Additional scattering from low-lying phonon modes in the thermal conductivity of the quasi-one-dimensional compound (${\mathrm{TaSe}}_4$${)}_2$I

We report on thermal conductivity κ measurements between 0.07 and 4 K of the quasi-one-dimensional compound (${\mathrm{TaSe}}_4$${)}_2$I and we compare them with the data for (${\mathrm{NbSe}}_4$${)}_3$I. Whereas the variation of κ for (${\mathrm{NbSe}}_4$${)}_3$I is rather usual for an insulating material, it exhibits a pronounced anomaly for (${\mathrm{TaSe}}_4$${)}_2$I. This anomaly has the shape of a broad minimum around 3 K which we ascribe to a strong phonon scattering of low-lying transverse acoustic modes with a very weak dispersion over a large part of the Brillouin zone, lying at an unusual low frequency.