  We present a theoretical study of the influence of the relativistic effects
on electronic band structure and thermopower of Mg2X(X= Si, Ge, Sn)
semiconductors. The full potential Korringa-Kohn-Rostoker (KKR) method is used,
and the detailed comparison between the fully relativistic and
semi-relativistic electronic structure features is done. We show that the
spin-orbit (S-O) interaction splits the valence band structure at Gamma point
in good agreement with the experimental data, and this effect strongly depends
on X atom. The S-O modifications of the topology of the Gamma-centered
hole-like Fermi surface pockets lead to a change in electron transport
properties, which are investigated using the Boltzmann approach. In addition,
the simple and efficient method is presented for the calculation of density of
states effective mass m*, and then used to examine the impact of relativistic
effects on m*. It is found that S-O coupling of the valence bands reduces
effective mass and therefore significantly lowers the thermopower, primarily in
Mg2Sn, but also in Mg2Ge. A detrimental influence of the S-O interaction on
thermoelectric performance of p-type Mg2X is analyzed in function of
temperature (10-900 K) and carrier concentration (10^18-10^22 cm-3).
Interestingly, similar calculations in n-type Mg2X, show negligible effect of
the S-O interaction on lowest conduction bands and consequently also on the
Seebeck coefficient.
