Electronic energy changes for the substitution of Mg atoms in the Mg8Si4 unit cell by Ca atoms and those for the substitution of Ca atoms in the Ca8Si4 unit cell by Mg atoms were calculated using density-functional theory to clarify the possible formation of a Mg2Si-Ca2Si solid solution with the expectation of a decrease in the concentration of donor-like positively charged Mg ions, the origin of persistent n-type conductivity in Mg2Si. CaMgSi with the Ca2Si-type structure, in which all the Ca atoms occupying one type of 4c site are completely substituted by Mg and all the other 4c sites remain occupied by Ca, is found to be energetically quite stable. This agrees with the observations that CaMgSi with this structure, usually referred to as the TiNiSi-type structure, is the only equilibrium phase in the Ca2Si-Mg2Si pseudobinary system. Electronic structure calculations of this phase showed that it is semimetallic, even though both Ca2Si and Mg2Si are semiconductors. (c) 2012 Elsevier B.V. All rights reserved.