energy ball mill Spex 8000. We use ceramic milling vials (alumina or zirconia), a single4-g ball, and a ball-to-powder weight ratio of typically2:1 to produce at least 2 g of powder. The average grain size was determined by the line broadening of X-ray diffraction (XRD) pat-terns (cf. Fig.1) for various milling times. A few hours