Scattering of phonons by a spatially correlated system of Fe atoms and the low-temperature anomaly of thermal conductivity in HgSe:Fe crystals

The thermal conductivity κ of HgSe:Fe samples with various content N _Fe of Fe impurity was studied in the temperature range of 8–60 K. It was found that the dependence of the thermal conductivity κ on N _Fe is unconventional at low temperatures. For T<12 K, the value of κ first decreases with an increase in the Fe concentration up to N _Fe=5×10¹⁸ cm^?3 and then increases and attains a maximum for N _Fe=(1–2)×10¹⁹ cm^?3. A further increase in Fe concentration brings about a steady decrease in thermal conductivity. The electron-and phonon-related thermal conductivity of HgSe:Fe crystals with consideration of the effects caused by the ordering of trivalent Fe ions was analyzed. It is shown that both the electron-and phonon-related contributions to thermal conductivity at low temperatures are increasing functions of Fe concentration in the range of 5×10¹⁸<N _Fe<(1–2)×10¹⁹ cm^?3. However, the electronic contribution is too small to account for the experimental increase in thermal conductivity. An analysis of the lattice contribution to thermal conductivity showed that an anomalous increase in thermal conductivity is caused by a reduction in the Rayleigh scattering of phonons by a system of Fe ions with mixed valence and is related to the spatial ordering of Fe³⁺ ions.