X-ray diffraction and energy dispersive x-ray spectroscopic analyses showed a natural galena (PbS) crystal from Freiberg in Saxony (Germany) to be a single phase specimen [rock salt (NaCl) structure type, space group $Fm\overline{3}m, a=5.932\left(1\right)$ Å] with stoichiometric composition and an enhanced dislocation density ($\delta \approx {10}^{11}{\mathrm{cm}}^{-2}$). The latter parameter leads to an increase of the electrical resistivity in the high-temperature regime, as well as to the appearance of phonon resonance with a characteristic frequency ${\omega }_{\mathrm{PR}}=3.8\left(1\right)$ THz. Being in the same range (i.e., 3–5.5 THz) with the sulfur optical modes of highest group velocities, it results in a drastic reduction (by $\sim$ 75%) of thermal conductivity ($\kappa$) at lower temperatures (i.e., $<100$ K), as well as in the appearance of a characteristic minimum in $\kappa$ at $T\approx$ 30 K. Furthermore, the studied galena is characterized by phonon-drag behavior and by temperature dependent switch of the charge carrier scattering mechanism regime (i.e., scattering on dislocations for $T<100$ K, on acoustic phonons for 100 K $<T<170$ K and on both acoustic and optical phonons for 170 K $<T<300$ K). The combined theoretical calculation and optical spectroscopic study confirm this mineral to be a direct gap degenerate semiconductor. The possible origins of the second-order Raman spectrum are discussed.

• Received 24 February 2023
• Revised 27 April 2023
• Accepted 11 May 2023

DOI:https://doi.org/10.1103/PhysRevB.107.195203