A comparative study of thermal transport properties in FeS₂ and RuS₂

The thermal properties of RuS₂have been investigated and compared to those of FeS₂, its well-documented 3d analogue. Particular care was required in the synthesis of RuS₂by solid-state reaction, due to the strongly exothermic nature of the reaction, in contrast to the easier preparation of FeS₂. Dense polycrystalline samples were obtained and characterized by x-ray diffraction, electron microscopy, and transport measurements. While FeS₂exhibits very high thermal conductivity, reaching ∼175 Wm^-1K^-1at 75 K, RuS₂shows much lower values, with<i>κ</i>∼ 20 Wm^-1K^-1at 300 K. First-principles calculations reveal that the intrinsic lattice conductivities of FeS₂and RuS₂are comparable, with a modest reduction in RuS₂arising from enhanced phonon-phonon Umklapp scattering. The much stronger experimental suppression is explained by extrinsic microstructural disorder, consistent with scanning electron microscope and x-ray powder diffraction evidence of smaller grains and reduced crystallinity. These results highlight both the synthetic challenges, the moderate role of Umklapp scattering, and the decisive role of microstructure in shaping the transport properties of 4d transition-metal pyrites.