4. Barium titanate (BaTiO3) has a tetragonal crystal symmetry (point group 4mm) at ro temperature. The appropriate piezoelectric strain coefficient matrix is therefore of the 0 0 0 0 d15 0 0 0 0 d15 form: dij= 0 0 d31 d31 d33 0 a. Determine the nature of the strain induced in the material when an electric field is applied along the crystallographic c axis. b. Determine the nature of the strain induced in the material when an electric field is applied along the crystallographic a axis.