A comparative study of thermal transport properties in FeS₂and RuS₂

The thermal properties of RuS₂ have been investigated and compared to those of FeS₂, its well-documented 3d analogue. Particular care was required in the synthesis of RuS₂ by 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 FeS₂ exhibits very high thermal conductivity, reaching ∼175 Wm⁻¹ K⁻¹ at 75 K, RuS₂ shows much lower values, with κ ∼ 20 Wm⁻¹ K⁻¹ at 300 K. First-principles calculations reveal that the intrinsic lattice conductivities of FeS₂ and RuS₂ are comparable, with a modest reduction in RuS₂ arising 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.