The elastic constants C ij needed to derive the mechanical properties were determined by performing finite distortions of the lattice and deriving C ij from the strain-stress relationship. Then fully the bulk B and shear G moduli are obtained by averaging the single-crystal elastic constants using, here, Voigt's method based on a uniform strain. The calculated sets of elastic constants are given in Table . All values are positive. Their combinations obeying the rules pertaining to the mechanical stability of the phase, and the equations providing the bulk B V and shear G V moduli are as follows.