A study of the vibrational and thermoelectric properties of silicon type I and II clathrates
ABSTRACT First principles calculations are employed to investigate both type I and II silicon clathrates. The phonon and infrared spectra of both types of clathrate are obtained. We find a localized vibrational mode of Si46 whose frequency is 375.0 cm -1 , where the vibration in the mode localizes in the hexagonal chain. The heat capacity of both clathrates is the same as that of the diamond phase Si (d -Si ) . When the temperature is lower than 100 K, the Debye temperatures of the clathrates are higher than that of d -Si ; however, the Debye temperatures of both clathrates at high temperature (≫100 K ) are lower than that of the d -Si . The mean free paths (λ) and thermal conductivities (κ) of the clathrates are larger than those of d -Si at low temperature. The Seebeck coefficients (S) of the clathrates are higher than that of d -Si in the temperature interval 300–1000 K; however, both clathrates exhibit a lower value of σ/τ when compared to the d -Si .