Thermoelectric Properties of Bi-Doped Magnesium Silicide Stannides

Mg₂(Si,Sn)-based compounds have shown great promise for thermoelectric (TE) applications, as they are nontoxic and comprised abundantly available constituent elements. In this work, the crystal structures and TE properties of polycrystalline materials with nominal compositions Mg₂Si_0.35Sn_{0.65- x}Bi _x ( x = 0, 0.015, 0.030, and 0.045) and Mg₂Si _ySn_{0.97- y}Bi_0.03 ( y = 0.30, 0.325, and 0.35) have been investigated. The electrical conductivity, Seebeck coefficient, and thermal conductivity are strongly affected by the presence of Bi. Undoped samples showed higher values of Seebeck coefficients (below 600 K), lower electrical conductivity, and lower thermal conductivity (above 600 K) in comparison to the Bi-doped samples. Furthermore, the signs of Seebeck coefficients are all negative, confirming that n-type conduction is dominant in these materials. Electrical conductivity was enhanced by increasing the Bi content up to 3% on the Si/Sn site because of the increasing amount of electron donors, and the absolute value of Seebeck coefficient decreased. When the Bi content is greater than 3%, lower zT values were obtained at 773 K. Thermal conductivity values might decrease with increasing Sn alloying for Mg₂Si _ySn_{0.97- y}Bi_0.03, as mass and strain fluctuation caused by alloying can effectively scatter phonons. However, a different behavior was observed in higher Sn content material, possibly because of the absence of Mg atoms at the interstitial site [Mg_i, on (1/2, 1/2, 1/2)] and vacancies of Mg atoms at the (1/4, 1/4, 1/4) site, as confirmed by Rietveld refinements. Outstanding figure of merit values in excess of unity were achieved with all samples, culminating in  zT_max = 1.35.