Solvothermally synthesized nanocrystalline CoSb3: Insights into lattice dynamics, thermal stability, and thermal conductivity

Abstract

Binary skutterudite CoSb₃ is considered as one of the most promising thermoelectric (TE) materials for power generation. However, relatively high thermal conductivity limits its application. The nanocrystalline phase addresses this issue by introducing numerous grain boundaries that scatter phonons and hinder their movement. This phonon scattering diminishes the material's heat conduction capability, effectively lowering its thermal conductivity while possibly preserving or improving its electrical properties. Hence, in search for lower thermal conductivity, we have successfully synthesized nanocrystalline CoSb₃ with an average grain size of ∼60 nm using solvothermal method. The nanocrystalline nature of the CoSb₃ powder is confirmed through various characterization techniques, including X-ray diffraction, high resolution transmission electron microscopy, and Raman spectroscopy. Our study focuses on understanding the temperature-dependent phase stability and lattice dynamics of nanocrystalline CoSb₃ skutterudites using Raman scattering. Raman spectroscopy reveals that the skutterudite phase in the nanocrystalline form remains stable up to 653 K, after which secondary phases start to develop due to oxidation in the air. Additionally, we discuss the temperature dependent Raman frequency shift for individual optical phonon modes that emphasizes the necessity of using mode-dependent parameters for the modelling of lattice thermal conductivity.