Local symmetry breaking and low-energy continuum in K$_2$ReCl$_6 $

Abstract

Using polarization selective spontaneous Raman scattering, we have investigated the 5$d$ transition metal compound K$_2$ReCl$_6$ 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 local fluctuations in the orientation of the ReCl$_6$ octahedra which locally break the long-range cubic symmetry. Consistent results from the related non-magnetic compound K$_2$SnCl$_6$ support this interpretation.