The bulk and shear moduli are usually obtained by averaging the single-crystal elastic constants using Voigt's method based on a uniform strain. The calculated sets of elastic constants are given in Table . All values are positive, which is the first indication of mechanical stability. Further proofs were obtained from C ij combinations obeying rules related to mechanical stability. The elastic constants C ij and the mechanical stability for the tetragonal system are then described as follows: The equations providing the bulk (B V ) and shear (G V ) moduli for the tetragonal system are as follows: ).