Finally, single crystal growth was performed using the OFZ, which is equipped with 4 x 1000 W halogen lamps. During the growth process feed and seed rods were counter-rotated (22 rpm) to minimize the diffusion zone close to the crystallization surface. The final crystal growth process was carried out under a controlled atmosphere of mixed Ar/O2 (80:20 ml min -1 ) gas flow at a pressure of 6 bar.