Lattice dynamics of commensurate and incommensurateK2SeO4

Abstract
K2SeO4 undergoes an incommensurate structural transformation at T0=128 K followed by a commensurate "lock-in" transformation at Tc=93 K. Below T0, the soft-phonon mode associated with the transformation is "split" into modes representing fluctuations in amplitude and phase of the complex order parameter. We have studied the dispersion and temperature dependence of these modes at T<~118 K. We have also seen intensity anomalies which can be explained by interactions between a transverse-acoustic-phonon branch and the low-lying "phason" branch.