Effect of vacancies on the thermoelectric properties of Mg2Si1−xSbx(0⩽x⩽0.1)

Abstract

Mg${}_{2}$Si${}_{1\ensuremath{-}x}$Sb${}_{x}$ compositions with $x$ ranging from 0 to 0.1 were prepared by induction melting and compacted by hot uniaxial pressing. The lattice parameter and the positional occupancy at the Mg site as a function of Sb substitution were calculated from Rietveld refinement. A monotonic increase in the $a$lattice constant value was observed with increasing Sb substitution. Also, a decrease in the Mg site occupancy was observed with increasing Sb substitution and was confirmed by energy dispersive analysis of x-ray measurements. Room temperature carrier concentration ($n$) measurements show an increase from $1.4\ifmmode\times\else\texttimes\fi{}{10}^{18}$ cm${}^{\ensuremath{-}3}$ for $x=0$ to $1.4\ifmmode\times\else\texttimes\fi{}{10}^{20}$ cm${}^{\ensuremath{-}3}$ for $x=0.025$ followed by a nearly constant $n$, indicating the influence of vacancy formation. Electrical conductivity ($\ensuremath{\sigma}$), Seebeck coefficient ($S$), and thermal conductivity ($\ensuremath{\kappa}$) values were measured from 300 to 773 K. The observed trends in the absolute values of $\ensuremath{\sigma}$ and $S$ can be explained based on the vacancy formation at the Mg site which counteracts the doping influence of Sb. The $\ensuremath{\kappa}$${}_{L}$ (phonon component of the thermal conductivity) values show a significant decrease with increasing Sb substitution. Calculation of the thermoelectric figure of merit ($\mathit{ZT}$) shows a maximum $\mathit{ZT}$ of 0.55 for $x=0.10$ compared to a $\mathit{ZT}$ of $~$0.05 for $x=0.0$.