Experimental and computational approaches to study the high temperature thermoelectric properties of novel topological semimetal CoSi

Abstract

Here, we study the thermoelectric properties of topological semimetal CoSi in the temperature range 300-800 K by using combined experimental and density functional theory (DFT) based methods. CoSi is synthesized using arc melting technique and the Rietveld refinement gives the lattice parameters of a = b = c = 4.445 Å. The measured values of Seebeck coefficient (S) shows the non-monotonic behaviour in the studied temperature range with the value of 1/4-81 μV K-1 at room temperature. The |S| first increases till 560 K (1/4-93 μV K-1) and then decreases up to 800 K (1/4-84 μV K-1) indicating the dominating n-type behaviour in the full temperature range. The electrical conductivity, σ (thermal conductivity, κ) shows the monotonic decreasing (increasing) behaviour with the values of 1/45.2×105 (12.1 W m-1 K-1) and 1/43.6×105 (14.2 W m-1 K-1) ω-1 m-1 at 300 K and 800 K, respectively. The κ exhibits the temperature dependency as, κ T 0.16. The DFT based Boltzmann transport theory is used to understand these behaviour. The multi-band electron and hole pockets appear to be mainly responsible for deciding the temperature dependent transport behaviour. Specifically, the decrease in the |S| above 560 K and change in the slope of σ around 450 K are due to the contribution of thermally generated charge carriers from the hole pockets. The temperature dependent relaxation time (τ) is computed by comparing the experimental σ with calculated σ/τ and it shows temperature dependency of 1/T 0.35. Further this value of τ is used to calculate the temperature dependent electronic part of thermal conductivity (κ e) and it gives a fairly good match with the experiment. Present study suggests that electronic band-structure obtained from DFT provides a reasonably good estimate of the transport coefficients of CoSi in the high temperature region of 300-800 K.