Pressure Coefficient of the Band Gap in Mg 2 Si, Mg 2 Ge, and Mg 2 Sn

The pressure deri vative of the band gap, (∂ E /∂ P ) T , has been determined to be + 3 × 10 −6 e Vbar −1 for Mg 2 Sn by measurements of the pressure derivative of the electrical conductivity. Similar measurements on Mg 2 Si and Mg 2 Ge were inconclusive, but estimates of this parameter were made from the shift of the optical obsorption edge with pressure. (∂ E /∂ P ) T = (0 ± 0.5) × 10 −6 e Vbar −1 for both compounds. The similarity of the coefficients for Mg 2 Si and Mg 2 Ge suggests that the symmetries of the band edges of both compounds are similar, while either the conduction band edge, the valence band edge, or both edges in Mg 2 Sn may have a different symmetry.