X-ray diffraction studies of Mg$_{2}$Si and Ag-doped Mg$_{2}$Si under pressure*

The magnesium disilicide (Mg$_{2}$Si) is one of the thermoelectric material in 500-800 K temperature rage. The ${p}$- and ${n}$-Mg$_{2}$Si materials are necessary for the high-performance thermoelectric device, however Mg$_{2}$Si is ${n}$-type semiconductor and stable ${p}$-type Mg$_{2}$Si has not developed. Because it was reported that Ag-dope Mg$_{2}$Si was ${p}$-type Mg$_{2}$Si, we performed the X-ray diffraction studies of Mg$_{2}$Si and Ag-doped Mg$_{2}$Si under high-pressure at NE-5C beam line (PF-AR). Four samples which were a high-purity Mg$_{2}$Si powder, a mixture of Mg and Si powders, and Ag-doped these powders were papered. Mg$_{2}$Si decomposed with increasing temperature, and new peaks of MgO and SiO$_{2}$ appeared beyond 673 K. The Mg$_{2}$Si with Ag also decomposed and the oxide peaks appeared, and Ag peaks did not disappear. In the case of Mg and Si powder, Mg$_{2}$Si was synthesized at 573 K and Mg peaks disappeared with increasing temperature but MgO or SiO$_{2}$ peaks did not appeared. In the case of Ag-doped Mg and Si powder, Mg$_{2}$Si was synthesized at 523 K, and Ag peaks disappeared at 823 K and MgO or SiO$_{2}$ peaks did not also appeared. The result means the possibility of the synthesis of Ag-doped Mg$_{2}$Si under pressure.