Abstract
We verify the Vegard law for ZnSiAs2-xPx solid solution. We follow by electroreflectance the Γ15 → Γ1 transitions in the entire range of composition and give the direct threshold Γ1 → Γ6 = 3.42 eV for ZnSiP2. From the quasicubic model we obtain the crystal field and the spin-orbit splitting which are compared with theory.