experiments. The neutron result is interpreted to mean that the TA mode shows crossover behavior through the regime of relaxational hydrogen motion. Another possible explanation of the anomalous behavior of the TA mode is coupling to a soft optical mode. A search for such a mode was performed, with negative results.