The investigation of mechanical characteristics was based on the calculations of the elastic properties determined by performing finite distortions of the lattice and deriving the elastic constants from the strain-stress relationship. Most compounds are polycrystalline, and generally considered as randomly oriented single crystalline grains. Consequently, on a large scale, such materials can be considered as statistically isotropic. They are then fully described by bulk (B) and shear (G) moduli obtained by averaging the single-crystal elastic constants. The method used here is Voigt's , based on a uniform strain. The calculated sets of elastic constants are given in Table .