obtained is then calcined the calcination temperature is expediently at from 1000 u00b0 to 1300 u00b0 c preferably at from 1100 u00b0 to 1200 u00b0 c the powder prepared according to the invention is notable for a high u03b1 alpha proportion a high specific surface and a crystallite size in the order of magnitude of 0 1 u03bcm after calcination the powder exhibits strong agglomeration and subsequently has to be subjected to a disagglomeration treatment the disagglomeration also is preferably carried out in an attritor a vibratory mill or a stirred mill the grinding medium again preferably comprising bodies made of aluminum oxide the particle size distribution after disaggglomeration depends on the duration of the disagglomeration treatment and on the proportion of solids in the treatment by suitable choice of the parameters it is possible for example with solids contents of 25 percent and treatment duration in the order of magnitude of 1 hour to achieve particle size distributions in which 90 percent of the material has a particle size of less than approximately 0 4 u03bcm the mean particle size in this instance is only slightly larger than the mean crystallite size and there is no significant size reduction of the individual crystals the following examples illustrate the working of the method according to the invention in an attritor 0 6 l 70 g of aluminum oxide calcined at low temperature type ax from martinswerk gmbh d w 5010 bergheim were milled and disagglomerated for.