The electronic and thermal transport properties of Mg 2 Sn crystals prepared by radio-frequency (RF) induction melting, were investigated in the temperature range 10À700 K to study the effect of adding excess silver to the melt on the microstructure and thermoelectric properties. As the Agcontent increases the amount of silver incorporated at doping sites tends to saturate so that the carrier concentration increases asymptotically to 6.3 Â 10 19 cm À3 . The excess amount of silver reacts with magnesium to form a fine MgAg þ Mg 2 Sn eutectic structure. The temperature dependence of the carrier mobility varies from $T À1.5 for the heavily doped samples in the extrinsic range to $T À2.8 for the undoped sample in the intrinsic range. From electrical conductivity and Hall measurements on the undoped samples we find the energy gap to be 0.36À0.38 eV. Doped samples show broad Seebeck peaks of 150À210 mV K À1 at T ¼ 350À550 K. The thermal conductivity decreases sharply as the addition of Ag is increased, and reaches a minimum value of k ¼ 3.4 W m À1 K À1 at 450 À 500 K for the doped sample containing a uniformly distributed eutectic phase, leading to a maximum figure of merit ZT max ¼ 0.30.

Mg 2 Sn compounds were prepared by the modified vertical Bridgman method, and were doped with Bi and Ag to obtain n-and p-type materials, respectively. Excess Mg was also added to some of the ingots to compensate for the loss of Mg during the preparation process. The Mg 2 Sn samples were characterized by x-ray diffraction (XRD) and scanning electron microscopy (SEM), and their power factors were calculated from the Seebeck coefficient and electrical conductivity, measured from 80 K to 700 K. The sample prepared with 4% excess Mg, which contains a small amount of Mg 2 Sn + Mg eutectic phase, had the highest power factor of 12 9 10 -3 W m -1 K -2 at 115 K, while the sample doped with 2% Ag, in which a small amount of eutectics also exists, has a power factor of 4 9 10 -3 W m -1 K -2 at 420 K.

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