In the present work, a comparative study is attempted, dealing with the influence of the grain size distribution on the microstructure and the free carrier concentration in Mg2SnXSi1-X (x=0.2) ternary compounds doped with Sb. Structural in-homogeneities were monitored by using Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) as well as Fourier transform infrared spectroscopy (FTIR) in the reflectivity mode.