Mg₂X (X = Si, Ge or Sn) based alloys are considered as promising candidates in the middle to high temperature range thermoelectric applications due to their low cost, nontoxicity and abundance of constituent elements. However, they exhibit relatively higher thermal conductivity compared to other thermoelectric materials. In this study, we present a simple synthetic method for a Mg_2 + xSi_0.5Sn_0.5Sb_0.075 material with a heterogeneous microstructure that reduces thermal conductivity. By controlling the amount of excess Mg during synthesis, a heterogeneous microstructure due to the formation of secondary phases was obtained. This heterogeneous microstructure reduced the thermal conductivity through phonon scattering, leading to an improved thermoelectric efficiency, particularly at high temperatures.