Local symmetry breaking and low-energy continuum in K2ReCl6

Using polarization selective spontaneous Raman scattering, we have investigated the 5d transition metal compound K2ReCl6 which displays a series of structural phase transitions. We observe a violation of the Raman selection rules in the cubic high temperature phase as well as a low-energy scattering continuum persistent throughout the investigated temperature range from 300 down to 5 K. The continuum couples to one of the phonon modes at temperatures above the lowest structural phase transition at 76 K. We propose a common origin of these observations caused by disorder in the orientation of the ReCl6 octahedra which locally breaks the long-range cubic symmetry. Consistent results from the related nonmagnetic compound K2SnCl6 support this interpretation.