value is in agreement with literature . The replacement of carbon by silicon possessing a larger radius and the overall resulting larger volume can explain the larger compressibility of Si 2 C 4 . Turning to the ternary phase i.e. Si 2 C 2 N 2 , the calculations of the two moduli B V and G V lead to significant increase that can be explained by the smaller d(Si-C) and d(Si-N) versus d(Si-C) in the carbide (Table ).